Add Full-duplex serial driver for ARM boards (#9842)

4 files changed, 506 insertions, 56 deletions

diff --git a/docs/serial_driver.md b/docs/serial_driver.md
index c98f4c117..359fc5955 100644
--- a/docs/serial_driver.md
+++ b/docs/serial_driver.md
@@ -3,16 +3,18 @@ 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
+Drivers in this category have the following characteristics:
+* bit bang and USART Half-duplex provide data and signaling over a single conductor
+* USART Full-duplex provide data and signaling over two conductors
+* They are all limited to single master and single slave communication scheme
 
 ## Supported Driver Types
 
 |                   | AVR                | ARM                |
-|-------------------|--------------------|--------------------|
+| ----------------- | ------------------ | ------------------ |
 | bit bang          | :heavy_check_mark: | :heavy_check_mark: |
 | USART Half-duplex |                    | :heavy_check_mark: |
+| USART Full-duplex |                    | :heavy_check_mark: |
 
 ## Driver configuration
 
@@ -42,7 +44,7 @@ Configure the driver via your config.h:
 Along with the generic options above, you must also turn on the `PAL_USE_CALLBACKS` feature in your halconf.h.
 
 ### 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:
+Targeting STM32 boards where communication is offloaded to a USART hardware device. The advantage over bitbang is that this provides fast and accurate timings. `SERIAL_PIN_TX` for this driver is the configured USART TX pin. As this Pin is configured in open-drain mode an **external pull-up resistor is needed to keep the line high** (resistor values of 1.5k to 8.2k are known to work). To configure it, add this to your rules.mk:
 
 ```make
 SERIAL_DRIVER = usart
@@ -50,7 +52,8 @@ SERIAL_DRIVER = usart
 
 Configure the hardware via your config.h:
 ```c
-#define SOFT_SERIAL_PIN B6  // USART TX pin
+#define SOFT_SERIAL_PIN B6         // USART TX pin
+//#define USART1_REMAP             // Remap USART TX and RX pins on STM32F103 MCUs, see table below.
 #define SELECT_SOFT_SERIAL_SPEED 1 // or 0, 2, 3, 4, 5
                                    //  0: about 460800 baud
                                    //  1: about 230400 baud (default)
@@ -58,7 +61,7 @@ Configure the hardware via your config.h:
                                    //  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_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
 #define SERIAL_USART_TIMEOUT 100 // USART driver timeout. default 100
 ```
@@ -68,3 +71,140 @@ You must also enable the ChibiOS `SERIAL` feature:
 * In your board's mcuconf.h: `#define STM32_SERIAL_USE_USARTn TRUE` (where 'n' matches the peripheral number of your selected USART on the MCU)
 
 Do note that the configuration required is for the `SERIAL` peripheral, not the `UART` peripheral.
+
+### USART Full-duplex
+Targeting STM32 boards where communication is offloaded to a USART hardware device. The advantage over bitbang is that this provides fast and accurate timings. USART Full-Duplex requires two conductors **without** pull-up resistors instead of one conductor with a pull-up resistor unlike the Half-duplex driver, but it is more efficent as it uses DMA transfers, which can result in even faster transmission speeds.
+
+#### Pin configuration
+
+`SERIAL_USART_TX_PIN` is the USART `TX` pin, `SERIAL_USART_RX_PIN` is the USART `RX` pin. No external pull-up resistors are needed as the `TX` pin operates in push-pull mode. To use this driver the usart peripherals `TX` and `RX` pins must be configured with the correct Alternate-functions. If you are using a Proton-C everything is already setup, same is true for STM32F103 MCUs. For MCUs which are using a modern flexible GPIO configuration you have to specify these by setting `SERIAL_USART_TX_PAL_MODE` and `SERIAL_USART_RX_PAL_MODE`. Refeer to the corresponding datasheets of your MCU or find those settings in the table below.
+
+#### Connecting the halves and Pin Swap
+Please note that `TX` of the master half has to be connected with the `RX` pin of the slave half and `RX` of the master half has to be connected with the `TX` pin of the slave half! Usually this pin swap has to be done outside of the MCU e.g. with cables or on the pcb. Some MCUs like the STM32F303 used on the Proton-C allow this pin swap directly inside the MCU, this feature can be enabled using `#define SERIAL_USART_PIN_SWAP` in your config.h.
