why is there a delay between (DR register written) and (data really showed) in UART on STM32F103CB?

[Olly Kao]

I'm curious about the delay time between the title mentioned, I toggled an IO when I wrote data into UART->DR, the delay time varies from 3 micro seconds to 10x micro seconds

int main(void)
{
/* initial code generated by STMCubeMX */
  HAL_Init();
  SystemClock_Config();
  MX_GPIO_Init();
  MX_USART1_UART_Init();
 
  while (1)
  {
    HAL_Delay(50);
 
    if (USART_GetFlagStatus(&huart1, USART_SR_TXE) == SET)
    {
      USART_SendData(&huart1, 'F');
    }
  }
}
 
 
void USART_SendData(UART_HandleTypeDef *huart, uint16_t Data)
{
  assert_param(IS_USART_ALL_PERIPH(USARTx));
 
  assert_param(IS_USART_DATA(Data));
 
  GPIOB->BSRR = GPIO_PIN_1;                                   // Tick an IO pin for debugging
  GPIOB->BSRR = (uint32_t)GPIO_PIN_1 << 16u;     // reset pin
 
  huart->Instance->DR = (uint8_t)(Data & (uint8_t)0x00FF);        // send data (write DR)
}

I'm not sure whether the time jitters is related with BAUD rate 9600(104 micro seconds/bit),

Isn't the data should be showed immediately when DR register written????

And why isn't the delay time all the same(or close)?

[Nikita91]

USART transmission involve 2 registers; TDR and a hiden shift register.

When you write your first data to DR,

• The DR content is immediately transferred to the shift register
• The data begins to shift out of the TX pin
• The DR register becomes free and generate TXE

So the delay you measure is very small.

When you write your second data to DR the shift register is already full, and the DR register will be empty (and generete TXE) only at the end of the 1st data transfer.

So the delay you measure is near the transmission delay of 1 data.

Try to set the HAL_delay to more than 104 us.

You can think of the DR register as a 1-data FIFO!

[Olly Kao]

@Nikita91​ Thanks for reply

HAL_delay(50) delays 50 millisecond

Do you know the time and process of the data begins to shift out of the TX pin???

[Nikita91]

Sorry for the 50ms!

I don't know the delay.

The only thing I get from the RM is "An idle frame will be sent after the TE bit is enabled" (page 792/794 of RM008 rev 17).

So I understand that the delay of the first data may be different from the others.

I do not see anything else.

[Nikita91 · Accepted solution]

I think the shift register is clocked by the baud rate generator. So the transfer from the DR to the shift register is clocked by the baud rate generartor not the bus clock.

Hence the delay from 0 to 1 bit time.