Going sequential - The program counter

A project log for The Cardboard Computer

My goal is a 4-bit cpu using discrete transistors. I am learning from scratch as I have never worked with hardware at the gate level.

Mark Nesselhaus 12/22/2016 at 09:163 Comments

in the summer of 2016 I needed to tackle sequential circuits. So started the real fun ( hair pulling ). On a single flip-flop level I started with the basic R-S to J-K to the D. However when I tried to make these circuits toggle then I began to understand the problems of signal levels, timing and edge vs. level triggering. No matter how many searches on the internet I did, I could not get things to toggle or at least toggle without random errors. I had built a clock using transistors but was not sure of the signal purity ( The only test equipment I have is a analog vom and a transistor tester ). I made the call to allow one chip for this project and that is a trusted 555 timer chip. Did that help, No way. Finally I came across a circuit that converts the square wave output of my clock to a very short pulse. Now my flip-flops started to toggle the way I wanted.

Below is my program counter connected directly to the clock board on the left. There are four toggle D flip-flops on the right and note the pulser boards to the left of each one. Without them I would not have any hair left.


Yann Guidon / YGDES wrote 12/22/2016 at 20:13 point

I don't recommend this kind of "pulse generator", at least for a potentially complex synchronous system. A master-slave flip-flop is "safer" as long as the clock signal is nicely square and propagates correctly everywhere.

I managed to hack a 10-transistors synchronous D flip-flop (not latch):   However it works with MOSFET, not bipolar transistors. I still search a method to adapt this topology to bipolar, I might have something but it's for ECL, not DTL levels...

Anyway, you can create a suitable edge detector with a RC circuit, no need of a series of inverters (RC is more flexible and you can tune the pulse width with a potentiometer).

Note : many designs used transparent latches controlled by a 4-phase clock.

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Ted Yapo wrote 12/23/2016 at 01:32 point

You can make a true edge-triggered D-flop from six single-transistor inverters and a bunch of diodes, too.  Diode-OR gates combined with the inverters makes NOR gates, which are easily fastened into an edge-triggered flop.  The flop triggers on the falling edge of the clock, which is different from the typical 7474-type flops, but you can easily work around that. I made one with diode inverters, described here:

Scroll down to the d-flop section, then just replace the diode (DDL) NOR gates with DTL NOR's, and you're all set :-)

The D-flops toggle if you feed the Qbar signal back to the D input.

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Yann Guidon / YGDES wrote 12/23/2016 at 01:48 point

Yay thanks ! I was looking for this circuit/schematic :-)

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