Configured USART1 of mcu to 500 kbit/sec using STM32CubeMX Version 4.21.0.
Code created by Cube seems to be correct.
Measurement shows that bitrate is 250 kbit ( a 10 bit character needs 40 µsec).
USART1 clock is 80 MHz. Comes from 16 MHz HSI RC, via PLL (x10, /2) PLLCLK to SYSCLK.
Do not know where the error is...
Best regards Juergen