Back in 2020, inspired by Umma95's build, I bought this Betafpv F405 V1.3 FC + ESC 2-4S 16A set. This week, I finally decided to build a drone. I know, I'm not fast lol
Betaflight identifies it as "AIRB/OMNIBUSF4SD(STM32F405), version: 0"

It seems like it has three UARTs:

But unfortunately, I couldn't find the RX6 pad on the board; there was only TX6. I really needed RX6 for GPS because I reserved UART1 and UART3 for CRSF and Vista.
In the resources, I found that RX6 is mapped to pin C07:
I looked up the CPU datasheet for the pin location and then found out that the pin seemingly wasn't connected to anything.
So, I soldered a thin wire to C07 and connected the GPS module's TX to it:

(I really don't need TX6 because I configured the GPS module manually, so I don't really need Betaflight to mess with it; I just want it to read the GNSS data stream.)

And it works well, even on 57600 bps! The CPU load didn't increase, so I guess it's not any kind of emulated serial UART.

But before I came to the solution described above, I had several failed attempts:

1) The most obvious idea was to free the resource for TX6 and use the already connected pin C06 as RX6:
   

It didn't work. Why?

2) I found out that pin B12 seemingly was intended for an SD card that I don't use, and it was connected to a convenient pad near the flash chip. So I freed B12 and mapped RX6 to it:


But this also didn't work. Why?

3) Then I read Oscar's "The Missing UART6" and tried all of the above with Serial UART: enabled it in the configuration, and tried to map SERIAL_RX 11 to C06 and then to B12. That also didn't work.

For each of the attempts above, I used a GPS configured to 9600 bps with a slow update rate of 500ms and set the speed manually in "Ports" to exclude any speed-related issues. And none of this helped. At the same time, the module worked well with UART1 or UART3.

So yeah, I figured it out, but it was a lot of work, and I'm still confused about how resource mapping works (to me it simply don't).

I hope the first part will help someone out there.

SERIAL_RX 11, "11" indicates soft serial or software serial. Not something you want to use for GPS. If you want to use softserial for UART6 TX and RX, I suggest you assign it B03 and B10 as Oscar indicated.

What you are trying to do is assign B12 as hardware UART. I believe with hardware UART, you also need to make sure there is a free TIMER and DMA assigned to B12. I am a little rusty, when it comes to this stuff now. Maybe someone else can correct me.

So I was correct, no TIMER assignment for B12

As for why C06 assigned to UART6 RX didn't work? C06 is an UART TX on the STM32405 MCU. So you cannot reassign it. In general, you can't reassign hardware UART to a different UART. But you can however free UART for other purposes.

voodoo614, thank you so much for your input! It made everything a bit clearer.

(10-Apr-2023, 04:27 AM)voodoo614 Wrote: SERIAL_RX 11, "11" indicates soft serial or software serial. Not something you want to use for GPS.

Hmm.. I understand that hardware uart is always better, but I thought it was Oscar who said that it's okay for GPS...
Yep, here, although maybe it's outdated.

Here's the hardware UARTs pinout for STM32F405 according to STM32 Cube IDE:

So yeah, it explains why I couldn't swap C6/C7 functions.

But it's still not clear to me why software serial didn't work with B12. Looks like it's a reassignable GPIO pin, and at a slow speed like 9600 bps, there shouldn't be any issues...

Softserial can be fickle. So Only certain pin works. Usually extra motor outputs, PPM and LED_STRIP are common pins that usually works for softserial. These pin have Timer assigned to them. Unfortunately B12 does not. Probably the reason it doesn't work.

Sounds like the hours I wasted trying to get T1 pad working on Succex-D AIO. Just wont work. Had to do similar - micro solder to the leg.