7 years, 2 months ago.

You're probably not going to get a 20MHz pulse via interrupt. However, use TIM1 or TIM8 in capture mode with APB1 at 108MHz and you should get a reasonable measure especially if you can use averaging. A single snapshot will have a slight variance each reading due to the 5:1 ratio of APB clock to external pulse (20MHz). ST has app notes on how to measure an external pulse. As well there have been several projects submitted by users showing how to do this with ST. You'll need to look at the HAL setup for capture timer.

You could always experiment with using interrupts but start your pulse rate at something <1MHz and validate the code. Then turn up the pulse speed slowly until you can't keep up anymore - then you'll have found the limit of external ints for measurement.