STM32F030C8 UART1 RS485 not working above 19200 baud rate

[Harshad31]

Hi everyone,

I'm working on a project using the STM32F030C8 microcontroller. I'm using USART1 for RS485 communication (both transmit and receive), and I’m facing a strange issue with the baud rate.

• At 19200 baud and below, everything works fine.

• When I try baud rates above 19200 (e.g., 38400, 57600, 115200), communication stops working—either I don't receive data correctly or nothing is transmitted.

• I'm using RS485 transceiver hardware, and the direction control is handled properly (I’ve verified the DE pin timing).

• On the other hand, USART2 works perfectly at all baud rates, including 115200.

Things I've tried:

• Checked system clock settings.

• Verified USART1 initialization code.

• Ensured the RS485 driver enable (DE) pin timing is correct.

• Tried different cables and RS485 adapters.

• Measured signal lines with an oscilloscope—signal seems distorted at higher baud on USART1.

My questions:

1. Is there any known issue with USART1 on STM32F030C8 at higher baud rates with RS485?

2. Could it be hardware limitations, alternate function pin mapping, or GPIO speed settings?

3. Any suggestions for debugging or workarounds?

Thanks in advance!

[Andrew Neil]

Welcome to the forum.

You need to give some more details - please see: How to write your question to maximize your chances to find a solution.

You haven't said what board you're using.

The schematic you've posted only shows the RS485 part - please also show the STM32 part(s).

Has your RS485 hardware been separately tested, and found to work at >19k2 ?

Before adding the RS485 stuff, have you checked that your STM32 correctly sends & receives at >19k2 ?

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@Harshad31 wrote:

• When I try baud rates above 19200 (e.g., 38400, 57600, 115200), communication stops working—either I don't receive data correctly or nothing is transmitted.

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So which is it: 

• Does your hardware transmit anything at all ?
• Does it receive anything at all ?

Have you used an oscilloscope to see what's actually happening on the wires?

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@Harshad31 wrote:

• On the other hand, USART2 works perfectly at all baud rates, including 115200.

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Is that through the same RS485 hardware?

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@Harshad31 wrote:

• Measured signal lines with an oscilloscope—signal seems distorted at higher baud on USART1.

---

Where, exactly, do you see that?

What investigations have you done into that?

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@Harshad31 wrote:

2. Could it be hardware limitations, alternate function pin mapping, or GPIO speed settings?

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Could be many things - you need to investigate systematically.

Post a minimal but complete example which demonstrates the issue - see: How to insert source code.

Some general debugging tips:

https://community.st.com/t5/community-guidelines/how-to-write-your-question-to-maximize-your-chances-to-find-a/tac-p/706966/highlight/true#M49 

[Harshad31]

Thanks for the response.

I'm using an STM32F030C8 microcontroller. UART communication works fine up to 19200 baud, but when I try higher baud rates (like 38400 or above), it doesn’t work reliably — I'm seeing communication issues or no response at all.

The schematic I shared earlier only includes the RS485 transceiver. I’ll update and share the full schematic showing how the STM32 is connected as well.

Regarding the RS485 hardware, I haven’t tested it independently at higher baud rates yet. That’s something I’ll check next to rule out any limitations with the transceiver or wiring.

Let me know if there’s anything specific I should look into on the STM32 side — maybe clock settings, UART timing, or RS485 control pin timing?

[Andrew Neil]

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@Harshad31 wrote:

Let me know if there’s anything specific I should look into on the STM32 side

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First, check the UART signals directly at the STM32 pins, with extra hardware disconnected.

If this is fine, then the problem is not in the STM32 ...