STM32MP135f-dk USART synchronous mode support under Linux – CK signal not driven

Icyjkk · ‎2026-02-05

Hello,

I am working with an STM32MP135f-dk device running Linux and trying to evaluate the possibility of using the USART peripheral in synchronous mode (TX/RX + CK).

From a hardware point of view, the STM32 USART clearly supports synchronous operation. However, when using the standard STM32 USART Linux kernel driver, the peripheral is only exposed as a /dev/ttySTM* device and operates strictly in asynchronous (UART) mode.

As far as I can see, the current driver does not enable or drive the CK clock signal, which makes true synchronous USART communication impossible under Linux, even though the underlying hardware is a USART.

My questions are:

1. Is this limitation expected by design in the current STM32 Linux USART driver?

2. Has anyone successfully implemented synchronous USART on STM32 under Linux, for example by: Extendimg the existing driver, writing a custom kernel driver, or using an alternative ?

mfgkw · ‎2026-02-05

Regarding 1.: yes

/dev/tty* seem to support only async mode, see https://wiki.st.com/stm32mpu/wiki/USART_internal_peripheral "4. Software frameworks and drivers":

Linux®: serial/tty framework; however, the Linux® kernel supports only the UART Asynchronous mode (Synchronous mode not supported)

So another driver will be required...