So, while I am waiting on parts to build my quadcopter, I am also working on building a 7400 series drum machine. That’s right, 7400 series drum machine. I wish I had designed this, but I am _very_ slowly building this: 7400 Drum Machine. Why the sudden interest in 7400 chips? My grandpa stopped by a couple days ago with some real goodies. An entire tackle-box PACKED with mostly 7400 chips. There are also some 4000 series and several LM566 VCOs, EPROMS, and various other chips. It turns out that I now have most of the chips needed to build that drum machine.
There are a couple snags, though. Matt (the creator) used a couple (nowadays) rare chips in the design. One of them is the 74C922 16-key encoder. Yes, you can still buy these from some places, but they go for about twenty bucks a pop. Ouch.
Since I’m not entering any contests, like Matt did, I figured I might cheat a little bit. The 74C922 is a pretty simple chip; all it does is scan a 4×4 switch matrix and output BCD – binary coded decimal. It stores the last key pressed and toggles a “data available” output when a valid entry is made. This sounds like the perfect task for an AVR with some custom code.
Thus the 74C922 Emulator was born. My goal is to create an *almost* drop in replacement using an ATTiny2313. It will not be perfect, because the ‘tiny is 20 pins and the 74C922 is 18, but one should easily be able to make an adapter to plug my chip in.
There will be some people who call this “dirty” or “uncool” because I reverted to a microcontroller to replace basic logic chips. Okay, get me a 74C922 for 5 bucks – shipped. It isn’t cheating when you have to use modern technology to replace things that are no longer available.
You can take a look at the code on github as I develop it. I just started before I typed up this post, so it is not even close to being done, but it is there anyway.