27 files changed, 998 insertions, 97 deletions

diff --git a/quantum/audio/audio.h b/quantum/audio/audio.h
index bc00cd19e..dccf03d5f 100644
--- a/quantum/audio/audio.h
+++ b/quantum/audio/audio.h
@@ -83,6 +83,7 @@ void increase_tempo(uint8_t tempo_change);
 void decrease_tempo(uint8_t tempo_change);
 
 void audio_init(void);
+void audio_startup(void);
 
 #ifdef PWM_AUDIO
 void play_sample(uint8_t* s, uint16_t l, bool r);
diff --git a/quantum/audio/audio_avr.c b/quantum/audio/audio_avr.c
index 5a96bf643..1bac43bb4 100644
--- a/quantum/audio/audio_avr.c
+++ b/quantum/audio/audio_avr.c
@@ -227,7 +227,9 @@ void audio_init() {
 
         audio_initialized = true;
     }
+}
 
+void audio_startup() {
     if (audio_config.enable) {
         PLAY_SONG(startup_song);
     }
diff --git a/quantum/audio/audio_chibios.c b/quantum/audio/audio_chibios.c
index 1f147f2c9..dddb8f135 100644
--- a/quantum/audio/audio_chibios.c
+++ b/quantum/audio/audio_chibios.c
@@ -86,13 +86,21 @@ static void gpt_cb8(GPTDriver *gptp);
 
 #define START_CHANNEL_1()        \
     gptStart(&GPTD6, &gpt6cfg1); \
-    gptStartContinuous(&GPTD6, 2U)
+    gptStartContinuous(&GPTD6, 2U); \
+    palSetPadMode(GPIOA, 4, PAL_MODE_INPUT_ANALOG)
 #define START_CHANNEL_2()        \
     gptStart(&GPTD7, &gpt7cfg1); \
-    gptStartContinuous(&GPTD7, 2U)
-#define STOP_CHANNEL_1() gptStopTimer(&GPTD6)
-#define STOP_CHANNEL_2() gptStopTimer(&GPTD7)
-#define RESTART_CHANNEL_1() \
+    gptStartContinuous(&GPTD7, 2U); \
+    palSetPadMode(GPIOA, 5, PAL_MODE_INPUT_ANALOG)
+#define STOP_CHANNEL_1()          \
+    gptStopTimer(&GPTD6);         \
+    palSetPadMode(GPIOA, 4, PAL_MODE_OUTPUT_PUSHPULL); \
+    palSetPad(GPIOA, 4)
+#define STOP_CHANNEL_2()         \
+    gptStopTimer(&GPTD7);        \
+    palSetPadMode(GPIOA, 5, PAL_MODE_OUTPUT_PUSHPULL); \
+    palSetPad(GPIOA, 5)
+    #define RESTART_CHANNEL_1() \
     STOP_CHANNEL_1();       \
     START_CHANNEL_1()
 #define RESTART_CHANNEL_2() \
@@ -274,6 +282,12 @@ void audio_init() {
     dacStart(&DACD2, &dac1cfg2);
 
     /*
+     * Start the note timer
+     */
+    gptStart(&GPTD8, &gpt8cfg1);
+    gptStartContinuous(&GPTD8, 2U);
+
+    /*
      * Starting GPT6/7 driver, it is used for triggering the DAC.
      */
     START_CHANNEL_1();
@@ -287,10 +301,12 @@ void audio_init() {
 
     audio_initialized = true;
 
+    stop_all_notes();
+}
+
+void audio_startup() {
     if (audio_config.enable) {
         PLAY_SONG(startup_song);
-    } else {
-        stop_all_notes();
     }
 }
 
@@ -630,6 +646,9 @@ bool is_playing_notes(void) { return playing_notes; }
 bool is_audio_on(void) { return (audio_config.enable != 0); }
 
 void audio_toggle(void) {
+    if (audio_config.enable) {
+        stop_all_notes();
+    }
     audio_config.enable ^= 1;
     eeconfig_update_audio(audio_config.raw);
     if (audio_config.enable) {
diff --git a/quantum/audio/audio_pwm.c b/quantum/audio/audio_pwm.c
index 545aef6dd..d93ac4bb4 100644
--- a/quantum/audio/audio_pwm.c
+++ b/quantum/audio/audio_pwm.c
@@ -29,6 +29,11 @@
 
 #define CPU_PRESCALER 8
 
+#ifndef STARTUP_SONG
+#    define STARTUP_SONG SONG(STARTUP_SOUND)
+#endif
+float startup_song[][2] = STARTUP_SONG;
+
 // Timer Abstractions
 
 // TIMSK3 - Timer/Counter #3 Interrupt Mask Register
@@ -155,6 +160,12 @@ void audio_init() {
     audio_initialized = true;
 }
 
+void audio_startup() {
+    if (audio_config.enable) {
+        PLAY_SONG(startup_song);
+    }
+}
+
 void stop_all_notes() {
     if (!audio_initialized) {
         audio_init();
diff --git a/quantum/audio/musical_notes.h b/quantum/audio/musical_notes.h
index 8ac6aafd3..0ba572c34 100644
--- a/quantum/audio/musical_notes.h
+++ b/quantum/audio/musical_notes.h
@@ -17,7 +17,9 @@
 #pragma once
 
 // Tempo Placeholder
-#define TEMPO_DEFAULT 100
+#ifndef TEMPO_DEFAULT
+#    define TEMPO_DEFAULT 100
+#endif
 
 #define SONG(notes...) \
     { notes }
@@ -60,8 +62,9 @@
 #define TIMBRE_25 0.250f
 #define TIMBRE_50 0.500f
 #define TIMBRE_75 0.750f
-#define TIMBRE_DEFAULT TIMBRE_50
-
+#ifndef TIMBRE_DEFAULT
+#    define TIMBRE_DEFAULT TIMBRE_50
+#endif
 // Notes - # = Octave
 
 #ifdef __arm__
diff --git a/quantum/backlight/backlight_avr.c b/quantum/backlight/backlight_avr.c
index 4d66da80b..2ecdd4f2c 100644
--- a/quantum/backlight/backlight_avr.c
+++ b/quantum/backlight/backlight_avr.c
@@ -68,7 +68,7 @@
 #        define COMxx1 COM3A1
 #        define OCRxx OCR3A
 #    endif
-#elif (defined(__AVR_ATmega16U2__) || defined(__AVR_ATmega32U2__)) && (BACKLIGHT_PIN == B7 || BACKLIGHT_PIN == C5 || BACKLIGHT_PIN == C6)
+#elif (defined(__AVR_AT90USB162__) || defined(__AVR_ATmega16U2__) || defined(__AVR_ATmega32U2__)) && (BACKLIGHT_PIN == B7 || BACKLIGHT_PIN == C5 || BACKLIGHT_PIN == C6)
 #    define HARDWARE_PWM
 #    define ICRx ICR1
 #    define TCCRxA TCCR1A
diff --git a/quantum/config_common.h b/quantum/config_common.h
index 5973232ef..bfaf7389e 100644
--- a/quantum/config_common.h
+++ b/quantum/config_common.h
@@ -39,7 +39,7 @@
 #        define PIND_ADDRESS 0x9
 #        define PINE_ADDRESS 0xC
 #        define PINF_ADDRESS 0xF
-#    elif defined(__AVR_ATmega32U2__) || defined(__AVR_ATmega16U2__) || defined(__AVR_ATmega328P__) || defined(__AVR_ATmega328__)
+#    elif defined(__AVR_AT90USB162__) || defined(__AVR_ATmega16U2__) || defined(__AVR_ATmega32U2__) || defined(__AVR_ATmega328P__) || defined(__AVR_ATmega328__)
 #        define ADDRESS_BASE 0x00
 #        define PINB_ADDRESS 0x3
 #        define PINC_ADDRESS 0x6
diff --git a/quantum/debounce/sym_defer_pk.c b/quantum/debounce/sym_defer_pk.c
index 6c0e3bb07..60513f98e 100644
--- a/quantum/debounce/sym_defer_pk.c
+++ b/quantum/debounce/sym_defer_pk.c
@@ -23,6 +23,12 @@ When no state changes have occured for DEBOUNCE milliseconds, we push the state.
 #include "quantum.h"
 #include <stdlib.h>
 
+#ifdef PROTOCOL_CHIBIOS
+#    if CH_CFG_USE_MEMCORE == FALSE
+#        error ChibiOS is configured without a memory allocator. Your keyboard may have set `#define CH_CFG_USE_MEMCORE FALSE`, which is incompatible with this debounce algorithm.
+#    endif
+#endif
+
 #ifndef DEBOUNCE
 #    define DEBOUNCE 5
 #endif
diff --git a/quantum/debounce/sym_eager_pk.c b/quantum/debounce/sym_eager_pk.c
index 93a40ad44..e66cf92d7 100644
--- a/quantum/debounce/sym_eager_pk.c
+++ b/quantum/debounce/sym_eager_pk.c
@@ -23,6 +23,12 @@ No further inputs are accepted until DEBOUNCE milliseconds have occurred.
 #include "quantum.h"
 #include <stdlib.h>
 
+#ifdef PROTOCOL_CHIBIOS
+#    if CH_CFG_USE_MEMCORE == FALSE
+#        error ChibiOS is configured without a memory allocator. Your keyboard may have set `#define CH_CFG_USE_MEMCORE FALSE`, which is incompatible with this debounce algorithm.
+#    endif
+#endif
+
 #ifndef DEBOUNCE
 #    define DEBOUNCE 5
 #endif
diff --git a/quantum/debounce/sym_eager_pr.c b/quantum/debounce/sym_eager_pr.c
index d12931fdd..20ccb46f1 100644
--- a/quantum/debounce/sym_eager_pr.c
+++ b/quantum/debounce/sym_eager_pr.c
@@ -23,6 +23,12 @@ No further inputs are accepted until DEBOUNCE milliseconds have occurred.
 #include "quantum.h"
 #include <stdlib.h>
 
