UART Half Duplex: How to get DMA working for both RX and TX on STM32F303CC?

Question

I am using UART1 in half duplex mode and DMA works for TX, but I cannot get it to work for RX (after calling HAL_HalfDuplex_EnableReceiver). Here is my initialization code: __HAL_RCC_GPIOA_CLK_ENABLE();
	m_GPIO_InitStructTX.Pin = GPIO_PIN_9;
	m_GPIO_InitStructTX.Mode = GPIO_MODE_AF_PP;
	m_GPIO_InitStructTX.Pull = GPIO_NOPULL;
	m_GPIO_InitStructTX.Speed = GPIO_SPEED_LOW;
	m_GPIO_InitStructTX.Alternate = GPIO_AF7_USART1;
	HAL_GPIO_Init(GPIOA, &m_GPIO_InitStructTX);
	
	__HAL_RCC_USART1_FORCE_RESET();
	__HAL_RCC_USART1_RELEASE_RESET();
	__HAL_RCC_USART1_CLK_ENABLE();
	m_uartHandle = UART_HandleTypeDef();
	m_uartHandle.Instance = USART1;
	m_uartHandle.Init.BaudRate = 9600;
	m_uartHandle.Init.WordLength = UART_WORDLENGTH_8B;
	m_uartHandle.Init.StopBits = UART_STOPBITS_2; // !!! 2 Stopbits
	m_uartHandle.Init.Parity = UART_PARITY_NONE;
	m_uartHandle.Init.HwFlowCtl = UART_HWCONTROL_NONE;
	m_uartHandle.Init.Mode = UART_MODE_TX;
	huart = &m_uartHandle;
	HAL_HalfDuplex_Init(&m_uartHandle);
	HAL_HalfDuplex_EnableTransmitter(&m_uartHandle);
	HAL_NVIC_ClearPendingIRQ(USART1_IRQn);
	HAL_NVIC_DisableIRQ(USART1_IRQn);
	HAL_NVIC_SetPriority(USART1_IRQn, 0, 0);
	HAL_NVIC_EnableIRQ(USART1_IRQn);
 
	__HAL_RCC_DMA1_CLK_ENABLE();
	m_DMAHandleTX.Instance = DMA1_Channel4;
	m_DMAHandleTX.Init.Direction = DMA_MEMORY_TO_PERIPH;
	m_DMAHandleTX.Init.PeriphInc = DMA_PINC_DISABLE;
	m_DMAHandleTX.Init.MemInc = DMA_MINC_ENABLE;
	m_DMAHandleTX.Init.PeriphDataAlignment = DMA_PDATAALIGN_BYTE;
	m_DMAHandleTX.Init.MemDataAlignment = DMA_MDATAALIGN_BYTE;
	m_DMAHandleTX.Init.Priority = DMA_PRIORITY_MEDIUM;
	m_DMAHandleTX.Init.Mode = DMA_NORMAL;
	hdmaTX = &m_DMAHandleTX;
	__HAL_LINKDMA(&m_uartHandle, hdmatx, m_DMAHandleTX);
	__HAL_DMA_ENABLE_IT(&m_DMAHandleTX, DMA_IT_TC);
	HAL_DMA_Init(&m_DMAHandleTX);
	HAL_NVIC_SetPriority(DMA1_Channel4_IRQn, 0, 0);
	HAL_NVIC_EnableIRQ(DMA1_Channel4_IRQn);DMA works fine for TX, but when I switch to HAL_HalfDuplex_EnableReceiver it does not work anymore. I also tried to reconfigure the DMA and UART by calling:m_uartHandle.Init.Mode = UART_MODE_RX;
HAL_HalfDuplex_Init(&m_uartHandle);
HAL_DMA_DeInit(&m_DMAHandleTX);
m_DMAHandleTX.Init.Direction = DMA_PERIPH_TO_MEMORY;
__HAL_LINKDMA(&m_uartHandle, hdmarx, m_DMAHandleTX);
HAL_DMA_Init(&m_DMAHandleTX);
HAL_HalfDuplex_EnableReceiver(&m_uartHandle);
HAL_UART_Receive_DMA(&m_uartHandle, buf, length);What am I missing?

S.Ma · Answer

For STM32L4R the reference manual says:USART single-wire Half-duplex communicationSingle-wire Half-duplex mode is selected by setting the HDSEL bit in the USART_CR3register. In this mode, the following bits must be kept cleared:• LINEN and CLKEN bits in the USART_CR2 register,• SCEN and IREN bits in the USART_CR3 register.The USART can be configured to follow a Single-wire Half-duplex protocol where the TXand RX lines are internally connected. The selection between half- and Full-duplexcommunication is made with a control bit HDSEL in USART_CR3.As soon as HDSEL is written to ‘1’:• The TX and RX lines are internally connected.• The RX pin is no longer used.• The TX pin is always released when no data is transmitted. Thus, it acts as a standardI/O in idle or in reception. It means that the I/O must be configured so that TX isconfigured as alternate function open-drain with an external pull-upUsually the DMA Channel for RX and TX are different... maybe this is the missing point.Make sure only one DMA channel is active at any time.

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