Arduino for STM32

It has NOT been answered there, it has been avoided there. The first answer given was clearly incorrect, and I provided a link to the ST website using your own documentation that proved it was incorrect. For some reason, that evidence seems to have disappeared and there is a second incorrect answer ...

"but there are so many timing sensitive things I need to do in this code that I ended up with more and more in the interrupts"

Just set more flags in the ISR. Put the body of the code in the main loop. If using this strategy, you have "too much to do" or "run out of time" then the simple truth is ...

You should never try to do "real work" inside an ISR. You should do the absolute minimum possible such as set a global flag (usually marked 'volatile') then get out immediately. You should certainly never try to call anything else that uses or requires interrupts. If you do then, well...you already ...

I'm interfacing an SPI SD reader, since the SDIO examples won't compile. The good news is that I have TWO connected successfully on SPI1 and SPI3. The bad news is that I was only able to do that after I realised that the SPI pin defnitions in ...Arduino15\packages\STM32\hardware\stm32\1.9.0\variants ...

Now down to one error:

"C:\\Users\\phil\\AppData\\Local\\Temp\\arduino_build_551967\\core\\core.a" -lc -Wl,--end-group -lm -lgcc -lstdc++
c:/users/phil/appdata/local/arduino15/packages/stm32/tools/xpack-arm-none-eabi-gcc/9.2.1-1.1/bin/../lib/gcc/arm-none-eabi/9.2.1/../../../../arm-none-eabi/bin/ld ...

Excellent - thank you.

But this is kind of my point - why isn't this in the documentation for STM32SD ?

Getting started trying to use FatFS and/or STM32SD. I have to say I find the lack of documentation and working examples very difficult. I see in another thread that someone has the same compile errors and this was because his board did not have SDIO.

I have a NUCLEOF429ZI which I *know* has SDMMC ...

Asynchronous TCP and solid full-featured MQTT (QoS0/1/2 + large payloads) are now available for STM32-NUCLEO boards using ArduinoIDE. Using the STM32Duino LwIP and STM32Ethernet libraries, the popular ESP8266 library ESPAsyncTCP has been ported to STM32 as STM32AsyncTCP.

While this allows any STM32 ...

I agree!

Yes, I suspect is is something in those other libraries - the sketch only start every fourth of fifth attempt on reset. And if Serial has a 64-byte buffer which it can't clear because interrupts are disabled, then I guess we'd be seeing this behaviour

FWIW: here's the whole sketch:

#include ...