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pi 2 or 3, Gpio for controls, questions

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ivwshane:
I'll be making a two player bartop and I'll be using either a raspberry pi 2 or 3. I'll be using recalbox which I believe supports gpio. I think the total number of inputs the 2 or 3 supports is 26 which should be enough. Am I correct? Are there any disadvantages to using gpio inputs other than compatibility (input lag? Ghosting? Reliability)?

lilshawn:
AFAIK the GPIO inputs are all completely separate and the polling rate of the GPIO pins is dependant on the OS/programming language used to access them. (anywhere from 40hz to 20khz) i remember reading someone tested the GPIO switching rate with various programming languages and different methods of access, but I don't recall who or where. but even at 40hz, 0.025 of a second is probably unnoticeable.

depending on how many buttons you plan on using per player and the use of admin buttons, 26 may or may not be enough. (26 is enough for a joy and 6 buttons and start buttons for 2 players and a few admin) if you go franken panel you might need more or have to shift buttons with a button to double up some admin buttons.

TL;DR - Ghosting won't be an issue...I don't think lag will be much of an issue.

ivwshane:
Thanks.

I'm thinking of going with a six button layout with a start and a coin button and two admin buttons, esc and pause.

Dave Bullet:
Retrogame is a GPIO to virtual keyboard emulator.  I'm hacking it to support more than one "pin combination" per key mapping.  Of course you are still constrained to physical buttons / switches, but combinations can trigger other key presses. 

eg. think of using say the player 1 start button as a "shift" key combined with joystick up or down to increase/decrease volume, or player 1 start + player 2 start to end the game (MAME ESC key).

When my PI arrives, I'll let you know if it works if you are interested.

ivwshane:
I use recalbox and it allows for a "shift" key. Technically its called the hotkey. I wasn't aware of any limitations with regards to the hotkey and the gpio?

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