Ghosting

Published on 2017-05-01 in PewPew FeatherWing.

When I realized that the IS31FL3733 chip is not going to handle the button scanning well enough for this use case, I decided to design a new prototype board, with the buttons connected directly to the GPIO pins of the Feather.

However, I want this to work with all Feather boards, including the ESP8266-based Feather HUZZAH, which means that I only have a few GPIO pins to work with. A quick count gave me 7 pins: GPIO0, GPIO2, GPIO12, GPIO13, GPIO14, GPIO15 and GPIO16. However, there are some caveats. The GPIO15 pin has to be pulled low at boot, otherwise the ESP8266 will boot into some weird mode. The same with GPIO0 and GPIO2 being pulled high. And finally the GPIO16 is a bit special and doesn’t have an internal pull-up.

I have 6 buttons, so with the naive approach I will need 6 of the GPIO pins. I made a quick go at the PCB for that, together with six external pull-down resistors. That turned out to be much more complex than anticipated. Also, I really can’t leave those pull-down resistors on GPIO2 or GPIO0, so I’m one pin short.

I considered my options. I could ditch the resistors (which would simplify the design a lot) and use internal pull-ups (which would also solve the problem of the pin state at boot). Hmmm… a quick redesign, and it already looks much better. But I can’t use GPIO16 (no pull-up), and the GPIO2 has a LED on it, which lights when you press the button. It would be nice to at least leave that one free…

Fortunately there other ways of connecting buttons. I can make a key matrix and scan it. Let’s see, I have 6 buttons, so the best matrix I can make is 3x2, requiring a total of 5 pins. Not much, but progress. I quickly made another PCB design using that, leaving the GPIO16 and GPIO2 pins free. Perfect! At this point I was pretty late, so I saved the design, generated the gerber files and ordered the prototype PCB at OSHPark. And went to bed.

And then, lying in the bed, I realized that my design has a fatal flaw: ghosting. It’s a game console, and people are not going to just press one button at a time like the cultured citizens they are. They are going to be mashing all the buttons at once, trying to beat the game. And the simple key matrices, the kind that I used in this design, have a problem when several keys are pressed at once: the keys connect together several rows and columns of the matrix, and in effect the microcontroller thinks that more buttons are pressed than actually are. A simple solution for that problem is to add diodes to every button, so that it won’t connect the columns and rows in both ways – this way preventing the columns that are not currently selected by the scanning algorithm from getting power. However, at this point the PCB was ordered and there was simply no room to add 6 additional diodes to it anyways.

For now I decided to just test the prototype when it arrives and work on the software and the games a little bit, ignoring the ghosting problem, and later I will look for a separate I²C chip that will handle the keys for me.