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STM32L476 Nucleo UART/HAL looses characters on TxD

Question asked by Juergen Marquardt on Oct 12, 2017
Latest reply on Oct 12, 2017 by Juergen Marquardt

trying to transmit data via USART1 of STM32L476 Nucleo evaluation board. Use PA9 for TxD.


Character count is 254*2+14 = 522  or   255*2+14 = 524.
522 always is ok, 524 gives errors at receiver site.


Data switches between 'all 0x00' and 'all 0xFF' every cycle.

Use HAL_UART_Transmit (...) for sending data.
Receiver always flags Error on 0xFF packets.


Took measurement on TxD with oscilloscope. That shows that with 'all 0xFF' always 8 characters are not sent!!! (s. attachement). 

What may be the cause of this?




By the way: Took a look into HAL_UART_Transmit () source code.
Found that writing TDR does not check TXE flag first (but that is not the cause of the 8 byte error).