New Clock and PC Board

Submitted by Andrew Wasson on Wed, 01/18/2012 - 6:58pm

Since I updated the site last week with the 4-bit Scratch Built CPU project, I decided to rebuild the board that holds the Clock, Program Counter, and improve the gates I'm using for my JUMP command. The improved board carries two 74LS161's (binary counters), a 74LS174 (upper 4 bit latch), a 74LS32 (NOR gate) and a 74LS132 (NAND Schmitt Trigger) and I'm using a 555 timer with a trim potentiometer for my adjustable clock generator.

Picture of the new program counter and clock circuits
Here's a picture of the completed PC/Clock/Logic board. The push button is a reset circuit.

The main reason for the rebuild was that I decided that I wanted to have a dedicated board just for the lights and switches that represent the 'front panel' of the computer. I also wanted to clean it up a bit. It's no secret that lots of wires zig-zagging around, particularly on a solderless breadboard can induce noise into the circuitry and I thought it would be a good idea to add decoupling capacitors to the chips too.

The goal is to have the Clock and Program Counter on one board, RAM on another, one dedicated to the Front Panel and then a final board for control ROM, MUX and the ALU. To achieve this I needed to add another solderless breadboard to the mix so that I would have four breadboards across where I currently have three.

I've pretty much finished and tested the new board so next on the agenda is adding it to the other boards, rebuilding the RAM board and then adding the Front Panel board which will consist of Address LEDs, Instruction LEDs, Data LEDs and some switches to control/reset/run the computer. I'll add a schematic soon.


by Ruby Wung on Fri, 07/27/2012 - 12:20am

Wonderful posting, it is nice to at last find creative info amongst the dross, I am honestly suprised to discover a blog that is not full of the ubiquitous garbage, bless you.

by bill rowe on Sat, 03/29/2014 - 10:32am

what will it run at andrew?

by Andrew Wasson on Sat, 03/29/2014 - 12:42pm

Hi Bill,

The clock is really dirty so it doesn't run fast at all. I've had the project on hold for a while as I make some space in the house for my shop. I've built a TTL hex keypad from a 74150 a clock and a couple of latches which I'll add to the front end and I'm going to put in a better clock as soon as I get the shop set up. I'm hoping to do that in the next 6 weeks if I can move the washer/dryer and take over the laundry room. LOL... 

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