Iguanas and Electronics

16-bit Counter Prototype

The original 8-bit computer design used the 74LS561 counter for the program counter. The 561 is definitely capable, but it is also a bit complex with synchronous and asynchronous operation in a 20-pin package. I tried using a few counters, but the ones I settled on are the 74LS193. This little 16-pin IC allows for loading, resetting to zeros, increment, and decrement. Most of the counter I tried are increment only, including the 561, so I ordered twently 193s and started prototyping when they arrived.

As you can see from the video (or the thumbnail, I won't judge), the 16-bit 193-based counter is working. I tested with load, clear, increment, and decrement successfully. This was with a 10Hz clock provided by the development board (middle, right, with all of the unconnected red wires), and I used one of the 4-digit hex displays for output.

The breadboard wiring looks complicated, but it worked almost perfectly from the start. A problem I did have was with the first 193 not counting. I eventually replaced it to the same effect and finally removed it. This necessitated adding a new forth IC to the left and moving the display wires, but the result was a successful prototype.


TTL 74LS193 up/down counter pinout

This IC is pretty simple, with a decent pin out. My preference would be for the data pins to be together and in order, but that is seldom the case with TTL components. The main things to look at with this IC are the count up/down pins, and the borrow/carry pins, as these are used to chain the ICs together to create a bigger counter... four of them, for a 16-bit version.

The increment and decrement signals are connected to the count up/down pins of the first (lowest 4-bits) counter IC. The carry from that counter is connected to the count up pin of the next IC, while the borrow is connected to the count down of the next IC. This is the same for the next two ICs, as they carry pin of one is connected to the count up of the next, and so on.

When incrementing, the counter will count to 15 before rolling back to a zero. When it does this, it sends a signal on the carry pin, which acts as the count up for the next IC. So, the first IC increments on every clock (10 times per second, in my prototype), but the second IC increments sixteen times slower.

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