+
+#### Setup
+To use the driver, add this to your rules.mk:
+
+```make
+SERIAL_DRIVER = usart_duplex
+```
+
+Next configure the hardware via your config.h:
+
+```c
+#define SERIAL_USART_TX_PIN B6     // USART TX pin
+#define SERIAL_USART_RX_PIN B7     // USART RX pin
+//#define USART1_REMAP             // Remap USART TX and RX pins on STM32F103 MCUs, see table below.
+//#define SERIAL_USART_PIN_SWAP    // Swap TX and RX pins if keyboard is master halve.
+                                   // Check if this feature is necessary with your keyboard design and available on the mcu.
+#define SELECT_SOFT_SERIAL_SPEED 1 // or 0, 2, 3, 4, 5
+                                   //  0: 460800 baud
+                                   //  1: 230400 baud (default)
+                                   //  2: 115200 baud
+                                   //  3: 57600 baud
+                                   //  4: 38400 baud
+                                   //  5: 19200 baud
+#define SERIAL_USART_DRIVER UARTD1 // USART driver of TX and RX pin. default: UARTD1
+#define SERIAL_USART_TX_PAL_MODE 7 // Pin "alternate function", see the respective datasheet for the appropriate values for your MCU. default: 7
+#define SERIAL_USART_RX_PAL_MODE 7 // Pin "alternate function", see the respective datasheet for the appropriate values for your MCU. default: 7
+#define SERIAL_USART_TIMEOUT 100 // USART driver timeout. default 100
+```
+
+You must also enable the ChibiOS `UART` with blocking api feature:
+* In your board's halconf.h: `#define HAL_USE_UART TRUE` and `#define UART_USE_WAIT TRUE`
+* In your board's mcuconf.h: `#define STM32_UART_USE_USARTn TRUE` (where 'n' matches the peripheral number of your selected USART on the MCU)
+
+Do note that the configuration required is for the `UART` peripheral, not the `SERIAL` peripheral.
+
+#### Pins for USART Peripherals with Alternate Functions for selected STM32 MCUs
+
+##### STM32F303 / Proton-C [Datasheet](https://www.st.com/resource/en/datasheet/stm32f303cc.pdf)
+
+Pin Swap available: :heavy_check_mark:
+
+|    Pin     | Function | Mode |
+| ---------- | -------- | ---- |
+| **USART1** |          |      |
+| PA9        | TX       | AF7  |
+| PA10       | RX       | AF7  |
+| PB6        | TX       | AF7  |
+| PB7        | RX       | AF7  |
+| PC4        | TX       | AF7  |
+| PC5        | RX       | AF7  |
+| PE0        | TX       | AF7  |
+| PE1        | RX       | AF7  |
+| **USART2** |          |      |
+| PA2        | TX       | AF7  |
+| PA3        | RX       | AF7  |
+| PA14       | TX       | AF7  |
+| PA15       | RX       | AF7  |
+| PB3        | TX       | AF7  |
+| PB4        | RX       | AF7  |
+| PD5        | TX       | AF7  |
+| PD6        | RX       | AF7  |
+| **USART3** |          |      |
+| PB10       | TX       | AF7  |
+| PB11       | RX       | AF7  |
+| PC10       | TX       | AF7  |
+| PC11       | RX       | AF7  |
+| PD8        | TX       | AF7  |
+| PD9        | RX       | AF7  |
+
+##### STM32F072 [Datasheet](https://www.st.com/resource/en/datasheet/stm32f072c8.pdf)
+
+Pin Swap available: :heavy_check_mark:
+
+|  Pin   | Function | Mode |
+| ------ | -------- | ---- |
+| USART1 |          |      |
+| PA9    | TX       | AF1  |
+| PA10   | RX       | AF1  |
+| PB6    | TX       | AF0  |
+| PB7    | RX       | AF0  |
+| USART2 |          |      |
+| PA2    | TX       | AF1  |
+| PA3    | RX       | AF1  |
+| PA14   | TX       | AF1  |
+| PA15   | RX       | AF1  |
+| USART3 |          |      |
+| PB10   | TX       | AF4  |
+| PB11   | RX       | AF4  |
+| PC4    | TX       | AF1  |
+| PC5    | RX       | AF1  |
+| PC10   | TX       | AF1  |
+| PC11   | RX       | AF1  |
+| PD8    | TX       | AF0  |
+| PD9    | RX       | AF0  |
+| USART4 |          |      |
+| PA0    | TX       | AF4  |
+| PA1    | RX       | AF4  |
+
+##### STM32F103 Medium Density (C8-CB) [Datasheet](https://www.st.com/resource/en/datasheet/stm32f103c8.pdf)
+
+Pin Swap available: N/A
+
+TX Pin is always Alternate Function Push-Pull, RX Pin is always regular input pin for any USART peripheral. **For STM32F103 no additional Alternate Function configuration is necessary. QMK is already configured.**
+
+Pin remapping:
+
+The pins of USART Peripherals use default Pins that can be remapped to use other pins using the AFIO registers. Default pins are marked **bold**. Add the appropriate defines to your config.h file.