+#ifdef PROTOCOL_CHIBIOS
+#    if CH_CFG_USE_MEMCORE == FALSE
+#        error ChibiOS is configured without a memory allocator. Your keyboard may have set `#define CH_CFG_USE_MEMCORE FALSE`, which is incompatible with this debounce algorithm.
+#    endif
+#endif
+
 #ifndef DEBOUNCE
 #    define DEBOUNCE 5
 #endif
diff --git a/quantum/dynamic_keymap.c b/quantum/dynamic_keymap.c
index 0608b469c..a860b9497 100644
--- a/quantum/dynamic_keymap.c
+++ b/quantum/dynamic_keymap.c
@@ -37,6 +37,8 @@
 #ifndef DYNAMIC_KEYMAP_EEPROM_MAX_ADDR
 #    if defined(__AVR_AT90USB646__) || defined(__AVR_AT90USB647__) || defined(__AVR_AT90USB1286__) || defined(__AVR_AT90USB1287__)
 #        define DYNAMIC_KEYMAP_EEPROM_MAX_ADDR 2047
+#    elif defined(__AVR_AT90USB162__)
+#        define DYNAMIC_KEYMAP_EEPROM_MAX_ADDR 511
 #    else
 #        define DYNAMIC_KEYMAP_EEPROM_MAX_ADDR 1023
 #    endif
diff --git a/quantum/encoder.c b/quantum/encoder.c
index 7ca31afed..2ed64c1e3 100644
--- a/quantum/encoder.c
+++ b/quantum/encoder.c
@@ -94,8 +94,9 @@ void encoder_init(void) {
 #endif
 }
 
-static void encoder_update(int8_t index, uint8_t state) {
-    uint8_t i = index;
+static bool encoder_update(int8_t index, uint8_t state) {
+    bool    changed = false;
+    uint8_t i       = index;
 
 #ifdef ENCODER_RESOLUTIONS
     int8_t resolution = encoder_resolutions[i];
@@ -109,40 +110,53 @@ static void encoder_update(int8_t index, uint8_t state) {
     encoder_pulses[i] += encoder_LUT[state & 0xF];
     if (encoder_pulses[i] >= resolution) {
         encoder_value[index]++;
+        changed = true;
         encoder_update_kb(index, ENCODER_COUNTER_CLOCKWISE);
     }
     if (encoder_pulses[i] <= -resolution) {  // direction is arbitrary here, but this clockwise
         encoder_value[index]--;
+        changed = true;
         encoder_update_kb(index, ENCODER_CLOCKWISE);
     }
     encoder_pulses[i] %= resolution;
+    return changed;
 }
 
-void encoder_read(void) {
+bool encoder_read(void) {
+    bool changed = false;
     for (uint8_t i = 0; i < NUMBER_OF_ENCODERS; i++) {
         encoder_state[i] <<= 2;
         encoder_state[i] |= (readPin(encoders_pad_a[i]) << 0) | (readPin(encoders_pad_b[i]) << 1);
-        encoder_update(i, encoder_state[i]);
+        changed |= encoder_update(i, encoder_state[i]);
     }
+    return changed;
 }
 
 #ifdef SPLIT_KEYBOARD
+void last_encoder_activity_trigger(void);
+
 void encoder_state_raw(uint8_t* slave_state) { memcpy(slave_state, &encoder_value[thisHand], sizeof(uint8_t) * NUMBER_OF_ENCODERS); }
 
 void encoder_update_raw(uint8_t* slave_state) {
+    bool changed = false;
     for (uint8_t i = 0; i < NUMBER_OF_ENCODERS; i++) {
         uint8_t index = i + thatHand;
         int8_t  delta = slave_state[i] - encoder_value[index];
         while (delta > 0) {
             delta--;
             encoder_value[index]++;
+            changed = true;
             encoder_update_kb(index, ENCODER_COUNTER_CLOCKWISE);
         }
         while (delta < 0) {
             delta++;
             encoder_value[index]--;
+            changed = true;
             encoder_update_kb(index, ENCODER_CLOCKWISE);
         }
     }
+
+    // Update the last encoder input time -- handled external to encoder_read() when we're running a split
+    if (changed) last_encoder_activity_trigger();
 }
 #endif
diff --git a/quantum/encoder.h b/quantum/encoder.h
index ec09a8cc4..db6f220da 100644
--- a/quantum/encoder.h
+++ b/quantum/encoder.h
@@ -20,7 +20,7 @@
 #include "quantum.h"
 
 void encoder_init(void);
-void encoder_read(void);
+bool encoder_read(void);
 
