STM32L052 USART: Problem Switchover from Output- to UART - TX - functionality produces a disturbing H/L - edge on TX

[FEber.1]

Two Usart - TX - lines shall be used either in PP- output- or in Usart - TX - mode.

The switchover from a normal output pin to a Uart - TX - pin produces

an High/ Low edge on TX followed by an interrupt on TX side.

On RX- side this H/L edge causes an unwanted Interrupt too. And that is the actual problem.

The cause is the following sequence

u32ModeBack = GPIOA->MODER;

u32ModeBack &= ~(GPIO_MODER_MODE2_Msk | GPIO_MODER_MODE15_Msk);      

u32ModeBack |= (0x02ul<<GPIO_MODER_MODE2_Pos);                  //Alternate output function

GPIOA->MODER = u32ModeBack;

Is there a possibility to avoid this additional output of pulses?

[TDK]

So it goes from a high GPIO pin, then low immediately when initialized as AF, then high again without changing anything?

Is the USART peripheral initialized prior to switching it to AF? External pullup present?

[waclawek.jan]

> followed by an interrupt on TX side

What interrupt?

Post a minimal but complete compilable example exhibiting the problem.

JW

[FEber.1]

Hi TDK and Jan,

Meantime I know, that from my point of view I made the error to use the flag

USART_ISR_TXE instead of USART_ISR_TC for controlling of usart - tx - interrupt - routine.

The flag USART_ISR_TXE created my initial problem: 

An interrupt without writing anything.

OK, now I useUSART_ISR_TC. That wastes time but is fast enough for max.19200 Baud.

The correct solution would be a combined using of USART_ISR_TC and USART_ISR_TXE I guess.

For the elimination of problem two I have taken into account the question of TDK regarding of pull-ups.

Before switching to alternate function the internal TX - pull up is switched on and some instruction

later switched off again. Perhaps this cast can be removed when I am more familiar with STM32- USARTs.

Many thank for your help 

[FEber.1 · Accepted solution]

Update:

Now I have achieved a clean switch from output to Uart-Tx by changing the order of IO initialization:

//RX + TX Pull Up

u32ModeBack = GPIOA->PUPDR;

u32ModeBack &= ~(GPIO_PUPDR_PUPD2_Msk | GPIO_PUPDR_PUPD15_Msk);

u32ModeBack |= (GPIO_PUPDR_PUPD2_0 | GPIO_PUPDR_PUPD15_0);            

GPIOA->PUPDR = u32ModeBack;

//Alternate function = Usart 2

GPIOA->AFR[0] &= ~(GPIO_AFRL_AFSEL2_Msk);                        

GPIOA->AFR[0] |= (GPIO_AF4_USART2<<GPIO_AFRL_AFSEL2_Pos);            

GPIOA->AFR[1] &= ~(GPIO_AFRH_AFSEL15_Msk);                        

GPIOA->AFR[1] |= (GPIO_AF4_USART2<<GPIO_AFRH_AFSEL15_Pos);        

//Switch to alternate function

u32ModeBack = GPIOA->MODER;

u32ModeBack &= ~(GPIO_MODER_MODE2_Msk | GPIO_MODER_MODE15_Msk);        

u32ModeBack |= (GPIO_MODER_MODE2_1 | GPIO_MODER_MODE15_1);          

GPIOA->MODER = u32ModeBack;

Many thanks for your help