+
+|    Pin     | Function | Mode |     USART_REMAP     |
+| ---------- | -------- | ---- | ------------------- |
+| **USART1** |          |      |                     |
+| **PA9**    | TX       | AFPP |                     |
+| **PA10**   | RX       | IN   |                     |
+| PB6        | TX       | AFPP | USART1_REMAP        |
+| PB7        | RX       | IN   | USART1_REMAP        |
+| **USART2** |          |      |                     |
+| **PA2**    | TX       | AFPP |                     |
+| **PA3**    | RX       | IN   |                     |
+| PD5        | TX       | AFPP | USART2_REMAP        |
+| PD6        | RX       | IN   | USART2_REMAP        |
+| **USART3** |          |      |                     |
+| **PB10**   | TX       | AFPP |                     |
+| **PB11**   | RX       | IN   |                     |
+| PC10       | TX       | AFPP | USART3_PARTIALREMAP |
+| PC11       | RX       | IN   | USART3_PARTIALREMAP |
+| PD8        | TX       | AFPP | USART3_FULLREMAP    |
+| PD9        | RX       | IN   | USART3_FULLREMAP    |
diff --git a/drivers/chibios/serial_usart.c b/drivers/chibios/serial_usart.c
index 7c81b1646..cae29388c 100644
--- a/drivers/chibios/serial_usart.c
+++ b/drivers/chibios/serial_usart.c
@@ -1,13 +1,20 @@
-#include "quantum.h"
-#include "serial.h"
-#include "print.h"
-
-#include <ch.h>
-#include <hal.h>
+/* Copyright 2021 QMK
+ *
+ * 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 3 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/>.
+ */
 
-#ifndef USART_CR1_M0
-#    define USART_CR1_M0 USART_CR1_M  // some platforms (f1xx) dont have this so
-#endif
+#include "serial_usart.h"
 
 #ifndef USE_GPIOV1
 // The default PAL alternate modes are used to signal that the pins are used for USART
@@ -20,50 +27,10 @@
 #    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
-
-#ifndef SERIAL_USART_TIMEOUT
-#    define SERIAL_USART_TIMEOUT 100
-#endif
-
-#define HANDSHAKE_MAGIC 7
-
 static inline msg_t sdWriteHalfDuplex(SerialDriver* driver, uint8_t* data, uint8_t size) {
     msg_t ret = sdWrite(driver, data, size);
 
@@ -123,6 +90,10 @@ __attribute__((weak)) void usart_init(void) {
 #else
     palSetLineMode(SERIAL_USART_TX_PIN, PAL_MODE_ALTERNATE(SERIAL_USART_TX_PAL_MODE) | PAL_STM32_OTYPE_OPENDRAIN);
 #endif
+
+#if defined(USART_REMAP)
+    USART_REMAP;
+#endif
 }
 
 void usart_master_init(void) {
diff --git a/drivers/chibios/serial_usart.h b/drivers/chibios/serial_usart.h
new file mode 100644
index 000000000..d35b5d12c
--- /dev/null
+++ b/drivers/chibios/serial_usart.h
@@ -0,0 +1,78 @@
+/* Copyright 2021 QMK
+ *
+ * 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 3 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 "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 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
+
+#if defined(USART1_REMAP)
+#    define USART_REMAP do { (AFIO->MAPR |= AFIO_MAPR_USART1_REMAP); } while(0)
+#elif defined(USART2_REMAP)