 void encoder_update_kb(int8_t index, bool clockwise);
 void encoder_update_user(int8_t index, bool clockwise);
diff --git a/quantum/keymap_extras/keymap_us_extended.h b/quantum/keymap_extras/keymap_us_extended.h
new file mode 100644
index 000000000..b2b3a734c
--- /dev/null
+++ b/quantum/keymap_extras/keymap_us_extended.h
@@ -0,0 +1,227 @@
+/* Copyright 2020
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "keymap.h"
+
+// clang-format off
+
+/*
+ * ┌───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───────┐
+ * │ ` │ 1 │ 2 │ 3 │ 4 │ 5 │ 6 │ 7 │ 8 │ 9 │ 0 │ - │ = │       │
+ * ├───┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─────┤
+ * │     │ Q │ W │ E │ R │ T │ Y │ U │ I │ O │ P │ [ │ ] │  \  │
+ * ├─────┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴─────┤
+ * │      │ A │ S │ D │ F │ G │ H │ J │ K │ L │ ; │ ' │        │
+ * ├──────┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴────────┤
+ * │        │ Z │ X │ C │ V │ B │ N │ M │ , │ . │ / │          │
+ * ├────┬───┴┬──┴─┬─┴───┴───┴───┴───┴───┴──┬┴───┼───┴┬────┬────┤
+ * │    │    │    │                        │    │    │    │    │
+ * └────┴────┴────┴────────────────────────┴────┴────┴────┴────┘
+ */
+// Row 1
+#define US_GRV  KC_GRV  // `
+#define US_1    KC_1    // 1
+#define US_2    KC_2    // 2
+#define US_3    KC_3    // 3
+#define US_4    KC_4    // 4
+#define US_5    KC_5    // 5
+#define US_6    KC_6    // 6
+#define US_7    KC_7    // 7
+#define US_8    KC_8    // 8
+#define US_9    KC_9    // 9
+#define US_0    KC_0    // 0
+#define US_MINS KC_MINS // -
+#define US_EQL  KC_EQL  // =
+// Row 2
+#define US_Q    KC_Q    // Q
+#define US_W    KC_W    // W
+#define US_E    KC_E    // E
+#define US_R    KC_R    // R
+#define US_T    KC_T    // T
+#define US_Y    KC_Y    // Y
+#define US_U    KC_U    // U
+#define US_I    KC_I    // I
+#define US_O    KC_O    // O
+#define US_P    KC_P    // P
+#define US_LBRC KC_LBRC // [
+#define US_RBRC KC_RBRC // ]
+#define US_BSLS KC_BSLS // (backslash)
+// Row 3
+#define US_A    KC_A    // A
+#define US_S    KC_S    // S
+#define US_D    KC_D    // D
+#define US_F    KC_F    // F
+#define US_G    KC_G    // G
+#define US_H    KC_H    // H
+#define US_J    KC_J    // J
+#define US_K    KC_K    // K
+#define US_L    KC_L    // L
+#define US_SCLN KC_SCLN // ;
+#define US_QUOT KC_QUOT // '
+// Row 4
+#define US_Z    KC_Z    // Z
+#define US_X    KC_X    // X
+#define US_C    KC_C    // C
+#define US_V    KC_V    // V
+#define US_B    KC_B    // B
+#define US_N    KC_N    // N
+#define US_M    KC_M    // M
+#define US_COMM KC_COMM // ,
+#define US_DOT  KC_DOT  // .
+#define US_SLSH KC_SLSH // /
+
+/* Shifted symbols
+ * ┌───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───────┐
+ * │ ~ │ ! │ @ │ # │ $ │ % │ ^ │ & │ * │ ( │ ) │ _ │ + │       │
+ * ├───┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─────┤
+ * │     │   │   │   │   │   │   │   │   │   │   │ { │ } │  |  │
+ * ├─────┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴─────┤
+ * │      │   │   │   │   │   │   │   │   │   │ : │ " │        │
+ * ├──────┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴────────┤
+ * │        │   │   │   │   │   │   │   │ < │ > │ ? │          │
+ * ├────┬───┴┬──┴─┬─┴───┴───┴───┴───┴───┴──┬┴───┼───┴┬────┬────┤
+ * │    │    │    │                        │    │    │    │    │
+ * └────┴────┴────┴────────────────────────┴────┴────┴────┴────┘
+ */
+// Row 1
+#define US_TILD S(US_GRV)  // ~
+#define US_EXLM S(US_1)    // !
+#define US_AT   S(US_2)    // @
+#define US_HASH S(US_3)    // #
+#define US_DLR  S(US_4)    // $
+#define US_PERC S(US_5)    // %
+#define US_CIRC S(US_6)    // ^
+#define US_AMPR S(US_7)    // &
+#define US_ASTR S(US_8)    // *
+#define US_LPRN S(US_9)    // (
+#define US_RPRN S(US_0)    // )
+#define US_UNDS S(US_MINS) // _
+#define US_PLUS S(US_EQL)  // +
+// Row 2
+#define US_LCBR S(US_LBRC) // {
+#define US_RCBR S(US_RBRC) // }
+#define US_PIPE S(US_BSLS) // |
+// Row 3
+#define US_COLN S(US_SCLN) // :
+#define US_DQUO S(US_QUOT) // "
+// Row 4
+#define US_LABK S(US_COMM) // <
+#define US_RABK S(US_DOT)  // >
+#define US_QUES S(US_SLSH) // ?
+
+/* AltGr symbols
+ * ┌───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───────┐
+ * │ ` │ ¹ │ ² │ ³ │ ¤ │ € │ ^ │ ̛  │ ¾ │ ‘ │ ’ │ ¥ │ × │       │
+ * ├───┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─────┤
+ * │     │ Ä │ Å │ É │ ® │ Þ │ Ü │ Ú │ Í │ Ó │ Ö │ « │ » │  ¬  │
+ * ├─────┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴─────┤
+ * │      │ Á │ ß │ Ð │   │   │   │ Ï │ Œ │ Ø │ ¶ │ ' │        │
+ * ├──────┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴────────┤
+ * │        │ Æ │   │ © │   │   │ Ñ │ µ │ Ç │ ˙ │ ¿ │          │
+ * ├────┬───┴┬──┴─┬─┴───┴───┴───┴───┴───┴──┬┴───┼───┴┬────┬────┤
+ * │    │    │    │                        │    │    │    │    │
+ * └────┴────┴────┴────────────────────────┴────┴────┴────┴────┘
+ */
+// Row 1
+#define US_DGRV ALGR(US_GRV)  // ` (dead)
+#define US_SUP1 ALGR(US_1)    // ¹
+#define US_SUP2 ALGR(US_2)    // ²
+#define US_SUP3 ALGR(US_3)    // ³
+#define US_CURR ALGR(US_4)    // ¤
+#define US_EURO ALGR(US_5)    // €
+#define US_DCIR ALGR(US_6)    // ^ (dead)
+#define US_HORN ALGR(US_7)    // ̛̛  (dead)
+#define US_OGON ALGR(US_8)    // ˛ (dead)
+#define US_LSQU ALGR(US_9)    // ‘
+#define US_RSQU ALGR(US_0)    // ’
+#define US_YEN  ALGR(US_MINS) // ¥
+#define US_MUL  ALGR(US_EQL)  // ×
+// Row 2
+#define US_ADIA ALGR(US_Q)    // Ä
+#define US_ARNG ALGR(US_W)    // Å
+#define US_EACU ALGR(US_E)    // É
+#define US_EDIA ALGR(US_R)    // Ë
+#define US_THRN ALGR(US_T)    // Þ
+#define US_UDIA ALGR(US_Y)    // Ü
+#define US_UACU ALGR(US_U)    // Ú
+#define US_IACU ALGR(US_I)    // Í
+#define US_OACU ALGR(US_O)    // Ó
+#define US_ODIA ALGR(US_P)    // Ö
+#define US_LDAQ ALGR(US_LBRC) // «
+#define US_RDAQ ALGR(US_RBRC) // »
+#define US_NOT  ALGR(US_BSLS) // ¬
+// Row 3
+#define US_AACU ALGR(US_A)    // Á
+#define US_SS   ALGR(US_S)    // ß
+#define US_ETH  ALGR(US_D)    // Ð
+#define US_IDIA ALGR(US_J)    // Ï
+#define US_OE   ALGR(US_K)    // Œ
+#define US_OSTR ALGR(US_L)    // Ø
+#define US_PILC ALGR(US_SCLN) // ¶
+#define US_ACUT ALGR(US_QUOT) // ´ (dead)
+// Row 4
+#define US_AE   ALGR(US_Z)    // Æ
+#define US_OE_2 ALGR(US_X)    // Œ
+#define US_COPY ALGR(US_C)    // ©
+#define US_REGD ALGR(US_V)    // ®
+#define US_NTIL ALGR(US_N)    // Ñ
+#define US_MICR ALGR(US_M)    // µ
+#define US_CCED ALGR(US_COMM) // Ç
+#define US_DOTA ALGR(US_DOT)  // ˙ (dead)
+#define US_IQUE ALGR(US_SLSH) // ¿
+
+/* Shift+AltGr symbols
+ * ┌───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───────┐
+ * │ ~ │ ¡ │ ˝ │ ¯ │ £ │ ¸ │ ¼ │ ½ │ ¾ │ ˘ │ ° │  ̣ │ ÷ │       │
+ * ├───┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─────┤
+ * │     │   │   │   │   │   │   │   │   │   │   │ “ │ ” │  ¦  │
+ * ├─────┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴─────┤
+ * │      │   │ § │   │   │   │   │   │   │   │ ° │ " │        │
+ * ├──────┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴────────┤
+ * │        │   │   │ ¢ │   │   │   │   │   │ ˇ │  ̉ │          │
+ * ├────┬───┴┬──┴─┬─┴───┴───┴───┴───┴───┴──┬┴───┼───┴┬────┬────┤
+ * │    │    │    │                        │    │    │    │    │
+ * └────┴────┴────┴────────────────────────┴────┴────┴────┴────┘
+ */
+// Row 1
+#define US_DTIL S(ALGR(US_GRV))  // ~ (dead)
+#define US_IEXL S(ALGR(US_1))    // ¡
+#define US_DACU S(ALGR(US_2))    // ˝ (dead)
+#define US_MACR S(ALGR(US_3))    // ¯ (dead)
+#define US_PND  S(ALGR(US_4))    // £
+#define US_CEDL S(ALGR(US_5))    // ¸ (dead)
+#define US_QRTR S(ALGR(US_6))    // ¼
+#define US_HALF S(ALGR(US_7))    // ½
+#define US_TQTR S(ALGR(US_8))    // ¾
+#define US_BREV S(ALGR(US_9))    // ˘ (dead)
+#define US_RNGA S(ALGR(US_0))    // ° (dead)
+#define US_DOTB S(ALGR(US_MINS)) // ̣  (dead)
+#define US_DIV  S(ALGR(US_EQL))  // ÷
+// Row 2
+#define US_LDQU S(ALGR(US_LBRC)) // “
+#define US_RDQU S(ALGR(US_LBRC)) // ”
+#define US_BRKP S(ALGR(US_BSLS)) // ¦
+// Row 3
+#define US_SECT S(ALGR(US_S))    // §
+#define US_DEG  S(ALGR(US_SCLN)) // °
+#define US_DIAE S(ALGR(US_QUOT)) // ¨ (dead)
+// Row 4
+#define US_CENT S(ALGR(US_C))    // ¢
+#define US_CARN S(ALGR(US_DOT))  // ˇ (dead)
+#define US_HOKA S(ALGR(US_SLSH)) //  ̉ (dead)
+
diff --git a/quantum/keymap_extras/keymap_us_international.h b/quantum/keymap_extras/keymap_us_international.h
index a3bc46597..49afcc4fb 100644
--- a/quantum/keymap_extras/keymap_us_international.h
+++ b/quantum/keymap_extras/keymap_us_international.h
@@ -26,7 +26,7 @@
  * ├───┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─────┤
  * │     │ Q │ W │ E │ R │ T │ Y │ U │ I │ O │ P │ [ │ ] │  \  │
  * ├─────┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴─────┤
- * │      │ A │ S │ D │ F │ G │ H │ J │ K │ L │ ; │ ' │        │
+ * │      │ A │ S │ D │ F │ G │ H │ J │ K │ L │ ; │ ´ │        │
  * ├──────┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴────────┤
  * │        │ Z │ X │ C │ V │ B │ N │ M │ , │ . │ / │          │
  * ├────┬───┴┬──┴─┬─┴───┴───┴───┴───┴───┴──┬┴───┼───┴┬────┬────┤
@@ -34,7 +34,7 @@
  * └────┴────┴────┴────────────────────────┴────┴────┴────┴────┘
  */
 // Row 1
-#define US_GRV  KC_GRV  // ` (dead)
+#define US_DGRV KC_GRV  // ` (dead)
 #define US_1    KC_1    // 1
 #define US_2    KC_2    // 2
 #define US_3    KC_3    // 3
@@ -72,7 +72,7 @@
 #define US_K    KC_K    // K
 #define US_L    KC_L    // L
 #define US_SCLN KC_SCLN // ;
-#define US_QUOT KC_QUOT // ' (dead)
+#define US_ACUT KC_QUOT // ´ (dead)
 // Row 4
 #define US_Z    KC_Z    // Z
 #define US_X    KC_X    // X
@@ -91,7 +91,7 @@
  * ├───┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─────┤
  * │     │   │   │   │   │   │   │   │   │   │   │ { │ } │  |  │
  * ├─────┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴─────┤
- * │      │   │   │   │   │   │   │   │   │   │ : │ " │        │
+ * │      │   │   │   │   │   │   │   │   │   │ : │ ¨ │        │
  * ├──────┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴────────┤
  * │        │   │   │   │   │   │   │   │ < │ > │ ? │          │
  * ├────┬───┴┬──┴─┬─┴───┴───┴───┴───┴───┴──┬┴───┼───┴┬────┬────┤
@@ -99,13 +99,13 @@
  * └────┴────┴────┴────────────────────────┴────┴────┴────┴────┘
  */
 // Row 1
-#define US_TILD S(US_GRV)  // ~ (dead)
+#define US_DTIL S(US_DGRV) // ~ (dead)
 #define US_EXLM S(US_1)    // !
-#define US_AT   S(US_2)    // "
+#define US_AT   S(US_2)    // @
 #define US_HASH S(US_3)    // #
 #define US_DLR  S(US_4)    // $
 #define US_PERC S(US_5)    // %
-#define US_CIRC S(US_6)    // ^
+#define US_DCIR S(US_6)    // ^ (dead)
 #define US_AMPR S(US_7)    // &
 #define US_ASTR S(US_8)    // *
 #define US_LPRN S(US_9)    // (
@@ -118,7 +118,7 @@
 #define US_PIPE S(US_BSLS) // |
 // Row 3
 #define US_COLN S(US_SCLN) // :
-#define US_DQUO S(US_QUOT) // " (dead)
+#define US_DIAE S(US_ACUT) // ¨ (dead)
 // Row 4
 #define US_LABK S(US_COMM) // <
 #define US_RABK S(US_DOT)  // >
@@ -170,7 +170,7 @@
 #define US_ETH  ALGR(US_D)    // Ð
 #define US_OSTR ALGR(US_L)    // Ø
 #define US_PILC ALGR(US_SCLN) // ¶
-#define US_ACUT ALGR(US_QUOT) // ´
+#define US_NDAC ALGR(US_ACUT) // ´
 // Row 4
 #define US_AE   ALGR(US_Z)    // Æ
 #define US_COPY ALGR(US_C)    // ©
@@ -201,6 +201,6 @@
 // Row 3
 #define US_SECT S(ALGR(US_S))    // §
 #define US_DEG  S(ALGR(US_SCLN)) // °
-#define US_DIAE S(ALGR(US_QUOT)) // ¨
+#define US_NDDR S(ALGR(US_ACUT)) // ¨
 // Row 4
 #define US_CENT S(ALGR(US_C))    // ¢
diff --git a/quantum/keymap_extras/keymap_us_international_linux.h b/quantum/keymap_extras/keymap_us_international_linux.h
new file mode 100644
index 000000000..2c3e23039
--- /dev/null
+++ b/quantum/keymap_extras/keymap_us_international_linux.h
@@ -0,0 +1,224 @@
+/* Copyright 2020
