How do I get the USART to work on an STM32C011?

[Neiliow]

I am relatively new to using the STM32 family and I am trying to use the USART on a STM32C011J4M6 device without success.

Initially I am just trying to send out a single character using the lines

 uint8_t TxData[] = {'@'};
 HAL_UART_Transmit(&huart1, TxData, 1, 10);

 after USART1 has been set up by CubeMX.

When it runs however, as soon as it tries to write to the Transmit Data Register, the processor resets. I have verified where it is going wrong by stepping through the disassembly - all the registers seem to be set up correctly but immediately on executing the STR instruction to write the character to 0x40013828 (USART1 TDR) it jumps to the reset handler.

I have been using STM32F042K6 and other devices up until now and the same code behaves correctly in those, exactly as expected.

I'm not sure what I am doing wrong so can anyone point to what it might be? If I remove the HAL_UART_Transmit() then the rest of the code runs (although it is just toggling a GPIO pin at the moment).

Thanks for any advice.

[Foued_KH]

Hello @Neiliow​ ,

uint8_t TxData[] = "@";
 HAL_UART_Transmit(&huart1, TxData, 1, 10);

Foued

[Neiliow]

Thanks Foued but that's not where the problem is. The '@' character is getting to the correct place and it fails when it is trying to write that into the Transmit Register. I can trace the disassembled program right up to that point so it is getting the character to send out. The problem happens when I write to TDR, it doesn't matter what data I try to write to it - '@' or anything else.

Neil

[Tesla DeLorean]

The processor shouldn't reset.

Double check board, make sure the UART TX pin isn't connected to anything that would reset the system, or cause a short or brown-out type event.​

​

C​ode lacks any broader context.

​

I​s this a chip with low pin count or pins bonded to multiple IO outputs or function?

[Neiliow]

The Tx pin is just connected to an oscilloscope and the chip is in a breadboard with the only connections being its power rails, an LED on a GPIO pin and the ST-Link on the SWD pins.

There isn't much code at the moment other than what was generated by Cube MX. The chip is an 8-pin device so the pins have multiple functions but only USART1 and SWD are enabled.

 

However you have got me wondering now - the TxD pin is also shared with nRST so maybe that is what's happening. When TxD goes low it is seeing that as a reset. I'll have a look into it. Thanks for the hint.

[Tesla DeLorean]

Yes, super low pin count device. Watch what else is bonded to pin.

​

Attach minimal / buildable project demonstrating issue as a ZIP file.​

[Neiliow]

OK, here you go...

This will demonstrate the problem, resetting as soon as it tries to send data on USART1.

I can't find anything about how to forcibly disable the reset function from that pin. I assumed it would be done if the pin was assigned as TxD for the USART.