+#    define USART_REMAP do { (AFIO->MAPR |= AFIO_MAPR_USART2_REMAP); } while(0)
+#elif defined(USART3_PARTIALREMAP)
+#    define USART_REMAP do { (AFIO->MAPR |= AFIO_MAPR_USART3_REMAP_PARTIALREMAP); } while(0)
+#elif defined(USART3_FULLREMAP)
+#    define USART_REMAP do { (AFIO->MAPR |= AFIO_MAPR_USART3_REMAP_FULLREMAP); } while(0)
+#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
+
+#ifndef SERIAL_USART_TIMEOUT
+#    define SERIAL_USART_TIMEOUT 100
+#endif
+
+#define HANDSHAKE_MAGIC 7
diff --git a/drivers/chibios/serial_usart_duplex.c b/drivers/chibios/serial_usart_duplex.c
new file mode 100644
index 000000000..cc9b889ac
--- /dev/null
+++ b/drivers/chibios/serial_usart_duplex.c
@@ -0,0 +1,261 @@
+/* Copyright 2021 QMK
+ *
+ * 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 3 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/>.
+ */
+
+#include "serial_usart.h"
+
+#include <stdatomic.h>
+
+#if !defined(USE_GPIOV1)
+// The default PAL alternate modes are used to signal that the pins are used for USART
+#    if !defined(SERIAL_USART_TX_PAL_MODE)
+#        define SERIAL_USART_TX_PAL_MODE 7
+#    endif
+#    if !defined(SERIAL_USART_RX_PAL_MODE)
+#        define SERIAL_USART_RX_PAL_MODE 7
+#    endif
+#endif
+
+#if !defined(SERIAL_USART_DRIVER)
+#    define SERIAL_USART_DRIVER UARTD1
+#endif
+
+#if !defined(SERIAL_USART_TX_PIN)
+#    define SERIAL_USART_TX_PIN A9
+#endif
+
+#if !defined(SERIAL_USART_RX_PIN)
+#    define SERIAL_USART_RX_PIN A10
+#endif
+
+#define SIGNAL_HANDSHAKE_RECEIVED 0x1
+
+void        handle_transactions_slave(uint8_t sstd_index);
+static void receive_transaction_handshake(UARTDriver* uartp, uint16_t received_handshake);
+
+/*
+ * UART driver configuration structure. We use the blocking DMA enabled API and
+ * the rxchar callback to receive handshake tokens but only on the slave halve.
+ */
+// clang-format off
+static UARTConfig uart_config = {
+    .txend1_cb = NULL,
+    .txend2_cb = NULL,
+    .rxend_cb = NULL,
+    .rxchar_cb = NULL,
+    .rxerr_cb = NULL,
+    .timeout_cb = NULL,
+    .speed = (SERIAL_USART_SPEED),
+    .cr1 = (SERIAL_USART_CR1),
+    .cr2 = (SERIAL_USART_CR2),
+    .cr3 = (SERIAL_USART_CR3)
+};
+// clang-format on
+
+static SSTD_t*              Transaction_table      = NULL;
+static uint8_t              Transaction_table_size = 0;
+static atomic_uint_least8_t handshake              = 0xFF;
+static thread_reference_t   tp_target              = NULL;
+
+/*
+ * This callback is invoked when a character is received but the application
+ * was not ready to receive it, the character is passed as parameter.
+ * Receive transaction table index from initiator, which doubles as basic handshake token. */
+static void receive_transaction_handshake(UARTDriver* uartp, uint16_t received_handshake) {
+    /* Check if received handshake is not a valid transaction id.
+     * Please note that we can still catch a seemingly valid handshake
+     * i.e. a byte from a ongoing transfer which is in the allowed range.
