4 files changed, 356 insertions, 0 deletions

diff --git a/docs/_summary.md b/docs/_summary.md
index b711ab7d9..0e82ab4f0 100644
--- a/docs/_summary.md
+++ b/docs/_summary.md
@@ -128,6 +128,7 @@
       * [SPI Driver](spi_driver.md)
       * [WS2812 Driver](ws2812_driver.md)
       * [EEPROM Driver](eeprom_driver.md)
+      * ['serial' Driver](serial_driver.md)
     * [GPIO Controls](internals_gpio_control.md)
     * [Keyboard Guidelines](hardware_keyboard_guidelines.md)
 
diff --git a/docs/serial_driver.md b/docs/serial_driver.md
new file mode 100644
index 000000000..395b3ec3f
--- /dev/null
+++ b/docs/serial_driver.md
@@ -0,0 +1,59 @@
+# 'serial' Driver
+This driver powers the [Split Keyboard](feature_split_keyboard.md) feature.
+
+!> Serial in this context should be read as **sending information one bit at a time**, rather than implementing UART/USART/RS485/RS232 standards.
+
+All drivers in this category have the following characteristics:
+* Provides data and signaling over a single conductor
+* Limited to single master, single slave
+
+## Supported Driver Types
+
+|                   | AVR                | ARM                |
+|-------------------|--------------------|--------------------|
+| bit bang          | :heavy_check_mark: | Soon™              |
+| USART Half-duplex |                    | :heavy_check_mark: |
+
+## Driver configuration
+
+### Bitbang
+Default driver, the absence of configuration assumes this driver. To configure it, add this to your rules.mk:
+
+```make
+SERIAL_DRIVER = bitbang
+```
+
+Configure the driver via your config.h:
+```c
+#define SOFT_SERIAL_PIN D0  // or D1, D2, D3, E6
+#define SELECT_SOFT_SERIAL_SPEED 1 // or 0, 2, 3, 4, 5
+                                   //  0: about 189kbps (Experimental only)
+                                   //  1: about 137kbps (default)
+                                   //  2: about 75kbps
+                                   //  3: about 39kbps
+                                   //  4: about 26kbps
+                                   //  5: about 20kbps
+```
+
+### USART Half-duplex
+Targeting STM32 boards where communication is offloaded to a USART hardware device. The advantage is that this provides fast and accurate timings. `SOFT_SERIAL_PIN` for this driver is the configured USART TX pin. **The TX pin must have appropriate pull-up resistors**. To configure it, add this to your rules.mk:
+
+```make
+SERIAL_DRIVER = usart
+```
+
+Configure the hardware via your config.h:
+```c
+#define SOFT_SERIAL_PIN B6  // USART TX pin
+#define SELECT_SOFT_SERIAL_SPEED 1 // or 0, 2, 3, 4, 5
+                                   //  0: about 460800 baud
+                                   //  1: about 230400 baud (default)
+                                   //  2: about 115200 baud
+                                   //  3: about 57600 baud
+                                   //  4: about 38400 baud
+                                   //  5: about 19200 baud
+#define SERIAL_USART_DRIVER SD1 // USART driver of TX pin. default: SD1
+#define SERIAL_USART_TX_PAL_MODE 7 // Pin "alternate function", see the respective datasheet for the appropriate values for your MCU. default: 7
+```
+
+You must also turn on the SERIAL feature in your halconf.h and mcuconf.h
diff --git a/drivers/chibios/serial.h b/drivers/chibios/serial.h
new file mode 100644
index 000000000..0c1857d52
--- /dev/null
+++ b/drivers/chibios/serial.h
@@ -0,0 +1,62 @@
+#pragma once
+
+#include <stdbool.h>
+
+// /////////////////////////////////////////////////////////////////
+// Need Soft Serial defines in config.h
+// /////////////////////////////////////////////////////////////////
+// ex.
