STM32F091 USART1 - Delay on first char to transmit after enabling transmitter (TE)

[JuM]

Trying to send over USART1 at 19200 bit/s.

USART is resting in receive mode (RE==1, TE==0).

After switching to transmit mode (TE:=1, RE:=0), we observe a ~500 us delay between storing ​and transmitting of first character. Subsequent chars are transmitted without delay.

Watch commented line // USART1->TDR = c; in first picture. No TDR, no Delays

[waclawek.jan]

JW

[JuM]

Thank you for clarification.

Seems idle frame is sent only if a character (the first character) is to be sent, not right after enabling transmitter (TE:=1). Nethertheless, we should not disable transmitter...