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "keymap.h"
+
+// clang-format off
+
+/*
+ * ┌───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───────┐
+ * │ ` │ 1 │ 2 │ 3 │ 4 │ 5 │ 6 │ 7 │ 8 │ 9 │ 0 │ - │ = │       │
+ * ├───┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─────┤
+ * │     │ Q │ W │ E │ R │ T │ Y │ U │ I │ O │ P │ [ │ ] │  \  │
+ * ├─────┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴─────┤
+ * │      │ A │ S │ D │ F │ G │ H │ J │ K │ L │ ; │ ´ │        │
+ * ├──────┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴────────┤
+ * │        │ Z │ X │ C │ V │ B │ N │ M │ , │ . │ / │          │
+ * ├────┬───┴┬──┴─┬─┴───┴───┴───┴───┴───┴──┬┴───┼───┴┬────┬────┤
+ * │    │    │    │                        │    │    │    │    │
+ * └────┴────┴────┴────────────────────────┴────┴────┴────┴────┘
+ */
+// Row 1
+#define US_DGRV KC_GRV  // ` (dead)
+#define US_1    KC_1    // 1
+#define US_2    KC_2    // 2
+#define US_3    KC_3    // 3
+#define US_4    KC_4    // 4
+#define US_5    KC_5    // 5
+#define US_6    KC_6    // 6
+#define US_7    KC_7    // 7
+#define US_8    KC_8    // 8
+#define US_9    KC_9    // 9
+#define US_0    KC_0    // 0
+#define US_MINS KC_MINS // -
+#define US_EQL  KC_EQL  // =
+// Row 2
+#define US_Q    KC_Q    // Q
+#define US_W    KC_W    // W
+#define US_E    KC_E    // E
+#define US_R    KC_R    // R
+#define US_T    KC_T    // T
+#define US_Y    KC_Y    // Y
+#define US_U    KC_U    // U
+#define US_I    KC_I    // I
+#define US_O    KC_O    // O
+#define US_P    KC_P    // P
+#define US_LBRC KC_LBRC // [
+#define US_RBRC KC_RBRC // ]
+#define US_BSLS KC_BSLS // (backslash)
+// Row 3
+#define US_A    KC_A    // A
+#define US_S    KC_S    // S
+#define US_D    KC_D    // D
+#define US_F    KC_F    // F
+#define US_G    KC_G    // G
+#define US_H    KC_H    // H
+#define US_J    KC_J    // J
+#define US_K    KC_K    // K
+#define US_L    KC_L    // L
+#define US_SCLN KC_SCLN // ;
+#define US_ACUT KC_QUOT // ´ (dead)
+// Row 4
+#define US_Z    KC_Z    // Z
+#define US_X    KC_X    // X
+#define US_C    KC_C    // C
+#define US_V    KC_V    // V
+#define US_B    KC_B    // B
+#define US_N    KC_N    // N
+#define US_M    KC_M    // M
+#define US_COMM KC_COMM // ,
+#define US_DOT  KC_DOT  // .
+#define US_SLSH KC_SLSH // /
+
+/* Shifted symbols
+ * ┌───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───────┐
+ * │ ~ │ ! │ @ │ # │ $ │ % │ ^ │ & │ * │ ( │ ) │ _ │ + │       │
+ * ├───┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─────┤
+ * │     │   │   │   │   │   │   │   │   │   │   │ { │ } │  |  │
+ * ├─────┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴─────┤
+ * │      │   │   │   │   │   │   │   │   │   │ : │ ¨ │        │
+ * ├──────┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴────────┤
+ * │        │   │   │   │   │   │   │   │ < │ > │ ? │          │
+ * ├────┬───┴┬──┴─┬─┴───┴───┴───┴───┴───┴──┬┴───┼───┴┬────┬────┤
+ * │    │    │    │                        │    │    │    │    │
+ * └────┴────┴────┴────────────────────────┴────┴────┴────┴────┘
+ */
+// Row 1
+#define US_DTIL S(US_DGRV) // ~ (dead)
+#define US_EXLM S(US_1)    // !
+#define US_AT   S(US_2)    // @
+#define US_HASH S(US_3)    // #
+#define US_DLR  S(US_4)    // $
+#define US_PERC S(US_5)    // %
+#define US_DCIR S(US_6)    // ^ (dead)
+#define US_AMPR S(US_7)    // &
+#define US_ASTR S(US_8)    // *
+#define US_LPRN S(US_9)    // (
+#define US_RPRN S(US_0)    // )
+#define US_UNDS S(US_MINS) // _
+#define US_PLUS S(US_EQL)  // +
+// Row 2
+#define US_LCBR S(US_LBRC) // {
+#define US_RCBR S(US_RBRC) // }
+#define US_PIPE S(US_BSLS) // |
+// Row 3
+#define US_COLN S(US_SCLN) // :
+#define US_DIAE S(US_ACUT) // ¨ (dead)
+// Row 4
+#define US_LABK S(US_COMM) // <
+#define US_RABK S(US_DOT)  // >
+#define US_QUES S(US_SLSH) // ?
+
+/* AltGr symbols
+ * ┌───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───────┐
+ * │ ` │ ¡ │ ² │ ³ │ ¤ │ € │ ¼ │ ½ │ ¾ │ ‘ │ ’ │ ¥ │ × │       │
+ * ├───┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─────┤
+ * │     │ Ä │ Å │ É │ ® │ Þ │ Ü │ Ú │ Í │ Ó │ Ö │ « │ » │  ¬  │
+ * ├─────┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴─────┤
+ * │      │ Á │ ß │ Ð │   │   │   │   │ Œ │ Ø │ ¶ │ ' │        │
+ * ├──────┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴────────┤
+ * │        │ Æ │   │ © │   │   │ Ñ │ µ │ Ç │ ˙ │ ¿ │          │
+ * ├────┬───┴┬──┴─┬─┴───┴───┴───┴───┴───┴──┬┴───┼───┴┬────┬────┤
+ * │    │    │    │                        │    │    │    │    │
+ * └────┴────┴────┴────────────────────────┴────┴────┴────┴────┘
+ */
+
+// Row 1
+#define US_GRV  ALGR(US_DGRV) // `
+#define US_IEXL ALGR(US_1)    // ¡
+#define US_SUP2 ALGR(US_2)    // ²
+#define US_SUP3 ALGR(US_3)    // ³
+#define US_CURR ALGR(US_4)    // ¤
+#define US_EURO ALGR(US_5)    // €
+#define US_QRTR ALGR(US_6)    // ¼
+#define US_HALF ALGR(US_7)    // ½
+#define US_TQTR ALGR(US_8)    // ¾
+#define US_LSQU ALGR(US_9)    // ‘
+#define US_RSQU ALGR(US_0)    // ’
+#define US_YEN  ALGR(US_MINS) // ¥
+#define US_MUL  ALGR(US_EQL)  // ×
+// Row 2
+#define US_ADIA ALGR(US_Q)    // Ä
+#define US_ARNG ALGR(US_W)    // Å
+#define US_EACU ALGR(US_E)    // É
+#define US_REGD ALGR(US_R)    // ®
+#define US_THRN ALGR(US_T)    // Þ
+#define US_UDIA ALGR(US_Y)    // Ü
+#define US_UACU ALGR(US_U)    // Ú
+#define US_IACU ALGR(US_I)    // Í
+#define US_OACU ALGR(US_O)    // Ó
+#define US_ODIA ALGR(US_P)    // Ö
+#define US_LDAQ ALGR(US_LBRC) // «
+#define US_RDAQ ALGR(US_RBRC) // »
+#define US_NOT  ALGR(US_BSLS) // ¬
+// Row 3
+#define US_AACU ALGR(US_A)    // Á
+#define US_SS   ALGR(US_S)    // ß
+#define US_ETH  ALGR(US_D)    // Ð
+#define US_OE   ALGR(US_K)    // Œ
+#define US_OSTR ALGR(US_L)    // Ø
+#define US_PILC ALGR(US_SCLN) // ¶
+#define US_QUOT ALGR(US_ACUT) // '
+// Row 4
+#define US_AE   ALGR(US_Z)    // Æ
+#define US_COPY ALGR(US_C)    // ©
+#define US_NTIL ALGR(US_N)    // Ñ
+#define US_MICR ALGR(US_M)    // µ
+#define US_CCED ALGR(US_COMM) // Ç
+#define US_DOTA ALGR(US_DOT)  // ˙ (dead)
+#define US_IQUE ALGR(US_SLSH) // ¿
+
+/* Shift+AltGr symbols
+ * ┌───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───────┐
+ * │ ~ │ ¹ │ ˝ │ ¯ │ £ │ ¸ │ ^ │ ̛  │ ˛ │ ˘ │ ° │  ̣ │ ÷ │       │
+ * ├───┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─────┤
+ * │     │   │   │   │   │   │   │   │   │   │   │ “ │ ” │  ¦  │
+ * ├─────┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴┬──┴─────┤
+ * │      │   │ § │   │   │   │   │   │   │   │ ° │ " │        │
+ * ├──────┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴─┬─┴────────┤
+ * │        │   │   │ ¢ │   │   │   │   │   │ ˇ │  ̉ │          │
+ * ├────┬───┴┬──┴─┬─┴───┴───┴───┴───┴───┴──┬┴───┼───┴┬────┬────┤
+ * │    │    │    │                        │    │    │    │    │
+ * └────┴────┴────┴────────────────────────┴────┴────┴────┴────┘
+ */
+// Row 1
+#define US_TILD S(ALGR(US_DGRV)) // ~
+#define US_SUP1 S(ALGR(US_1))    // ¹
+#define US_DACU S(ALGR(US_2))    // ˝ (dead)
+#define US_MACR S(ALGR(US_3))    // ¯ (dead)
+#define US_PND  S(ALGR(US_4))    // £
+#define US_CEDL S(ALGR(US_5))    // ¸ (dead)
+#define US_CIRC S(ALGR(US_6))    // ^
+#define US_HORN S(ALGR(US_7))    //  ̛  (dead)
+#define US_OGON S(ALGR(US_8))    // ˛ (dead)
+#define US_BREV S(ALGR(US_9))    // ˘ (dead)
+#define US_RNGA S(ALGR(US_0))    // ° (dead)
+#define US_DOTB S(ALGR(US_MINS)) //  ̣ (dead)
+#define US_DIV  S(ALGR(US_EQL))  // ÷
+// Row 2
+#define US_LDQU S(ALGR(US_LBRC)) // “
+#define US_RDQU S(ALGR(US_LBRC)) // ”
+#define US_BRKP S(ALGR(US_BSLS)) // ¦
+// Row 3
+#define US_SECT S(ALGR(US_S))    // §
+#define US_DEG  S(ALGR(US_SCLN)) // °
+#define US_DQUO S(ALGR(US_ACUT)) // "
+// Row 4
+#define US_CENT S(ALGR(US_C))    // ¢
+#define US_CARN S(ALGR(US_DOT))  // ˇ (dead)
+#define US_HOKA S(ALGR(US_SLSH)) //  ̉ (dead)
diff --git a/quantum/keymap_extras/sendstring_us_international.h b/quantum/keymap_extras/sendstring_us_international.h
new file mode 100644
index 000000000..53a5891fb
--- /dev/null
+++ b/quantum/keymap_extras/sendstring_us_international.h
@@ -0,0 +1,100 @@
+/* Copyright 2019 Rys Sommefeldt
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+// Sendstring lookup tables for UK layouts
+
+#pragma once
+
+#include "keymap_us_international.h"
+#include "quantum.h"
+
+// clang-format off
+
+const uint8_t ascii_to_shift_lut[16] PROGMEM = {
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+
+    KCLUT_ENTRY(0, 1, 1, 1, 1, 1, 1, 0),
+    KCLUT_ENTRY(1, 1, 1, 1, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 1, 0, 1, 0, 1, 1),
+    KCLUT_ENTRY(1, 1, 1, 1, 1, 1, 1, 1),
+    KCLUT_ENTRY(1, 1, 1, 1, 1, 1, 1, 1),
+    KCLUT_ENTRY(1, 1, 1, 1, 1, 1, 1, 1),
+    KCLUT_ENTRY(1, 1, 1, 0, 0, 0, 1, 1),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 1, 1, 1, 1, 0),
+};
+
+__attribute__((weak)) const uint8_t ascii_to_dead_lut[16] PROGMEM = {
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+
+    KCLUT_ENTRY(0, 0, 1, 0, 0, 0, 0, 1),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 1, 0),
+    KCLUT_ENTRY(1, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 1, 0),
+};
+
+const uint8_t ascii_to_keycode_lut[128] PROGMEM = {
+    // NUL   SOH      STX      ETX      EOT      ENQ      ACK      BEL
+    XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX,
+    // BS    TAB      LF       VT       FF       CR       SO       SI
+    KC_BSPC, KC_TAB,  KC_ENT,  XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX,
+    // DLE   DC1      DC2      DC3      DC4      NAK      SYN      ETB
+    XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX,
+    // CAN   EM       SUB      ESC      FS       GS       RS       US
+    XXXXXXX, XXXXXXX, XXXXXXX, KC_ESC,  XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX,
+
+    //       !        "        #        $        %        &        '
+    KC_SPC,  US_1,    US_ACUT, US_3,    US_4,    US_5,    US_7,    US_ACUT,
+    // (     )        *        +        ,        -        .        /
+    US_9,    US_0,    US_8,    US_EQL,  US_COMM, US_MINS, US_DOT,  US_SLSH,
+    // 0     1        2        3        4        5        6        7
+    US_0,    US_1,    US_2,    US_3,    US_4,    US_5,    US_6,    US_7,
+    // 8     9        :        ;        <        =        >        ?
+    US_8,    US_9,    US_SCLN, US_SCLN, US_COMM, US_EQL,  US_DOT,  US_SLSH,
+    // @     A        B        C        D        E        F        G
+    US_2,    US_A,    US_B,    US_C,    US_D,    US_E,    US_F,    US_G,
+    // H     I        J        K        L        M        N        O
+    US_H,    US_I,    US_J,    US_K,    US_L,    US_M,    US_N,    US_O,
+    // P     Q        R        S        T        U        V        W
+    US_P,    US_Q,    US_R,    US_S,    US_T,    US_U,    US_V,    US_W,
+    // X     Y        Z        [        \        ]        ^        _
+    US_X,    US_Y,    US_Z,    US_LBRC, US_BSLS, US_RBRC, US_6,    US_MINS,
+    // `     a        b        c        d        e        f        g
+    US_DGRV, US_A,    US_B,    US_C,    US_D,    US_E,    US_F,    US_G,
+    // h     i        j        k        l        m        n        o
+    US_H,    US_I,    US_J,    US_K,    US_L,    US_M,    US_N,    US_O,
+    // p     q        r        s        t        u        v        w
+    US_P,    US_Q,    US_R,    US_S,    US_T,    US_U,    US_V,    US_W,
+    // x     y        z        {        |        }        ~        DEL
+    US_X,    US_Y,    US_Z,    US_LBRC, US_BSLS, US_RBRC, US_DGRV, KC_DEL
+};
diff --git a/quantum/matrix.c b/quantum/matrix.c
index 9083ff386..c027b7bf2 100644
--- a/quantum/matrix.c
+++ b/quantum/matrix.c
@@ -101,9 +101,9 @@ static bool read_cols_on_row(matrix_row_t current_matrix[], uint8_t current_row)
     // Start with a clear matrix row
     matrix_row_t current_row_value = 0;
 