+//  #define SOFT_SERIAL_PIN ??   // ?? = D0,D1,D2,D3,E6
+//  OPTIONAL: #define SELECT_SOFT_SERIAL_SPEED ? // ? = 1,2,3,4,5
+//                                               //  1: about 137kbps (default)
+//                                               //  2: about 75kbps
+//                                               //  3: about 39kbps
+//                                               //  4: about 26kbps
+//                                               //  5: about 20kbps
+//
+// //// USE simple API (using signle-type transaction function)
+//   /* nothing */
+// //// USE flexible API (using multi-type transaction function)
+//   #define SERIAL_USE_MULTI_TRANSACTION
+//
+// /////////////////////////////////////////////////////////////////
+
+// Soft Serial Transaction Descriptor
+typedef struct _SSTD_t {
+    uint8_t *status;
+    uint8_t  initiator2target_buffer_size;
+    uint8_t *initiator2target_buffer;
+    uint8_t  target2initiator_buffer_size;
+    uint8_t *target2initiator_buffer;
+} SSTD_t;
+#define TID_LIMIT(table) (sizeof(table) / sizeof(SSTD_t))
+
+// initiator is transaction start side
+void soft_serial_initiator_init(SSTD_t *sstd_table, int sstd_table_size);
+// target is interrupt accept side
+void soft_serial_target_init(SSTD_t *sstd_table, int sstd_table_size);
+
+// initiator result
+#define TRANSACTION_END 0
+#define TRANSACTION_NO_RESPONSE 0x1
+#define TRANSACTION_DATA_ERROR 0x2
+#define TRANSACTION_TYPE_ERROR 0x4
+#ifndef SERIAL_USE_MULTI_TRANSACTION
+int soft_serial_transaction(void);
+#else
+int soft_serial_transaction(int sstd_index);
+#endif
+
+// target status
+// *SSTD_t.status has
+//   initiator:
+//       TRANSACTION_END
+//    or TRANSACTION_NO_RESPONSE
+//    or TRANSACTION_DATA_ERROR
+//   target:
+//       TRANSACTION_DATA_ERROR
+//    or TRANSACTION_ACCEPTED
+#define TRANSACTION_ACCEPTED 0x8
+#ifdef SERIAL_USE_MULTI_TRANSACTION
+int soft_serial_get_and_clean_status(int sstd_index);
+#endif
diff --git a/drivers/chibios/serial_usart.c b/drivers/chibios/serial_usart.c
new file mode 100644
index 000000000..62b4913cb
--- /dev/null
+++ b/drivers/chibios/serial_usart.c
@@ -0,0 +1,234 @@
+#include "quantum.h"
+#include "serial.h"
+#include "printf.h"
+
+#include "ch.h"
+#include "hal.h"
+
+#ifndef USART_CR1_M0
+#    define USART_CR1_M0 USART_CR1_M  // some platforms (f1xx) dont have this so
+#endif
+
+#ifndef USE_GPIOV1
+// The default PAL alternate modes are used to signal that the pins are used for USART
+#    ifndef SERIAL_USART_TX_PAL_MODE
+#        define SERIAL_USART_TX_PAL_MODE 7
+#    endif
+#endif
+
+#ifndef SERIAL_USART_DRIVER
+#    define SERIAL_USART_DRIVER SD1
+#endif
+
+#ifndef SERIAL_USART_CR1
+#    define SERIAL_USART_CR1 (USART_CR1_PCE | USART_CR1_PS | USART_CR1_M0)  // parity enable, odd parity, 9 bit length
+#endif
+
+#ifndef SERIAL_USART_CR2
+#    define SERIAL_USART_CR2 (USART_CR2_STOP_1)  // 2 stop bits
+#endif
+
+#ifndef SERIAL_USART_CR3
+#    define SERIAL_USART_CR3 0
+#endif
+
+#ifdef SOFT_SERIAL_PIN
+#    define SERIAL_USART_TX_PIN SOFT_SERIAL_PIN
+#endif
+
+#ifndef SELECT_SOFT_SERIAL_SPEED
+#    define SELECT_SOFT_SERIAL_SPEED 1
