I am developing a product that uses an STM32F103. I spent the last two days debugging the USART1 port due to multiple issues with the controlling SBC and the ARM GNU stdio lib.

> that was documented in your errata sheet

This is a primarily user-driven forum, with casual ST presence, so you perhaps want to talk directly to ST, through the web support form, or through FAE.

> The only way I could get it to work was to change the RTS GPIO pin from an alternate function push-pull to an alternate function open drain output

This is very unlikely. Open drain versus push-pull is a functionality localized within the GPIO module and is not related to the connected peripheral. That being said, a weird design error can't be excluded of course, just it sounds very unlikely. As you are apparently using some "libraries", I wouldn't be surprised if the error would be in there.

Can you please, using debugger, read out the USART and related GPIO registers with the working project; then do nothing else just set the project back to the push-pull-AF state, check it is not working, read out those registers and compare/check/post?

JW

PS. Please change your username to a normal nick.

JW,

I'm an old school bit hacker and I program the peripherals directly using the reference manual. But you got me interested so I ran some more tests:

PA12 AF OD Works with the pull up resistor:

GPIOA_CRH : 400e44b2

PA12 AF PP Works with the pull up resistor:

GPIOA_CRH : 400a44b2

Then I disconnected the pull up resistor and it kept on working. So I am perplexed too, because like you said it makes no sense, but I still spent a lot of time with a logic analyzer trying to get the RTS line to go HI, and now it works. I dunno, maybe my prototype is quasi stable, because originally I was using push-pull and only tried OD out of desperation.

Thanks for your answer,

Phil

This parts are over decade​ old, the newer parts fixed a lot of the teething issues learned from the first generation of parts.

​

If you​ can't emulate functionality with GPIOs something wrong at a board level.

​

I'd start with the schematic not the software.​

First thing I do is check for shorted circuit traces and then I toggled the output as a GPIO because I don't trust my hand assembled prototype PCBs at all. It worked fine. I wasted many hours before I learned about the USART1 RTS / CAN defect, and when I figured that out, I thought I was on the home stretch, but nooooo. Serial is a PITA because it's not just the peripheral, you got the legacy Linux terminal driver and stdio to contend with too. So many things to get right...