[Coco] CoCo's TIMER and 60hz

Allen Huffman alsplace at pobox.com
Wed May 29 12:32:58 EDT 2019

How precise is the CoCo’s 60hz IRQ from machine to machine? I’ve really only used two CoCo 3s in my lifetime, and I don’t recall the software clock under OS-9 being terribly inaccurate.

I did some tests with Roger Tayloer’s Matchbox CoCo FPGA and saw that the time gets off pretty badly under NitrOS-9.

I then did some BASIC tests between it and a stopwatch, assuming TIMER is 60/second. It also gets off.

Now I am testing Matchbox CoCo against my real CoCo 3, and see Matchbox is already 2 seconds behind in the first 8 minutes.

Since I don’t have multiple CoCos to compare, I wonder if this is just normal drift? Since we are using the video outputs 60hz, I assume it has to be pretty precise and consistent, but maybe not perfect from machine to machine? NTSC was never perfect, yes?

I can share my simple test program if someone would be interested in trying it on two real CoCos side-by-side.

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