+     * So this check mainly prevents any obviously wrong handshakes and
+     * subsequent wakeups of the receiving thread, which is a costly operation. */
+    if (received_handshake > Transaction_table_size) {
+        return;
+    }
+
+    handshake = (uint8_t)received_handshake;
+    chSysLockFromISR();
+    /* Wakeup receiving thread to start a transaction. */
+    chEvtSignalI(tp_target, (eventmask_t)SIGNAL_HANDSHAKE_RECEIVED);
+    chSysUnlockFromISR();
+}
+
+__attribute__((weak)) void usart_init(void) {
+#if defined(USE_GPIOV1)
+    palSetLineMode(SERIAL_USART_TX_PIN, PAL_MODE_STM32_ALTERNATE_PUSHPULL);
+    palSetLineMode(SERIAL_USART_RX_PIN, PAL_MODE_INPUT);
+#else
+    palSetLineMode(SERIAL_USART_TX_PIN, PAL_MODE_ALTERNATE(SERIAL_USART_TX_PAL_MODE) | PAL_STM32_OTYPE_PUSHPULL | PAL_STM32_OSPEED_HIGHEST);
+    palSetLineMode(SERIAL_USART_RX_PIN, PAL_MODE_ALTERNATE(SERIAL_USART_RX_PAL_MODE) | PAL_STM32_OTYPE_PUSHPULL | PAL_STM32_OSPEED_HIGHEST);
+#endif
+}
+
+/*
+ * This thread runs on the slave half and reacts to transactions initiated from the master.
+ */
+static THD_WORKING_AREA(waSlaveThread, 1024);
+static THD_FUNCTION(SlaveThread, arg) {
+    (void)arg;
+    chRegSetThreadName("slave_usart_tx_rx");
+
+    while (true) {
+        /* We sleep as long as there is no handshake waiting for us. */
+        chEvtWaitAny((eventmask_t)SIGNAL_HANDSHAKE_RECEIVED);
+        handle_transactions_slave(handshake);
+    }
+}
+
+void soft_serial_target_init(SSTD_t* const sstd_table, int sstd_table_size) {
+    Transaction_table      = sstd_table;
+    Transaction_table_size = (uint8_t)sstd_table_size;
+    usart_init();
+
+#if defined(USART_REMAP)
+    USART_REMAP;
+#endif
+
+    tp_target = chThdCreateStatic(waSlaveThread, sizeof(waSlaveThread), HIGHPRIO, SlaveThread, NULL);
+
+    // Start receiving handshake tokens on slave halve
+    uart_config.rxchar_cb = receive_transaction_handshake;
+    uartStart(&SERIAL_USART_DRIVER, &uart_config);
+}
+
+/**
+ * @brief React to transactions started by the master.
+ * This version uses duplex send and receive usart pheriphals and DMA backed transfers.
+ */
+void inline handle_transactions_slave(uint8_t sstd_index) {
+    size_t  buffer_size = 0;
+    msg_t   msg         = 0;
+    SSTD_t* trans       = &Transaction_table[sstd_index];
+
+    /* Send back the handshake which is XORed as a simple checksum,
+     to signal that the slave is ready to receive possible transaction buffers  */
+    sstd_index ^= HANDSHAKE_MAGIC;
+    buffer_size = (size_t)sizeof(sstd_index);
+    msg         = uartSendTimeout(&SERIAL_USART_DRIVER, &buffer_size, &sstd_index, TIME_MS2I(SERIAL_USART_TIMEOUT));
+
+    if (msg != MSG_OK) {
+        if (trans->status) {
+            *trans->status = TRANSACTION_NO_RESPONSE;
+        }
+        return;
+    }
+
+    /* Receive transaction buffer from the master. If this transaction requires it.*/
+    buffer_size = (size_t)trans->initiator2target_buffer_size;
+    if (buffer_size) {
+        msg = uartReceiveTimeout(&SERIAL_USART_DRIVER, &buffer_size, trans->initiator2target_buffer, TIME_MS2I(SERIAL_USART_TIMEOUT));
+        if (msg != MSG_OK) {
+            if (trans->status) {
+                *trans->status = TRANSACTION_NO_RESPONSE;
+            }
+            return;
+        }
+    }
+
