Electronic – TXE bit won’t clear UART ARM M0

interruptsstm32uart

I'm trying to get UART working on a STM32F0 part and can't figure out the interrupts. According to the reference manual 

Bit 7 TXE: Transmit data register empty
This bit is set by hardware when the content of the USARTx_TDR register has been 
transferred into the shift register. It is cleared by a write to the USARTx_TDR register.
An interrupt is generated if the TXEIE bit =1 in the USARTx_CR1 register.

However, I can never see the TXE flag clear. Here is what I have in my IRQ Handler:

  if(USART_GetITStatus(EVAL_COM1, USART_IT_TXE) != RESET)
  {   
    /* Write one byte to the transmit data register */
    if (TxBuffer[TxCount] != '\0')
    {
      USART_SendData(EVAL_COM1, TxBuffer[TxCount++]);
    }
    //else{
    //  USART_ITConfig(EVAL_COM1, USART_IT_TXE, DISABLE);
    //}
  }

Unless I uncomment the else statement, the send interrupt keeps triggering endlessly even after I stop sending bytes to the TDR.
Is there a way to stop the interrupts without disabling them? Or do I need to re-enable every time I want to send something?

Best Answer

When USARTx_TDR register is empty USART_ISR_TXE flag is set to 1. When USARTx_CR1_TXEIE (TXE Interrupt Enable flag) is enabled and TXE=1 it will pend interrupt.

So you code sample with disabling interrupt after completed transmission is good solution.

When starting transmission periodically just write first byte (it will clear TXE) and enable interrupt.

Or if possible use DMA - a bit more code, but after you calculate strlen and configure it, it is rather fire and forget ;)

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