Interpretation of TXEIE bit (UART programming)

[sidney]

Posted on July 04, 2013 at 13:33

Hi all,

The documentation of the ''GetITStatus'' function, as found in the file 'stm32f10x_uasrt.c'', describes the functionality as follows:

    ''Checks whether the specified USART interrupt has occurred or not.''

However, when I follow what happens why I call GetITStatus(UART1, USART_IT_TXE), it appears that it simply returns bit 7 of CR1 (TXEIE) which, according to the STM32 reference manual (RM0008), is the ''TXE interrupt enable''.

It seems these are two different things; the GetITStatus() documentation claims that the bit returns if the interrupt *has happened*, while the reference manual gives the meaning as: the interrupt is enabled (i.e., it *can happen*).

Can anyone explain which of these two interpretations is the correct one?

#uart

[zzdz2]

Posted on July 04, 2013 at 14:11

If I follow it correctly it returns SET when two bits are set TXE in SR and TXEIE in CR1.

It seems the only way to check if the interrupt has occurred.

[Tesla DeLorean]

Posted on July 04, 2013 at 14:39

USART_GetITStatus() gets the flagging from USARTx->SR, and masks that with if the bits are actually enabled.

[sidney]

Posted on July 04, 2013 at 16:03

> If I follow it correctly it returns SET when two bits are set TXE in SR and TXEIE in CR1.

Yes, on second inspection that appears to be what it does: anding the appropriate bits from the SR and the appropriate CR{1|2|3}.

This means that the documented behavior (''Checks whether the specified USART interrupt has occurred or not'') is not quite correct.

For example, writing to DR just before executing ''USART_GetITStatus()''  will make it return false, even if we're actually handling the TXE interrupt.