Add half-duplex support to HardwareSerial.

cores/arduino/HardwareSerial.cpp

@@ -347,6 +347,11 @@ void HardwareSerial::begin(unsigned long baud, byte config)
   }
 
   uart_init(&_serial, (uint32_t)baud, databits, parity, stopbits);
+  if (isHalfDuplex()) {
+    // Start half duplex communication in receive mode.
+    _rx_enabled = true;
+    uart_enable_rx(&_serial);
+  }
   uart_attach_rx_callback(&_serial, _rx_complete_irq);
 }
 
@@ -377,6 +382,15 @@ int HardwareSerial::peek(void)
 
 int HardwareSerial::read(void)
 {
+  if (isHalfDuplex()) {
+    // In half duplex mode we have to wait for all TX characters to
+    // be transmitted before we can receive data.
+    flush();
+    if (!_rx_enabled) {
+      _rx_enabled = true;
+      uart_enable_rx(&_serial);
+    }
+  }
   // if the head isn't ahead of the tail, we don't have any characters
   if (_serial.rx_head == _serial.rx_tail) {
     return -1;
@@ -417,6 +431,12 @@ void HardwareSerial::flush()
 size_t HardwareSerial::write(uint8_t c)
 {
   _written = true;
+  if (isHalfDuplex()) {
+    if (_rx_enabled) {
+      _rx_enabled = false;
+      uart_enable_tx(&_serial);
+    }
+  }
 
   tx_buffer_index_t i = (_serial.tx_head + 1) % SERIAL_TX_BUFFER_SIZE;
 

cores/arduino/HardwareSerial.h

@@ -143,12 +143,24 @@ class HardwareSerial : public Stream {
     void setRx(PinName _rx);
     void setTx(PinName _tx);
 
+    // Enable half-duplex mode by setting the Rx pin to NC
+    // This needs to be done before the call to begin()
+    inline void setHalfDuplex()
+    {
+      setRx(NC);
+    }
+    inline bool isHalfDuplex() const
+    {
+      return _serial.pin_rx == NC;
+    }
+
     friend class STM32LowPower;
 
     // Interrupt handlers
     static void _rx_complete_irq(serial_t *obj);
     static int _tx_complete_irq(serial_t *obj);
   private:
+    bool _rx_enabled;
     uint8_t _config;
     unsigned long _baud;
     void init(void);

cores/arduino/stm32/uart.c

@@ -131,7 +131,7 @@ void uart_init(serial_t *obj, uint32_t baudrate, uint32_t databits, uint32_t par
   USART_TypeDef *uart_rx = pinmap_peripheral(obj->pin_rx, PinMap_UART_RX);
 
   /* Pins Rx/Tx must not be NP */
-  if (uart_rx == NP || uart_tx == NP) {
+  if ((obj->pin_rx != NC && uart_rx == NP) || uart_tx == NP) {
     core_debug("ERROR: at least one UART pin has no peripheral\n");
     return;
   }
@@ -288,7 +288,9 @@ void uart_init(serial_t *obj, uint32_t baudrate, uint32_t databits, uint32_t par
 
   /* Configure UART GPIO pins */
   pinmap_pinout(obj->pin_tx, PinMap_UART_TX);
-  pinmap_pinout(obj->pin_rx, PinMap_UART_RX);
+  if (uart_rx != NP) {
+    pinmap_pinout(obj->pin_rx, PinMap_UART_RX);
+  }
 
   /* Configure uart */
   uart_handlers[obj->index] = huart;
@@ -359,7 +361,11 @@ void uart_init(serial_t *obj, uint32_t baudrate, uint32_t databits, uint32_t par
   }
 #endif
 
-  if (HAL_UART_Init(huart) != HAL_OK) {
+  if (uart_rx == NP) {
+    if (HAL_HalfDuplex_Init(huart) != HAL_OK) {
+      return;
+    }
+  } else if (HAL_UART_Init(huart) != HAL_OK) {
     return;
   }
 }
@@ -732,6 +738,32 @@ void uart_attach_tx_callback(serial_t *obj, int (*callback)(serial_t *))
   HAL_NVIC_EnableIRQ(obj->irq);
 }
 
+/**
+ * Enable transmitter for half-duplex mode. NOOP in full-fuplex mode
+ *
+ * @param obj : pointer to serial_t structure
+ * @retval none
+ */
+void uart_enable_tx(serial_t *obj)
+{
+  if (obj != NULL && obj->pin_rx == NC) {
+    HAL_HalfDuplex_EnableTransmitter(uart_handlers[obj->index]);
+  }
+}
+
+/**
+ * Enable receiver for half-duplex mode. NOOP in full-fuplex mode
+ *
+ * @param obj : pointer to serial_t structure
+ * @retval none
+ */
+void uart_enable_rx(serial_t *obj)
+{
+  if (obj != NULL && obj->pin_rx == NC) {
+    HAL_HalfDuplex_EnableReceiver(uart_handlers[obj->index]);
+  }
+}
+
 /**
   * @brief  Return index of the serial handler
   * @param  UartHandle pointer on the uart reference

cores/arduino/stm32/uart.h

@@ -181,6 +181,9 @@ void uart_attach_tx_callback(serial_t *obj, int (*callback)(serial_t *));
 uint8_t serial_tx_active(serial_t *obj);
 uint8_t serial_rx_active(serial_t *obj);
 
+void uart_enable_tx(serial_t *obj);
+void uart_enable_rx(serial_t *obj);
+
 size_t uart_debug_write(uint8_t *data, uint32_t size);
 
 #endif /* HAL_UART_MODULE_ENABLED */