-    // Select row and wait for row selecton to stabilize
+    // Select row
     select_row(current_row);
-    matrix_io_delay();
+    matrix_output_select_delay();
 
     // For each col...
     for (uint8_t col_index = 0; col_index < MATRIX_COLS; col_index++) {
@@ -116,6 +116,9 @@ static bool read_cols_on_row(matrix_row_t current_matrix[], uint8_t current_row)
 
     // Unselect row
     unselect_row(current_row);
+    if (current_row + 1 < MATRIX_ROWS) {
+        matrix_output_unselect_delay();  // wait for row signal to go HIGH
+    }
 
     // If the row has changed, store the row and return the changed flag.
     if (current_matrix[current_row] != current_row_value) {
@@ -147,9 +150,9 @@ static void init_pins(void) {
 static bool read_rows_on_col(matrix_row_t current_matrix[], uint8_t current_col) {
     bool matrix_changed = false;
 
-    // Select col and wait for col selecton to stabilize
+    // Select col
     select_col(current_col);
-    matrix_io_delay();
+    matrix_output_select_delay();
 
     // For each row...
     for (uint8_t row_index = 0; row_index < MATRIX_ROWS; row_index++) {
@@ -175,6 +178,9 @@ static bool read_rows_on_col(matrix_row_t current_matrix[], uint8_t current_col)
 
     // Unselect col
     unselect_col(current_col);
+    if (current_col + 1 < MATRIX_COLS) {
+        matrix_output_unselect_delay();  // wait for col signal to go HIGH
+    }
 
     return matrix_changed;
 }
diff --git a/quantum/matrix_common.c b/quantum/matrix_common.c
index 15f1e0e82..01d2b38e5 100644
--- a/quantum/matrix_common.c
+++ b/quantum/matrix_common.c
@@ -1,3 +1,4 @@
+#include "quantum.h"
 #include "matrix.h"
 #include "debounce.h"
 #include "wait.h"
@@ -83,8 +84,12 @@ uint8_t matrix_key_count(void) {
     return count;
 }
 
+/*　`matrix_io_delay ()` exists for backwards compatibility. From now on, use matrix_output_unselect_delay().　*/
 __attribute__((weak)) void matrix_io_delay(void) { wait_us(MATRIX_IO_DELAY); }
 
+__attribute__((weak)) void matrix_output_select_delay(void) { waitInputPinDelay(); }
+__attribute__((weak)) void matrix_output_unselect_delay(void) { matrix_io_delay(); }
+
 // CUSTOM MATRIX 'LITE'
 __attribute__((weak)) void matrix_init_custom(void) {}
 
diff --git a/quantum/mcu_selection.mk b/quantum/mcu_selection.mk
index 6b11eb498..81c467c65 100644
--- a/quantum/mcu_selection.mk
+++ b/quantum/mcu_selection.mk
@@ -279,7 +279,73 @@ ifneq ($(findstring STM32F411, $(MCU)),)
   DFU_SUFFIX_ARGS ?= -v 0483 -p DF11
 endif
 
-ifneq (,$(filter $(MCU),atmega16u2 atmega32u2 atmega16u4 atmega32u4 at90usb646 at90usb647 at90usb1286 at90usb1287))
+ifneq ($(findstring STM32G431, $(MCU)),)
+  # Cortex version
+  MCU = cortex-m4
+
+  # ARM version, CORTEX-M0/M1 are 6, CORTEX-M3/M4/M7 are 7
+  ARMV = 7
+
+  ## chip/board settings
+  # - the next two should match the directories in
+  #   <chibios>/os/hal/ports/$(MCU_FAMILY)/$(MCU_SERIES)
+  MCU_FAMILY = STM32
+  MCU_SERIES = STM32G4xx
+
+  # Linker script to use
+  # - it should exist either in <chibios>/os/common/ports/ARMCMx/compilers/GCC/ld/
+  #   or <keyboard_dir>/ld/
+  MCU_LDSCRIPT ?= STM32G431xB
+
+  # Startup code to use
+  #  - it should exist in <chibios>/os/common/startup/ARMCMx/compilers/GCC/mk/
+  MCU_STARTUP ?= stm32g4xx
+
+  # Board: it should exist either in <chibios>/os/hal/boards/,
+  # <keyboard_dir>/boards/, or drivers/boards/
+  BOARD ?= GENERIC_STM32_G431XB
+
+  USE_FPU ?= yes
+
+  # Options to pass to dfu-util when flashing
+  DFU_ARGS ?= -d 0483:DF11 -a 0 -s 0x08000000:leave
+  DFU_SUFFIX_ARGS ?= -v 0483 -p DF11
+endif
+
+ifneq ($(findstring STM32G474, $(MCU)),)
+  # Cortex version
+  MCU = cortex-m4
+
+  # ARM version, CORTEX-M0/M1 are 6, CORTEX-M3/M4/M7 are 7
+  ARMV = 7
+
+  ## chip/board settings
+  # - the next two should match the directories in
+  #   <chibios>/os/hal/ports/$(MCU_FAMILY)/$(MCU_SERIES)
+  MCU_FAMILY = STM32
+  MCU_SERIES = STM32G4xx
+
+  # Linker script to use
+  # - it should exist either in <chibios>/os/common/ports/ARMCMx/compilers/GCC/ld/
+  #   or <keyboard_dir>/ld/
+  MCU_LDSCRIPT ?= STM32G474xE
+
+  # Startup code to use
+  #  - it should exist in <chibios>/os/common/startup/ARMCMx/compilers/GCC/mk/
+  MCU_STARTUP ?= stm32g4xx
+
+  # Board: it should exist either in <chibios>/os/hal/boards/,
+  # <keyboard_dir>/boards/, or drivers/boards/
+  BOARD ?= GENERIC_STM32_G474XE
+
+  USE_FPU ?= yes
+
+  # Options to pass to dfu-util when flashing
+  DFU_ARGS ?= -d 0483:DF11 -a 0 -s 0x08000000:leave
+  DFU_SUFFIX_ARGS ?= -v 0483 -p DF11
+endif
+
+ifneq (,$(filter $(MCU),at90usb162 atmega16u2 atmega32u2 atmega16u4 atmega32u4 at90usb646 at90usb647 at90usb1286 at90usb1287))
   PROTOCOL = LUFA
 
