Gameboy Development Forum

Hello Game Boy developers!

I have joypad input working, but I was wondering what exactly the required timing for reading the joypad register is.
I've looked at several sources and they all say that after writing to memory address $FF00, one must wait a little before the data stabilizes.
From what I've gathered, most games implement the little delay by reading the register multiple times consecutively.
My question is, how big does that little delay even have to be?


Looking at the dissasembly, it seems as if the Tetris ROM I got does 4 reads for the buttons, followed by 10 reads for the D-pad.
With the reads using the ldh a, [$FF00] instruction, each read takes 3 cycles.
This puts it at 12 cycles to read the first set of four inputs, and 30 cycles to read the second set of four inputs.

I also looked at a ROM of Pokemon Red, which used 6 read instructions for the buttons, then 10 read instructions for the D-pad.
This means that it takes 18 cycles for the first one, and 30 cycles for the second one.

The BGB emulator seems to work fine even with a single nop of delay, followed by ld a, [$FF00 + c] to load the value from memory.
Together, this makes a working read take 3 cycles.


So, my questions are:
How short of a delay can I get away with?
How much should I use to be safe if I decide to run my game on actual hardware?
Has anyone tested this on actual hardware?

I figure I can just copy what others do, but I find it odd that I have not yet stumbled upon any documentation mentioning actual values.


I'd like to give big thanks to the community. The tools, docs, and forum threads have been very helpful.

- Borna