RTEMS | arm/stm32f4: extend USART DR field to 9 bits (!1141)

Mon Mar 16 15:49:43 UTC 2026

Mohamed Ayman created a merge request: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1141

Project:Branches: mohamedayman23/rtems:extend-USART-DR-field-to-9-bits to rtems/rtos/rtems:main
Author:   Mohamed Ayman

## Summary

arm/stm32f4: extend USART DR field to 9 bits

The STM32F4 USART data register supports up to 9-bit data when CR1.M is set. The current DR macros mask only bits \[7:0\], which truncates the MSB in 9-bit mode. Extend the field width to \[8:0\] to match the hardware definition while remaining compatible with 8-bit mode.

## ![Screenshot from 2026-03-16 17-45-26.png](/uploads/ccb5185d560db1c35031bfb7e47d93fc/Screenshot_from_2026-03-16_17-45-26.png){width=812 height=600}

AI usage: yes\
Prompt used: check the registers by the USART register map which i include\
AI model: GPT-5 mini\
How AI was used: just for checking if this registers a valid to the this family reference manual or there is copy-paste issue

-- 
View it on GitLab: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1141
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