+    /* Send transaction buffer to the master. If this transaction requires it. */
+    buffer_size = (size_t)trans->target2initiator_buffer_size;
+    if (buffer_size) {
+        msg = uartSendFullTimeout(&SERIAL_USART_DRIVER, &buffer_size, trans->target2initiator_buffer, TIME_MS2I(SERIAL_USART_TIMEOUT));
+        if (msg != MSG_OK) {
+            if (trans->status) {
+                *trans->status = TRANSACTION_NO_RESPONSE;
+            }
+            return;
+        }
+    }
+
+    if (trans->status) {
+        *trans->status = TRANSACTION_ACCEPTED;
+    }
+}
+
+void soft_serial_initiator_init(SSTD_t* const sstd_table, int sstd_table_size) {
+    Transaction_table      = sstd_table;
+    Transaction_table_size = (uint8_t)sstd_table_size;
+    usart_init();
+
+#if defined(SERIAL_USART_PIN_SWAP)
+    uart_config.cr2 |= USART_CR2_SWAP;  // master has swapped TX/RX pins
+#endif
+
+#if defined(USART_REMAP)
+    USART_REMAP;
+#endif
+
+    uartStart(&SERIAL_USART_DRIVER, &uart_config);
+}
+
+/**
+ * @brief Start transaction from the master to the slave.
+ * This version uses duplex send and receive usart pheriphals and DMA backed transfers.
+ *
+ * @param index Transaction Table index of the transaction to start.
+ * @return int TRANSACTION_NO_RESPONSE in case of Timeout.
+ *             TRANSACTION_TYPE_ERROR in case of invalid transaction index.
+ *             TRANSACTION_END in case of success.
+ */
+#if !defined(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* const trans       = &Transaction_table[sstd_index];
+    msg_t         msg         = 0;
+    size_t        buffer_size = (size_t)sizeof(sstd_index);
+
+    /* Send transaction table index to the slave, which doubles as basic handshake token. */
+    uartSendFullTimeout(&SERIAL_USART_DRIVER, &buffer_size, &sstd_index, TIME_MS2I(SERIAL_USART_TIMEOUT));
+
+    uint8_t sstd_index_shake = 0xFF;
+    buffer_size              = (size_t)sizeof(sstd_index_shake);
+
+    /* Receive the handshake token from the slave. The token was XORed by the slave as a simple checksum.
+     If the tokens match, the master will start to send and receive possible transaction buffers. */
+    msg = uartReceiveTimeout(&SERIAL_USART_DRIVER, &buffer_size, &sstd_index_shake, TIME_MS2I(SERIAL_USART_TIMEOUT));
+    if (msg != MSG_OK || (sstd_index_shake != (sstd_index ^ HANDSHAKE_MAGIC))) {
+        dprintln("USART: Handshake Failed");
+        return TRANSACTION_NO_RESPONSE;
+    }
+
+    /* Send transaction buffer to the slave. If this transaction requires it. */
+    buffer_size = (size_t)trans->initiator2target_buffer_size;
+    if (buffer_size) {
+        msg = uartSendFullTimeout(&SERIAL_USART_DRIVER, &buffer_size, trans->initiator2target_buffer, TIME_MS2I(SERIAL_USART_TIMEOUT));
+        if (msg != MSG_OK) {
+            dprintln("USART: Send Failed");
+            return TRANSACTION_NO_RESPONSE;
+        }
+    }
+
+    /* Receive transaction buffer from the slave. If this transaction requires it. */
+    buffer_size = (size_t)trans->target2initiator_buffer_size;
+    if (buffer_size) {
+        msg = uartReceiveTimeout(&SERIAL_USART_DRIVER, &buffer_size, trans->target2initiator_buffer, TIME_MS2I(SERIAL_USART_TIMEOUT));
+        if (msg != MSG_OK) {
+            dprintln("USART: Receive Failed");
+            return TRANSACTION_NO_RESPONSE;
+        }
+    }
+
+    return TRANSACTION_END;
+}