Arduino for STM32

500 nsecs could be challenging, but nevertheless you have a H5 practically a newest chip, hence it is still worth trying.

for exti, I dug through the codes and found this:
https://github.com/stm32duino/Arduino_Core_STM32/blob/main/cores/arduino/WInterrupts.h
https://github.com/stm32duino/Arduino ...

show your codes that initialize and use Serial to provide a context.

and try

void setup() {
Serial.dtr(false);
Serial.begin();
}


this is assuming you are using
https://github.com/stm32duino/Arduino_Core_STM32/

if that still doesn't work try another terminal app e.g. putty
https://www ...

EXTI (external interrupts) ISR normally can't run too fast, usually has some limits say like (well) below 1 mhz. code latencies etc.


I didn't see anything in the documentation that led me to believe that an ISR from an external interrupt would be much slower than a peripheral interrupt, can ...

oh about those interrupts and hardware etc stuff
note that STM32 has verbose very detailed well written reference manuals
https://www.st.com/resource/en/reference_manual/rm0481-stm32h52333xx-stm32h56263xx-and-stm32h573xx-armbased-32bit-mcus-stmicroelectronics.pdf
as well as review the specs ...

if what is needed is a keyboard, there are some alternatives e.g. ps/2 keyboard
https://en.wikipedia.org/wiki/PS/2_port
https://www.aliexpress.com/w/wholesale-ps2-keyboard.html
https://www.burtonsys.com/ps2_chapweske.htm
https://oe7twj.at/images/6/6a/PS2_Keyboard.pdf
https://github.com ...

stm32h5 series is fairly new
the STM 'official' core (implementation) is one that supports them
https://github.com/stm32duino/Arduino_Core_STM32?tab=readme-ov-file#generic-stm32h5-boards
https://github.com/stm32duino/Arduino_Core_STM32/wiki
https://github.com/stm32duino/Arduino_Core_STM32/wiki ...

USB enumeration is triggered by a *usb reset*
which is a single-ended-zero (i.e. both d- d+ low ) > 2.5 uS
https://www.engineersgarage.com/signal-and-encoding-of-usb-system-part-5-6/

many times when things don't work, it could be a defective usb cable (more common)

to proof if it is after all ...

I've not tried it out but
it may be 'easier' with the library
https://github.com/Arduino-IRremote/Arduino-IRremote
the ir receivers:
https://github.com/Arduino-IRremote/Arduino-IRremote?tab=readme-ov-file#irreceiver-pinouts
for transmit, I'd guess a regular ir led would do
if you are using the ...

Provide the board details, stm32 chip used etc.
there are a large number of stm32 soc all with different features, capabilities.
board details is also important, if it is well known board, a board on the list
https://github.com/stm32duino/Arduino_Core_STM32
mention it, otherwise provide more details ...

fpiSTM is correct

it is a bit surprising that they put a usb-uart (ch340n) at pa9 / pa10 which is in turn connected to usb, as normally stm32f103 has its own usb.
stm32f103mini_sch.jpg
in this case, you'd need to use the normal uart option for Serial in your sketch

and it use buttons instead ...