STM32H563ZIT6 cannot receive RS485 at high baud rates (e.g. 12 Mbaud)

[MES98]

TX works but RX interrupt not triggered

Hello,

I am currently using a custom board based on STM32H563ZIT6. My goal is to establish communication over RS485 between the STM32 and a PC (using Hterm). The system is designed to receive a command and send a response.

Communication Details:

• Interface: RS485 (half-duplex)
• PC Tool: Hterm
• Converter: USB COM 485 PLUS 4
• Packet size: Very small (typically 4 bytes)
• Transmission interval: ≥ 300 ms

Current Situation:

• Communication works perfectly between 115200 and 2,000,000 baud
• I am trying to increase the baud rate up to 12 Mbaud

Problem:

At 12 Mbaud:

• STM32 → PC communication works correctly
  (I can see transmitted data on Hterm)
• PC → STM32 communication does NOT work
  • STM32 does not receive any data
  • RX interrupt is not triggered at all
  • It behaves as if no data is arriving

Additional Notes:

• I have checked the termination resistor, and it seems correct
• The data sent from PC is valid (verified from Hterm)
• UART baud rate is configured up to 12.5 Mbaud in CubeMX
• The issue only appears at very high baud rates

What I Suspect:

• Possible limitation of RS485 converter at high baud rates
• UART sampling/timing issues at 12 Mbaud
• Interrupt-based reception may not be sufficient at such speeds
• Hardware limitations (signal integrity, cable, etc.)

Questions:

1. Is it realistic to achieve 12 Mbaud over RS485 in this setup?
2. Could this be a limitation of the USB-RS485 converter?
3. Are there any specific STM32 UART settings required for such high speeds?
4. Should DMA be used instead of interrupt for reliable reception?
5. Are there known limitations for STM32H5 series UART at these speeds?

Any insights or similar experiences would be very helpful.

Thanks in advance!

[unsigned_char_array]

And it's why fractional baudrate exists. If a desired baudrate is close to 2 possible baudrates the peripheral can periodically switch between those baudrates per bit so the sample point doesn't drift too much. It's the same principle we use for leap years. 

[David Littell]

Maybe I missed it in this (long) thread but I don't think you've ever fully explained why you're fixated on such a high baud rate.  I think I remember you saying your packets are 4 bytes or so - what is the actual packet rate?  And again, what is driving the "need" for such a high baud rate?