Add optional PCB connection between GPIO0 and GPIO1 to facilitate use with Dr Gusman's Logic Analyzer

[nitro2k01]

This feature request requires some background. Ever since I got my GB Interceptor I've had the idea that it could also be useful as a logic analyzer for the GB CPU bus. The other day I found Dr Gusman's Logic Analyzer which is a Raspberry Pi Pico logic analyzer project. I tried it with the GB Interceptor and it worked... almost. Two things were wrong.

1. Dr Gusman's firmware only supported 24 channels, whereas there are 28 useful channels of information. I sent a feature request and I'm now working with him now to get this supported, so that should be finished soon.
2. His project is using two separate PIO state machines to capture the data and generate trigger data for complex trigger setups. The PIOs can't communicate directly, so he's using a hack where two digital IOs are connected on the board to pass the signal out and then back in again. (This is also used for chaining devices in his setup.) So to make full use of his firmware, you would need to connect those two pins together.

So, looking at how GPIO0 and GPIO1 are used in the GB Interceptor to see if they could be safely connected to each other, and driven low/high for triggering:
GPIO0: Used as power sense for the GB. Connected through a voltage divider using 2.2k and 3.3k. Driving the middle of the divider to 0 V would produce a 1.5 mA current from the GB's Vcc. Driving this to 3.3 V would produce some similarly small current. This doesn't hurt either the IO pin or GB.
GPIO1: Used for an LED from Vcc, and a button to ground. Driving this to 0 V is the normal state and lights up the LED. No issue. Driving it to 3.3 V would turn off the LED, but there's one hazard here. Pressing the button in this state would short 3.3 V to ground and potentially damage the GPIO pin. A logic high is output by GPIO0 to signal a complex trigger event, so pressing the button while such an event happens would be a bad idea. But you could also... just not do that.

Connecting GPIO0 and GPIO1 together would also obviously interfere with the original function of the pins, but would be 100% safe (as in permanent component damage) when the pins are configured by the firmware as a GB Interceptor, and "95% safe" when configured as Dr Gusman's Logic Analyzer.

I've HW modded my Interceptor, and this is what I came up with, see image. I confirmed that the mod works with the logic analyzer firmware. I cut out pads in the ground place for a pin header strip used for choosing the mode with a jumper. So, what I'm requesting in the end for a future board version is a similar set of pads for connecting GPIO0 and GPIO1 using something like pin header like shown here, or maybe a small sliding switch. Just anything to make the mode a little easier if you would choose to make it. 99% of people would have no use for it. That's ok, and in this case this would just be two unpopulated pads on the board, that don't do anything.

I'll also mention in passing that while looking at the schematic, I noticed that the +5V trace is the same minimum size as the signal traces. I would personally have beefed that up. But that's a different issue.

[Staacks]

Thanks, that is very cool. In fact I might try that at some point myself.

However, I do not see making a hardware revision anytime soon. I am struggling a bit to find time to update several of my projects at the moment, including a few things for the interceptor (nothing big, though, mostly housekeeping).

A big problem with new PCB revisions is that I am not experienced enough (see 5V lanes :) ) to trust them without testing. So, I am also hesitant of blindly merging changes, either for these small optimizations or for replacing unavailable components. But I will happily link or include variants of the tested design.

I would say, for now I will leave this issue open to either remind me of it if I ever do a new hardware version (could be when ordering a new batch, but still got a stock of PCBs) or until someone shares an adapted version, which I would put on the wiki and readme.