+#endif
+
+#ifdef SERIAL_USART_SPEED
+// Allow advanced users to directly set SERIAL_USART_SPEED
+#elif SELECT_SOFT_SERIAL_SPEED == 0
+#    define SERIAL_USART_SPEED 460800
+#elif SELECT_SOFT_SERIAL_SPEED == 1
+#    define SERIAL_USART_SPEED 230400
+#elif SELECT_SOFT_SERIAL_SPEED == 2
+#    define SERIAL_USART_SPEED 115200
+#elif SELECT_SOFT_SERIAL_SPEED == 3
+#    define SERIAL_USART_SPEED 57600
+#elif SELECT_SOFT_SERIAL_SPEED == 4
+#    define SERIAL_USART_SPEED 38400
+#elif SELECT_SOFT_SERIAL_SPEED == 5
+#    define SERIAL_USART_SPEED 19200
+#else
+#    error invalid SELECT_SOFT_SERIAL_SPEED value
+#endif
+
+#define TIMEOUT 100
+#define HANDSHAKE_MAGIC 7
+
+static inline msg_t sdWriteHalfDuplex(SerialDriver* driver, uint8_t* data, uint8_t size) {
+    msg_t ret = sdWrite(driver, data, size);
+
+    // Half duplex requires us to read back the data we just wrote - just throw it away
+    uint8_t dump[size];
+    sdRead(driver, dump, size);
+
+    return ret;
+}
+#undef sdWrite
+#define sdWrite sdWriteHalfDuplex
+
+static inline msg_t sdWriteTimeoutHalfDuplex(SerialDriver* driver, uint8_t* data, uint8_t size, uint32_t timeout) {
+    msg_t ret = sdWriteTimeout(driver, data, size, timeout);
+
+    // Half duplex requires us to read back the data we just wrote - just throw it away
+    uint8_t dump[size];
+    sdReadTimeout(driver, dump, size, timeout);
+
+    return ret;
+}
+#undef sdWriteTimeout
+#define sdWriteTimeout sdWriteTimeoutHalfDuplex
+
+static inline void sdClear(SerialDriver* driver) {
+    while (sdGetTimeout(driver, TIME_IMMEDIATE) != MSG_TIMEOUT) {
+        // Do nothing with the data
+    }
+}
+
+static SerialConfig sdcfg = {
+    (SERIAL_USART_SPEED),  // speed - mandatory
+    (SERIAL_USART_CR1),    // CR1
+    (SERIAL_USART_CR2),    // CR2
+    (SERIAL_USART_CR3)     // CR3
+};
+
+void handle_soft_serial_slave(void);
+
+/*
+ * This thread runs on the slave and responds to transactions initiated
+ * by the master
+ */
+static THD_WORKING_AREA(waSlaveThread, 2048);
+static THD_FUNCTION(SlaveThread, arg) {
+    (void)arg;
+    chRegSetThreadName("slave_transport");
+
+    while (true) {
+        handle_soft_serial_slave();
+    }
+}
+
+__attribute__((weak)) void usart_init(void) {
+#if defined(USE_GPIOV1)
+    palSetLineMode(SERIAL_USART_TX_PIN, PAL_MODE_STM32_ALTERNATE_OPENDRAIN);
+#else
+    palSetLineMode(SERIAL_USART_TX_PIN, PAL_MODE_ALTERNATE(SERIAL_USART_TX_PAL_MODE) | PAL_STM32_OTYPE_OPENDRAIN);
+#endif
+}
+
+void usart_master_init(void) {
+    usart_init();
+
+    sdcfg.cr3 |= USART_CR3_HDSEL;
+    sdStart(&SERIAL_USART_DRIVER, &sdcfg);
+}
+
+void usart_slave_init(void) {
+    usart_init();
+
+    sdcfg.cr3 |= USART_CR3_HDSEL;
+    sdStart(&SERIAL_USART_DRIVER, &sdcfg);
+
+    // Start transport thread
+    chThdCreateStatic(waSlaveThread, sizeof(waSlaveThread), HIGHPRIO, SlaveThread, NULL);
+}
+
+static SSTD_t* Transaction_table      = NULL;
+static uint8_t Transaction_table_size = 0;
+
+void soft_serial_initiator_init(SSTD_t* sstd_table, int sstd_table_size) {
+    Transaction_table      = sstd_table;
+    Transaction_table_size = (uint8_t)sstd_table_size;