   # Processor frequency.
@@ -317,7 +383,7 @@ ifneq (,$(filter $(MCU),atmega16u2 atmega32u2 atmega16u4 atmega32u4 at90usb646 a
   ifeq (,$(filter $(NO_INTERRUPT_CONTROL_ENDPOINT),yes))
     OPT_DEFS += -DINTERRUPT_CONTROL_ENDPOINT
   endif
-  ifneq (,$(filter $(MCU),atmega16u2 atmega32u2))
+  ifneq (,$(filter $(MCU),at90usb162 atmega16u2 atmega32u2))
     NO_I2C = yes
   endif
 endif
diff --git a/quantum/quantum.c b/quantum/quantum.c
index cf16e953a..8ae487bec 100644
--- a/quantum/quantum.c
+++ b/quantum/quantum.c
@@ -391,6 +391,29 @@ __attribute__((weak)) const uint8_t ascii_to_altgr_lut[16] PROGMEM = {
     KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
 };
 
+/* Bit-Packed look-up table to convert an ASCII character to whether
+ * [Space] needs to be sent after the keycode
+ */
+__attribute__((weak)) const uint8_t ascii_to_dead_lut[16] PROGMEM = {
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+    KCLUT_ENTRY(0, 0, 0, 0, 0, 0, 0, 0),
+};
+
 /* Look-up table to convert an ASCII character to a keycode.
  */
 __attribute__((weak)) const uint8_t ascii_to_keycode_lut[128] PROGMEM = {
@@ -531,6 +554,7 @@ void send_char(char ascii_code) {
     uint8_t keycode    = pgm_read_byte(&ascii_to_keycode_lut[(uint8_t)ascii_code]);
     bool    is_shifted = PGM_LOADBIT(ascii_to_shift_lut, (uint8_t)ascii_code);
     bool    is_altgred = PGM_LOADBIT(ascii_to_altgr_lut, (uint8_t)ascii_code);
+    bool    is_dead    = PGM_LOADBIT(ascii_to_dead_lut, (uint8_t)ascii_code);
 
     if (is_shifted) {
         register_code(KC_LSFT);
@@ -545,6 +569,9 @@ void send_char(char ascii_code) {
     if (is_shifted) {
         unregister_code(KC_LSFT);
     }
+    if (is_dead) {
+        tap_code(KC_SPACE);
+    }
 }
 
 void set_single_persistent_default_layer(uint8_t default_layer) {
@@ -629,6 +656,26 @@ void matrix_init_quantum() {
 }
 
 void matrix_scan_quantum() {
+#if defined(AUDIO_ENABLE)
+    // There are some tasks that need to be run a little bit
+    // after keyboard startup, or else they will not work correctly
+    // because of interaction with the USB device state, which
+    // may still be in flux...
+    //
+    // At the moment the only feature that needs this is the
+    // startup song.
+    static bool     delayed_tasks_run  = false;
+    static uint16_t delayed_task_timer = 0;
+    if (!delayed_tasks_run) {
+        if (!delayed_task_timer) {
+            delayed_task_timer = timer_read();
+        } else if (timer_elapsed(delayed_task_timer) > 300) {
+            audio_startup();
+            delayed_tasks_run = true;
+        }
+    }
+#endif
+
 #if defined(AUDIO_ENABLE) && !defined(NO_MUSIC_MODE)
     matrix_scan_music();
 #endif
diff --git a/quantum/quantum.h b/quantum/quantum.h
index f4df5bf15..370a65fe0 100644
--- a/quantum/quantum.h
+++ b/quantum/quantum.h
@@ -53,6 +53,7 @@
 #include "eeconfig.h"
 #include "bootloader.h"
 #include "timer.h"
+#include "sync_timer.h"
 #include "config_common.h"
 #include "led.h"
 #include "action_util.h"
@@ -209,6 +210,13 @@ typedef uint8_t pin_t;
 
 #    define togglePin(pin) (PORTx_ADDRESS(pin) ^= _BV((pin)&0xF))
 
+/*   The AVR series GPIOs have a one clock read delay for changes in the digital input signal.
+ *   But here's more margin to make it two clocks. */
+#    if !defined(GPIO_INPUT_PIN_DELAY)
+#        define GPIO_INPUT_PIN_DELAY 2
+#    endif
+#    define waitInputPinDelay() wait_cpuclock(GPIO_INPUT_PIN_DELAY)
+
 #elif defined(PROTOCOL_CHIBIOS)
 typedef ioline_t pin_t;
 
@@ -224,6 +232,28 @@ typedef ioline_t pin_t;
 #    define readPin(pin) palReadLine(pin)
 
 #    define togglePin(pin) palToggleLine(pin)
+
+#endif
+
+#if defined(__ARMEL__) || defined(__ARMEB__)
+/* For GPIOs on ARM-based MCUs, the input pins are sampled by the clock of the bus
+ * to which the GPIO is connected.
+ * The connected buses differ depending on the various series of MCUs.
+ * And since the instruction execution clock of the CPU and the bus clock of GPIO are different,
+ * there is a delay of several clocks to read the change of the input signal.
+ *
+ * Define this delay with the GPIO_INPUT_PIN_DELAY macro.
+ * If the GPIO_INPUT_PIN_DELAY macro is not defined, the following default values will be used.
+ * (A fairly large value of 0.25 microseconds is set.)
+ */
+#    if !defined(GPIO_INPUT_PIN_DELAY)
+#        if defined(STM32_SYSCLK)
+#            define GPIO_INPUT_PIN_DELAY (STM32_SYSCLK / 1000000L / 4)
+#        elif defined(KINETIS_SYSCLK_FREQUENCY)
+#            define GPIO_INPUT_PIN_DELAY (KINETIS_SYSCLK_FREQUENCY / 1000000L / 4)
+#        endif
+#    endif
+#    define waitInputPinDelay() wait_cpuclock(GPIO_INPUT_PIN_DELAY)
 #endif
 
 // Atomic macro to help make GPIO and other controls atomic.
@@ -288,6 +318,7 @@ static __inline__ void __interrupt_enable__(const uint8_t *__s) {
 extern const uint8_t ascii_to_keycode_lut[128];
 extern const uint8_t ascii_to_shift_lut[16];
 extern const uint8_t ascii_to_altgr_lut[16];
+extern const uint8_t ascii_to_dead_lut[16];
 // clang-format off
 #define KCLUT_ENTRY(a, b, c, d, e, f, g, h) \
     ( ((a) ? 1 : 0) << 0 \
diff --git a/quantum/rgb_matrix.c b/quantum/rgb_matrix.c
index 04af3ae9e..a945df68e 100644
--- a/quantum/rgb_matrix.c
+++ b/quantum/rgb_matrix.c
@@ -266,9 +266,9 @@ static bool rgb_matrix_none(effect_params_t *params) {
 
 static void rgb_task_timers(void) {
 #if defined(RGB_MATRIX_KEYREACTIVE_ENABLED) || RGB_DISABLE_TIMEOUT > 0
-    uint32_t deltaTime = timer_elapsed32(rgb_timer_buffer);
+    uint32_t deltaTime = sync_timer_elapsed32(rgb_timer_buffer);
 #endif  // defined(RGB_MATRIX_KEYREACTIVE_ENABLED) || RGB_DISABLE_TIMEOUT > 0
-    rgb_timer_buffer = timer_read32();
+    rgb_timer_buffer = sync_timer_read32();
 
     // Update double buffer timers
 #if RGB_DISABLE_TIMEOUT > 0
@@ -296,7 +296,7 @@ static void rgb_task_timers(void) {
 
 static void rgb_task_sync(void) {
     // next task
-    if (timer_elapsed32(g_rgb_timer) >= RGB_MATRIX_LED_FLUSH_LIMIT) rgb_task_state = STARTING;
+    if (sync_timer_elapsed32(g_rgb_timer) >= RGB_MATRIX_LED_FLUSH_LIMIT) rgb_task_state = STARTING;
 }
 
 static void rgb_task_start(void) {
diff --git a/quantum/rgblight.c b/quantum/rgblight.c
index ac4ff9bfd..7d7d015ba 100644
--- a/quantum/rgblight.c
+++ b/quantum/rgblight.c
@@ -29,7 +29,7 @@
 #endif
 #include "wait.h"
 #include "progmem.h"
-#include "timer.h"
+#include "sync_timer.h"
 #include "rgblight.h"
 #include "color.h"
 #include "debug.h"
@@ -42,6 +42,9 @@
 #ifndef MIN
 #    define MIN(a, b) (((a) < (b)) ? (a) : (b))
 #endif
+#ifndef MAX
+#    define MAX(a, b) (((a) > (b)) ? (a) : (b))
+#endif
 
 #ifdef RGBLIGHT_SPLIT
 /* for split keyboard */
@@ -700,18 +703,16 @@ static void rgblight_layers_write(void) {
 
 #    ifdef RGBLIGHT_LAYER_BLINK
 rgblight_layer_mask_t _blinked_layer_mask = 0;
-uint16_t              _blink_duration     = 0;
 static uint16_t       _blink_timer;
 
 void rgblight_blink_layer(uint8_t layer, uint16_t duration_ms) {
     rgblight_set_layer_state(layer, true);
     _blinked_layer_mask |= (rgblight_layer_mask_t)1 << layer;
-    _blink_timer    = timer_read();
-    _blink_duration = duration_ms;
+    _blink_timer = sync_timer_read() + duration_ms;
 }
 
 void rgblight_unblink_layers(void) {
-    if (_blinked_layer_mask != 0 && timer_elapsed(_blink_timer) > _blink_duration) {
+    if (_blinked_layer_mask != 0 && timer_expired(sync_timer_read(), _blink_timer)) {
         for (uint8_t layer = 0; layer < RGBLIGHT_MAX_LAYERS; layer++) {
             if ((_blinked_layer_mask & (rgblight_layer_mask_t)1 << layer) != 0) {
                 rgblight_set_layer_state(layer, false);
@@ -886,7 +887,7 @@ void rgblight_timer_enable(void) {
     if (!is_static_effect(rgblight_config.mode)) {
         rgblight_status.timer_enabled = true;
     }
-    animation_status.last_timer = timer_read();
+    animation_status.last_timer = sync_timer_read();
     RGBLIGHT_SPLIT_SET_CHANGE_TIMER_ENABLE;
     dprintf("rgblight timer enabled.\n");
 }
@@ -989,24 +990,25 @@ void rgblight_task(void) {
 #    endif
 #    ifdef RGBLIGHT_EFFECT_TWINKLE
         else if (rgblight_status.base_mode == RGBLIGHT_MODE_TWINKLE) {
-            interval_time = get_interval_time(&RGBLED_TWINKLE_INTERVALS[delta % 3], 5, 50);
+            interval_time = get_interval_time(&RGBLED_TWINKLE_INTERVALS[delta % 3], 5, 30);
             effect_func   = (effect_func_t)rgblight_effect_twinkle;
         }
 #    endif
         if (animation_status.restart) {
             animation_status.restart    = false;
-            animation_status.last_timer = timer_read() - interval_time - 1;
+            animation_status.last_timer = sync_timer_read();
             animation_status.pos16      = 0;  // restart signal to local each effect
         }
-        if (timer_elapsed(animation_status.last_timer) >= interval_time) {
+        uint16_t now = sync_timer_read();
+        if (timer_expired(now, animation_status.last_timer)) {
 #    if defined(RGBLIGHT_SPLIT) && !defined(RGBLIGHT_SPLIT_NO_ANIMATION_SYNC)
             static uint16_t report_last_timer = 0;
             static bool     tick_flag         = false;
             uint16_t        oldpos16;
             if (tick_flag) {
                 tick_flag = false;
-                if (timer_elapsed(report_last_timer) >= 30000) {
-                    report_last_timer = timer_read();
+                if (timer_expired(now, report_last_timer)) {
+                    report_last_timer += 30000;
                     dprintf("rgblight animation tick report to slave\n");
                     RGBLIGHT_SPLIT_ANIMATION_TICK;
                 }
@@ -1030,8 +1032,7 @@ void rgblight_task(void) {
 
