Understanding the Colecovision clock

Other Hardware General Electronics
1

Hi all, whilst doing research into various aspects for a project of mine I needed to learn how various 8Bit computers generated their clock frequencies and looked into the colecovision's schematic to get a general idea as its a fairly simple machine.

I'm using the redrawn and reverse engineered schems which I found here so I am aware these might not be accurate.

The problem I face is that on the surface of it, it doesn't make sense to me and even trying to replicate the circuit, I dont get any clock outputs for the CPU or Audio, only the video clock but that seems to be doing it's own thing. I tried the same circuit in the Circuit Simulator at falstad (and generating a couple of signals with logic) and it confirms my test (pic below of equivalent circuit)

Wondering if anyone can shed some light or maybe has an idea what is wrong with this? More to learn than to implement if anything.

2

Hi,
OPs Circuit.

colec.jpg
colec.jpg730×350 75.8 KB

Tom.. :slight_smile:

colec.jpg
colec.jpg730×350 75.8 KB

3

The block in the upper left the simulator circuit is a divide by 24 circuit. I don’t see this in the origional circuit.
A 74LS04 is a logic inverter, putting an inductorand capcitor between input and output is a piss poor way of making an oscillator which will only work with certain manafacature’s parts back in the day and probably not with today’s manufacturing techniques.

4

The divide by 24 was a crude way of getting a Z80 M1 signal into the circuit as it can either be every 2/3 clock cycles that it goes high and into the 7474. Doesnt make a lot of difference though