+
+    usart_master_init();
+}
+
+void soft_serial_target_init(SSTD_t* sstd_table, int sstd_table_size) {
+    Transaction_table      = sstd_table;
+    Transaction_table_size = (uint8_t)sstd_table_size;
+
+    usart_slave_init();
+}
+
+void handle_soft_serial_slave(void) {
+    uint8_t sstd_index = sdGet(&SERIAL_USART_DRIVER);  // first chunk is always transaction id
+    SSTD_t* trans      = &Transaction_table[sstd_index];
+
+    // Always write back the sstd_index as part of a basic handshake
+    sstd_index ^= HANDSHAKE_MAGIC;
+    sdWrite(&SERIAL_USART_DRIVER, &sstd_index, sizeof(sstd_index));
+
+    if (trans->initiator2target_buffer_size) {
+        sdRead(&SERIAL_USART_DRIVER, trans->initiator2target_buffer, trans->initiator2target_buffer_size);
+    }
+
+    if (trans->target2initiator_buffer_size) {
+        sdWrite(&SERIAL_USART_DRIVER, trans->target2initiator_buffer, trans->target2initiator_buffer_size);
+    }
+
+    if (trans->status) {
+        *trans->status = TRANSACTION_ACCEPTED;
+    }
+}
+
+/////////
+//  start transaction by initiator
+//
+// int  soft_serial_transaction(int sstd_index)
+//
+// Returns:
+//    TRANSACTION_END
+//    TRANSACTION_NO_RESPONSE
+//    TRANSACTION_DATA_ERROR
+#ifndef SERIAL_USE_MULTI_TRANSACTION
+int soft_serial_transaction(void) {
+    uint8_t sstd_index = 0;
+#else
+int soft_serial_transaction(int index) {
+    uint8_t sstd_index = index;
+#endif
+
+    if (sstd_index > Transaction_table_size) return TRANSACTION_TYPE_ERROR;
+    SSTD_t* trans = &Transaction_table[sstd_index];
+    msg_t   res   = 0;
+
+    sdClear(&SERIAL_USART_DRIVER);
+
+    // First chunk is always transaction id
+    sdWriteTimeout(&SERIAL_USART_DRIVER, &sstd_index, sizeof(sstd_index), TIME_MS2I(TIMEOUT));
+
+    uint8_t sstd_index_shake = 0xFF;
+
+    // Which we always read back first so that we can error out correctly
+    //   - due to the half duplex limitations on return codes, we always have to read *something*
+    //   - without the read, write only transactions *always* succeed, even during the boot process where the slave is not ready
+    res = sdReadTimeout(&SERIAL_USART_DRIVER, &sstd_index_shake, sizeof(sstd_index_shake), TIME_MS2I(TIMEOUT));
+    if (res < 0 || (sstd_index_shake != (sstd_index ^ HANDSHAKE_MAGIC))) {
+        dprintf("serial::usart_shake NO_RESPONSE\n");
+        return TRANSACTION_NO_RESPONSE;
+    }
+
+    if (trans->initiator2target_buffer_size) {
+        res = sdWriteTimeout(&SERIAL_USART_DRIVER, trans->initiator2target_buffer, trans->initiator2target_buffer_size, TIME_MS2I(TIMEOUT));
+        if (res < 0) {
+            dprintf("serial::usart_transmit NO_RESPONSE\n");
+            return TRANSACTION_NO_RESPONSE;
+        }
+    }
+
+    if (trans->target2initiator_buffer_size) {
+        res = sdReadTimeout(&SERIAL_USART_DRIVER, trans->target2initiator_buffer, trans->target2initiator_buffer_size, TIME_MS2I(TIMEOUT));
+        if (res < 0) {
+            dprintf("serial::usart_receive NO_RESPONSE\n");
+            return TRANSACTION_NO_RESPONSE;
+        }
+    }
+
+    return TRANSACTION_END;
+}