STM32H745 input fast read

[EsKi]

Hi, I have problem. I have idea: my STM32H745 must read GPIO state as quickly as possible. FCPU = ~400MHz

I wrote the program in assembler for M7

LED_ON()

ldr r2, [r3, #IDR]

ldr r2, [r3, #IDR]

... ~650x

LED_OFF()

...

only read, IDR, no write (no record yet)

And when I measure with an oscilloscope the led output is on for 25us -> one LDR 15 cycles clock CPU. When I read from RAM it takes about 12us, but GPIO 25us.

This is too slow for me. I tried all the tricks and combinations of LDR; STR ... LDR LDR STR STR ... LDR AND LDR STR AND ... Minimum is 25us.

LDR gpio and STR ram are without comment at all. Some astronomical values ​​come out that can be obtained from AVR or DSPIC.

I tried FMC to read and it was even worse.

What is the fastest way to read the state of a ~40MHz pin?

[gbm]

Two possibilities:

- For single pin - use SPI MISO line and SPI RX DMA. This guarantees fixed sampling frequency.

- For multiple pins - use timer-triggered DMA from port to memory. Some frequency variations and jitter will appear.