UART Issues

1. UART stops receiving but works fine after reset.

It is caused if interrupt is not handled properly or receiving flow goes out of sync. To resolve this issue, you can either use HAL_UART_ErrorCallback or check DMA status or both.

1.1. Use HAL_UART_ErrorCallback:

The callback function HAL_UART_ErrorCallback is called when an error occurs. You can use this callback to handle the error. For example:

void HAL_UART_ErrorCallback(UART_HandleTypeDef *huart)
{
  // Handle error
  if (huart->Instance == huart2.Instance)
  {
      HAL_UART_Receive_DMA(&huart2, (uint8_t *)&c, sizeof(c)); // Receive again
  }
  else if (huart->Instance == joystick.uart.get_uart_instance())
  {
      // Handle error by initializing the joystick again
      joystick.init();
  }
}

1.2. Check DMA status:

You can check the DMA status to ensure that the DMA is working properly. While receiving data, the DMA status is HAL_DMA_STATE_BUSY. If it is not, you can reinitialize the DMA by triggering DMA receive again inside looping function

void run_robot()
{
   if (huart->hdmarx->State != HAL_DMA_STATE_BUSY)
   {
       HAL_UART_Receive_DMA(&huart2, (uint8_t *)&c, sizeof(c)); // Receive again
   }
   else if (joystick.uart.hdmarx->State != HAL_DMA_STATE_BUSY)
   {
       joystick.init();
   }
 }

2. HAL_UART_TxCpltCallback is not called.

It is neccessary to enable the UART TX interrupt. If the interrupt is not enabled, the callback function will not be called. You can use STM32CubeMX to enable the UART TX interrupt.

Open your project on STM32CubeMX.
Go to Comminication > USARTx > Configuration > NVIC Settings.
Enable USARTx global interrupt.

3. Receiving data even pins are not connected.

It is caused by the noise in the circuit like:

Soldering flux not cleaned properly.
Touching the pins with fingers.
UART wires are too close to the power or actuator wires.