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?

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)()
• Yep, I answered my own question :) If I had found a list like this earlier, it would have saved me a good amount of time. I hope it helps someone! Mar 14 '16 at 22:17
• This answer is gold; very clear reference of how to clear each of these interrupts. Sep 22 '16 at 23:21

Just want to add some my experience on this problem, I follow the instructions:

USART_IT_ORE - "Overrun error"

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

Is seems not work, and the following command work for me instead:

If you look into the functions:

You will find what exactly Bitsmack wrote before... "First read the USARTx_SR register, then read the USARTx_DR register."

Hopefully it work for you and save lot more time on this issue.=)

• Thanks for your information! I don't recognize USART_GetFlat(). Can you tell me where it comes from? Oct 12 '18 at 17:05
• I am sorry that I wrongly typed on the command,, it should be USART_GetFlagStatus() followed by USART_ReceiveData(), the original post was corrected. Thanks Bitsmack. Oct 18 '18 at 1:59
• Excellent, thank you! It looks like the function names have changed. Are you using a Standard Peripheral Library or one of the STMCube HALs? For which family of microcontrollers? Oct 18 '18 at 7:27
• Thanks for the good answer. BUT: I receive one character on STM32L476, no further bytes are received (no signal on oszi), but permanent receive a FE (framing error) interrupt. Cannot even clear this interrupt. Any ideas what's the reason? May 1 '21 at 19:21

reading status and data registers clears overrun error.

uint32_t cnt_rx2_overrun;
void USART2_IRQHandler(void)
{
uint32_t status = USART2->SR;
uint8_t ch;
if (status & USART_SR_RXNE) { // rx data
ch = USART2->DR;
// parse data
}
else if (status & USART_SR_ORE){ // overrun error
ch = USART2->DR; // reading clears the ORE flag
cnt_rx2_overrun++;
}

}
$$$$
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