20mhz GPIO signal: possible on Raspberry Pi models?

Question

I am wondering whether any model of the Raspberry Pi can reliably output an 8-bit 20mhz signal into 8 GPIO pins at the same time. Basically I am trying to read 8-bits from a file at a time out to 8 pins of a raspberry pi, at 20mhz (my purpose is to feed those 8 bits into a current-output DAC).

I am not sure which model of Raspberry Pi would be best for this. I'd also love to know if there are some C/C++ code examples that do this already.

Any insight or advice on this would be greatly appreciated — thanks!

Answer 1

No.

The benchmarks you see showing impressive Pi GPIO speeds in the multi megahertz region are for simply toggling a single pin. I.e. a loop writing 0 then 1. Once you add logic in that loop the speed drops. Once you try to time the loop the speed will drop by several orders of magnitude.

I suggest you do your own experiments reading from a file and writing to GPIO.

If you don't mind timing jitter you might reach something like 20 thousand Hz in Python (not the 20 million Hz you want).

Answer 2

I seriously doubt that you could output data at that speed; the fastest code I have used can (just) exceed 20Mbps on a Pi4 (just toggling a pin). (Outputting multiple bits with custom code is just as fast or faster than most single bit writes.)

Maybe with DMA BUT you couldn't even read data from a file at that speed.