 #endif /* RGBLIGHT_USE_TIMER */
 
-// Effects
-#ifdef RGBLIGHT_EFFECT_BREATHING
+#if defined(RGBLIGHT_EFFECT_BREATHING) || defined(RGBLIGHT_EFFECT_TWINKLE)
 
 #    ifndef RGBLIGHT_EFFECT_BREATHE_CENTER
 #        ifndef RGBLIGHT_BREATHE_TABLE_SIZE
@@ -1040,17 +1041,24 @@ void rgblight_task(void) {
 #        include <rgblight_breathe_table.h>
 #    endif
 
-__attribute__((weak)) const uint8_t RGBLED_BREATHING_INTERVALS[] PROGMEM = {30, 20, 10, 5};
-
-void rgblight_effect_breathing(animation_status_t *anim) {
-    float val;
-
+static uint8_t breathe_calc(uint8_t pos) {
     // http://sean.voisen.org/blog/2011/10/breathing-led-with-arduino/
 #    ifdef RGBLIGHT_EFFECT_BREATHE_TABLE
-    val = pgm_read_byte(&rgblight_effect_breathe_table[anim->pos / table_scale]);
+    return pgm_read_byte(&rgblight_effect_breathe_table[pos / table_scale]);
 #    else
-    val = (exp(sin((anim->pos / 255.0) * M_PI)) - RGBLIGHT_EFFECT_BREATHE_CENTER / M_E) * (RGBLIGHT_EFFECT_BREATHE_MAX / (M_E - 1 / M_E));
+    return (exp(sin((pos / 255.0) * M_PI)) - RGBLIGHT_EFFECT_BREATHE_CENTER / M_E) * (RGBLIGHT_EFFECT_BREATHE_MAX / (M_E - 1 / M_E));
 #    endif
+}
+
+#endif
+
+// Effects
+#ifdef RGBLIGHT_EFFECT_BREATHING
+
+__attribute__((weak)) const uint8_t RGBLED_BREATHING_INTERVALS[] PROGMEM = {30, 20, 10, 5};
+
+void rgblight_effect_breathing(animation_status_t *anim) {
+    uint8_t val = breathe_calc(anim->pos);
     rgblight_sethsv_noeeprom_old(rgblight_config.hue, rgblight_config.sat, val);
     anim->pos = (anim->pos + 1);
 }
@@ -1302,48 +1310,54 @@ void rgblight_effect_alternating(animation_status_t *anim) {
 #endif
 
 #ifdef RGBLIGHT_EFFECT_TWINKLE
-__attribute__((weak)) const uint8_t RGBLED_TWINKLE_INTERVALS[] PROGMEM = {50, 25, 10};
+__attribute__((weak)) const uint8_t RGBLED_TWINKLE_INTERVALS[] PROGMEM = {30, 15, 5};
 
 typedef struct PACKED {
     HSV     hsv;
     uint8_t life;
-    bool    up;
+    uint8_t max_life;
 } TwinkleState;
 
 static TwinkleState led_twinkle_state[RGBLED_NUM];
 
 void rgblight_effect_twinkle(animation_status_t *anim) {
-    bool random_color = anim->delta / 3;
-    bool restart      = anim->pos == 0;
-    anim->pos         = 1;
+    const bool random_color = anim->delta / 3;
+    const bool restart      = anim->pos == 0;
+    anim->pos               = 1;
+
+    const uint8_t bottom = breathe_calc(0);
+    const uint8_t top    = breathe_calc(127);
+
+    uint8_t frac(uint8_t n, uint8_t d) { return (uint16_t)255 * n / d; }
+    uint8_t scale(uint16_t v, uint8_t scale) { return (v * scale) >> 8; }
 
     for (uint8_t i = 0; i < rgblight_ranges.effect_num_leds; i++) {
         TwinkleState *t = &(led_twinkle_state[i]);
         HSV *         c = &(t->hsv);
+
+        if (!random_color) {
+            c->h = rgblight_config.hue;
+            c->s = rgblight_config.sat;
+        }
+
         if (restart) {
             // Restart
-            t->life  = 0;
-            t->hsv.v = 0;
+            t->life = 0;
+            c->v    = 0;
         } else if (t->life) {
             // This LED is already on, either brightening or dimming
             t->life--;
-            uint8_t on = t->up ? RGBLIGHT_EFFECT_TWINKLE_LIFE - t->life : t->life;
-            c->v       = (uint16_t)rgblight_config.val * on / RGBLIGHT_EFFECT_TWINKLE_LIFE;
-            if (t->life == 0 && t->up) {
-                t->up   = false;
-                t->life = RGBLIGHT_EFFECT_TWINKLE_LIFE;
-            }
-            if (!random_color) {
-                c->h = rgblight_config.hue;
-                c->s = rgblight_config.sat;
-            }
-        } else if (rand() < RAND_MAX * RGBLIGHT_EFFECT_TWINKLE_PROBABILITY) {
+            uint8_t unscaled = frac(breathe_calc(frac(t->life, t->max_life)) - bottom, top - bottom);
+            c->v             = scale(rgblight_config.val, unscaled);
+        } else if (rand() < scale((uint16_t)RAND_MAX * RGBLIGHT_EFFECT_TWINKLE_PROBABILITY, 127 + rgblight_config.val / 2)) {
             // This LED is off, but was randomly selected to start brightening
-            c->h    = random_color ? rand() % 0xFF : rgblight_config.hue;
-            c->s    = random_color ? (rand() % (rgblight_config.sat / 2)) + (rgblight_config.sat / 2) : rgblight_config.sat;
-            c->v    = 0;
-            t->life = RGBLIGHT_EFFECT_TWINKLE_LIFE;
-            t->up   = true;
+            if (random_color) {
+                c->h = rand() % 0xFF;
+                c->s = (rand() % (rgblight_config.sat / 2)) + (rgblight_config.sat / 2);
+            }
+            c->v        = 0;
+            t->max_life = MAX(20, MIN(RGBLIGHT_EFFECT_TWINKLE_LIFE, rgblight_config.val));
+            t->life     = t->max_life;
         } else {
             // This LED is off, and was NOT selected to start brightening
         }
diff --git a/quantum/rgblight.h b/quantum/rgblight.h
index 1854fee99..028b3ea41 100644
--- a/quantum/rgblight.h
+++ b/quantum/rgblight.h
@@ -150,7 +150,7 @@ enum RGBLIGHT_EFFECT_MODE {
 #    endif
 
 #    ifndef RGBLIGHT_EFFECT_TWINKLE_LIFE
-#        define RGBLIGHT_EFFECT_TWINKLE_LIFE 75
+#        define RGBLIGHT_EFFECT_TWINKLE_LIFE 200
 #    endif
 
 #    ifndef RGBLIGHT_EFFECT_TWINKLE_PROBABILITY
diff --git a/quantum/split_common/matrix.c b/quantum/split_common/matrix.c
index 51bf8b109..bad762b49 100644
--- a/quantum/split_common/matrix.c
+++ b/quantum/split_common/matrix.c
@@ -114,9 +114,9 @@ static bool read_cols_on_row(matrix_row_t current_matrix[], uint8_t current_row)
     // Start with a clear matrix row
     matrix_row_t current_row_value = 0;
 
-    // Select row and wait for row selecton to stabilize
+    // Select row
     select_row(current_row);
-    matrix_io_delay();
+    matrix_output_select_delay();
 
     // For each col...
     for (uint8_t col_index = 0; col_index < MATRIX_COLS; col_index++) {
@@ -129,6 +129,9 @@ static bool read_cols_on_row(matrix_row_t current_matrix[], uint8_t current_row)
 
     // Unselect row
     unselect_row(current_row);
+    if (current_row + 1 < MATRIX_ROWS) {
+        matrix_output_unselect_delay();  // wait for row signal to go HIGH
+    }
 
     // If the row has changed, store the row and return the changed flag.
     if (current_matrix[current_row] != current_row_value) {
@@ -160,9 +163,9 @@ static void init_pins(void) {
 static bool read_rows_on_col(matrix_row_t current_matrix[], uint8_t current_col) {
     bool matrix_changed = false;
 
-    // Select col and wait for col selecton to stabilize
+    // Select col
     select_col(current_col);
-    matrix_io_delay();
+    matrix_output_select_delay();
 
     // For each row...
     for (uint8_t row_index = 0; row_index < ROWS_PER_HAND; row_index++) {
@@ -188,6 +191,9 @@ static bool read_rows_on_col(matrix_row_t current_matrix[], uint8_t current_col)
 
     // Unselect col
     unselect_col(current_col);
+    if (current_col + 1 < MATRIX_COLS) {
+        matrix_output_unselect_delay();  // wait for col signal to go HIGH
+    }
 
     return matrix_changed;
 }
@@ -245,21 +251,33 @@ void matrix_init(void) {
     split_post_init();
 }
 
-void matrix_post_scan(void) {
+bool matrix_post_scan(void) {
+    bool changed = false;
     if (is_keyboard_master()) {
         static uint8_t error_count;
 
