Electronic – Clearing USART (UART) interrupt flags in an STM32


I am using an STM32F105 to communicate with a Linx GPS chip using a UART.

If I don't use interrupts (if I just poll the RX flag) then it works just fine. But I'm getting unexpected results when trying to use interrupts.

For example, if I only enable the RXNE ("RX not empty") interrupt using USART_ITConfig(USARTx, USART_IT_RXNE), then the code should only vector to the ISR for this one specific event. But the interrupt is being triggered for an Overrun condition, too.

As far as clearing the flags, it seems that the method depends on the flag.
To clear the Overrun flag (USART_IT_ORE), the User Manual explains that I should first read the USARTx_SR register, then read the USARTx_DR register. This does work; the flag is cleared.

There is also a USART_ClearITPendingBit() function, but it only accepts a small subset of flags.

There are eight different interrupt sources that can be selectively enabled, and ten different flags. Is there a summary somewhere of how to manage all of these flags?

Best Answer

Generally, you only need to handle the interrupt flags which you have specifically enabled with USART_ITConfig().

However, if you enable the RXNE interrupt (USART_ITConfig(USARTx, USART_IT_RXNE)) then this also enables the Overrun interrupt! So you must handle both of those.


The USART flags can be confusing. There are separate status flags and interrupt flags and they share similar names. For example: USART_IT_RXNE and USART_FLAG_RXNE.

In addition, there are various methods to clear these flags. For example, the USART_ClearITPendingBit() function only works for four (of the ten) possible flags.

Here is a summary of the interrupt flags and how to use them. These are specific for the STM32F105, but are representative:

USART_IT_TXE - "Transmit Data register empty"

  • It is cleared automatically when calling USART_SendData()

USART_IT_RXNE - "Receive Data register not empty"

  • It is cleared automatically when calling USART_ReceiveData(USARTx)

  • It can be cleared manually by calling USART_ClearITPendingBit(USARTx, USART_IT_RXNE)

USART_IT_TC - "Transmission complete"

  • It is cleared automatically by:

    • USART_GetITStatus(USARTx, USART_IT_TC) followed by
    • USART_SendData()
  • It can be also be cleared manually by calling USART_ClearITPendingBit(USARTx, USART_IT_TC)

USART_IT_CTS - "CTS change"

  • Cleared by calling USART_ClearITPendingBit(USARTx, USART_IT_CTS)

USART_IT_LBD - "LIN Break detected"

  • Cleared by calling USART_ClearITPendingBit(USARTx, USART_IT_LBD)

USART_IT_PE - "Parity error"

  • Cleared by:
    • USART_GetITStatus(USARTx, USART_IT_PE) followed by
    • USART_ReceiveData(USARTx)

USART_IT_NE - "Noise error"

  • Cleared by:
    • USART_GetITStatus(USARTx, USART_IT_NE) followed by
    • USART_ReceiveData(USARTx)

USART_IT_ORE - "Overrun error"

  • Cleared by:
    • USART_GetITStatus(USARTx, USART_IT_ORE) followed by
    • USART_ReceiveData(USARTx)()

USART_IT_IDLE - "Idle line detected"

  • Cleared by:
    • USART_GetITStatus(USARTx, USART_IT_IDLE) followed by
    • USART_ReceiveData(USARTx)()