-        if (!transport_master(matrix + thatHand)) {
+        matrix_row_t slave_matrix[ROWS_PER_HAND] = {0};
+        if (!transport_master(slave_matrix)) {
             error_count++;
 
             if (error_count > ERROR_DISCONNECT_COUNT) {
                 // reset other half if disconnected
                 for (int i = 0; i < ROWS_PER_HAND; ++i) {
                     matrix[thatHand + i] = 0;
+                    slave_matrix[i]      = 0;
                 }
+
+                changed = true;
             }
         } else {
             error_count = 0;
+
+            for (int i = 0; i < ROWS_PER_HAND; ++i) {
+                if (matrix[thatHand + i] != slave_matrix[i]) {
+                    matrix[thatHand + i] = slave_matrix[i];
+                    changed              = true;
+                }
+            }
         }
 
         matrix_scan_quantum();
@@ -268,25 +286,27 @@ void matrix_post_scan(void) {
 
         matrix_slave_scan_user();
     }
+
+    return changed;
 }
 
 uint8_t matrix_scan(void) {
-    bool changed = false;
+    bool local_changed = false;
 
 #if defined(DIRECT_PINS) || (DIODE_DIRECTION == COL2ROW)
     // Set row, read cols
     for (uint8_t current_row = 0; current_row < ROWS_PER_HAND; current_row++) {
-        changed |= read_cols_on_row(raw_matrix, current_row);
+        local_changed |= read_cols_on_row(raw_matrix, current_row);
     }
 #elif (DIODE_DIRECTION == ROW2COL)
     // Set col, read rows
     for (uint8_t current_col = 0; current_col < MATRIX_COLS; current_col++) {
-        changed |= read_rows_on_col(raw_matrix, current_col);
+        local_changed |= read_rows_on_col(raw_matrix, current_col);
     }
 #endif
 
-    debounce(raw_matrix, matrix + thisHand, ROWS_PER_HAND, changed);
+    debounce(raw_matrix, matrix + thisHand, ROWS_PER_HAND, local_changed);
 
-    matrix_post_scan();
-    return (uint8_t)changed;
+    bool remote_changed = matrix_post_scan();
+    return (uint8_t)(local_changed || remote_changed);
 }
diff --git a/quantum/split_common/transport.c b/quantum/split_common/transport.c
index 467ff81a9..b45ba92c3 100644
--- a/quantum/split_common/transport.c
+++ b/quantum/split_common/transport.c
@@ -6,6 +6,7 @@
 #include "quantum.h"
 
 #define ROWS_PER_HAND (MATRIX_ROWS / 2)
+#define SYNC_TIMER_OFFSET 2
 
 #ifdef RGBLIGHT_ENABLE
 #    include "rgblight.h"
@@ -27,8 +28,20 @@ static pin_t encoders_pad[] = ENCODERS_PAD_A;
 #    include "i2c_slave.h"
 
 typedef struct _I2C_slave_buffer_t {
+#    ifndef DISABLE_SYNC_TIMER
+    uint32_t sync_timer;
+#    endif
     matrix_row_t smatrix[ROWS_PER_HAND];
-    uint8_t      backlight_level;
+#    ifdef SPLIT_MODS_ENABLE
+    uint8_t real_mods;
+    uint8_t weak_mods;
+#        ifndef NO_ACTION_ONESHOT
+    uint8_t oneshot_mods;
+#        endif
+#    endif
+#    ifdef BACKLIGHT_ENABLE
+    uint8_t backlight_level;
+#    endif
 #    if defined(RGBLIGHT_ENABLE) && defined(RGBLIGHT_SPLIT)
     rgblight_syncinfo_t rgblight_sync;
 #    endif
@@ -42,9 +55,13 @@ typedef struct _I2C_slave_buffer_t {
 
 static I2C_slave_buffer_t *const i2c_buffer = (I2C_slave_buffer_t *)i2c_slave_reg;
 
+#    define I2C_SYNC_TIME_START offsetof(I2C_slave_buffer_t, sync_timer)
+#    define I2C_KEYMAP_START offsetof(I2C_slave_buffer_t, smatrix)
+#    define I2C_REAL_MODS_START offsetof(I2C_slave_buffer_t, real_mods)
+#    define I2C_WEAK_MODS_START offsetof(I2C_slave_buffer_t, weak_mods)
+#    define I2C_ONESHOT_MODS_START offsetof(I2C_slave_buffer_t, oneshot_mods)
 #    define I2C_BACKLIGHT_START offsetof(I2C_slave_buffer_t, backlight_level)
 #    define I2C_RGB_START offsetof(I2C_slave_buffer_t, rgblight_sync)
-#    define I2C_KEYMAP_START offsetof(I2C_slave_buffer_t, smatrix)
 #    define I2C_ENCODER_START offsetof(I2C_slave_buffer_t, encoder_state)
 #    define I2C_WPM_START offsetof(I2C_slave_buffer_t, current_wpm)
 
@@ -91,10 +108,43 @@ bool transport_master(matrix_row_t matrix[]) {
         }
     }
 #    endif
+
+#    ifdef SPLIT_MODS_ENABLE
+    uint8_t real_mods = get_mods();
+    if (real_mods != i2c_buffer->real_mods) {
+        if (i2c_writeReg(SLAVE_I2C_ADDRESS, I2C_REAL_MODS_START, (void *)&real_mods, sizeof(real_mods), TIMEOUT) >= 0) {
+            i2c_buffer->real_mods = real_mods;
+        }
+    }
+
+    uint8_t weak_mods = get_weak_mods();
+    if (weak_mods != i2c_buffer->weak_mods) {
+        if (i2c_writeReg(SLAVE_I2C_ADDRESS, I2C_WEAK_MODS_START, (void *)&weak_mods, sizeof(weak_mods), TIMEOUT) >= 0) {
+            i2c_buffer->weak_mods = weak_mods;
+        }
+    }
+
+#        ifndef NO_ACTION_ONESHOT
+    uint8_t oneshot_mods = get_oneshot_mods();
+    if (oneshot_mods != i2c_buffer->oneshot_mods) {
+        if (i2c_writeReg(SLAVE_I2C_ADDRESS, I2C_ONESHOT_MODS_START, (void *)&oneshot_mods, sizeof(oneshot_mods), TIMEOUT) >= 0) {
+            i2c_buffer->oneshot_mods = oneshot_mods;
+        }
+    }
+#        endif
+#    endif
+
+#    ifndef DISABLE_SYNC_TIMER
+    i2c_buffer->sync_timer = sync_timer_read32() + SYNC_TIMER_OFFSET;
+    i2c_writeReg(SLAVE_I2C_ADDRESS, I2C_SYNC_TIME_START, (void *)&i2c_buffer->sync_timer, sizeof(i2c_buffer->sync_timer), TIMEOUT);
+#    endif
     return true;
 }
 
 void transport_slave(matrix_row_t matrix[]) {
+#    ifndef DISABLE_SYNC_TIMER
+    sync_timer_update(i2c_buffer->sync_timer);
+#    endif
     // Copy matrix to I2C buffer
     memcpy((void *)i2c_buffer->smatrix, (void *)matrix, sizeof(i2c_buffer->smatrix));
 
@@ -118,6 +168,14 @@ void transport_slave(matrix_row_t matrix[]) {
 #    ifdef WPM_ENABLE
     set_current_wpm(i2c_buffer->current_wpm);
 #    endif
+
+#    ifdef SPLIT_MODS_ENABLE
+    set_mods(i2c_buffer->real_mods);
+    set_weak_mods(i2c_buffer->weak_mods);
+#        ifndef NO_ACTION_ONESHOT
+    set_oneshot_mods(i2c_buffer->oneshot_mods);
+#        endif
+#    endif
 }
 
 void transport_master_init(void) { i2c_init(); }
@@ -139,11 +197,21 @@ typedef struct _Serial_s2m_buffer_t {
 } Serial_s2m_buffer_t;
 
 typedef struct _Serial_m2s_buffer_t {
+#    ifdef SPLIT_MODS_ENABLE
+    uint8_t  real_mods;
+    uint8_t  weak_mods;
+#        ifndef NO_ACTION_ONESHOT
+    uint8_t  oneshot_mods;
+#        endif
+#    endif
+#    ifndef DISABLE_SYNC_TIMER
+    uint32_t sync_timer;
+#    endif
 #    ifdef BACKLIGHT_ENABLE
-    uint8_t backlight_level;
+    uint8_t  backlight_level;
 #    endif
 #    ifdef WPM_ENABLE
-    uint8_t current_wpm;
+    uint8_t  current_wpm;
 #    endif
 } Serial_m2s_buffer_t;
 
@@ -249,13 +317,28 @@ bool transport_master(matrix_row_t matrix[]) {
 
 #    ifdef WPM_ENABLE
     // Write wpm to slave
-    serial_m2s_buffer.current_wpm = get_current_wpm();
+    serial_m2s_buffer.current_wpm  = get_current_wpm();
+#    endif
+
+#    ifdef SPLIT_MODS_ENABLE
+    serial_m2s_buffer.real_mods    = get_mods();
+    serial_m2s_buffer.weak_mods    = get_weak_mods();
+#        ifndef NO_ACTION_ONESHOT
+    serial_m2s_buffer.oneshot_mods = get_oneshot_mods();
+#        endif
+#    endif
+#    ifndef DISABLE_SYNC_TIMER
+    serial_m2s_buffer.sync_timer   = sync_timer_read32() + SYNC_TIMER_OFFSET;
 #    endif
     return true;
 }
 
 void transport_slave(matrix_row_t matrix[]) {
     transport_rgblight_slave();
+#    ifndef DISABLE_SYNC_TIMER
+    sync_timer_update(serial_m2s_buffer.sync_timer);
+#    endif
+
     // TODO: if MATRIX_COLS > 8 change to pack()
     for (int i = 0; i < ROWS_PER_HAND; ++i) {
         serial_s2m_buffer.smatrix[i] = matrix[i];
@@ -271,6 +354,14 @@ void transport_slave(matrix_row_t matrix[]) {
 #    ifdef WPM_ENABLE
     set_current_wpm(serial_m2s_buffer.current_wpm);
 #    endif
+
+#    ifdef SPLIT_MODS_ENABLE
+    set_mods(serial_m2s_buffer.real_mods);
+    set_weak_mods(serial_m2s_buffer.weak_mods);
+#        ifndef NO_ACTION_ONESHOT
+    set_oneshot_mods(serial_m2s_buffer.oneshot_mods);
+#        endif
+#    endif
 }
 
 #endif