[Coco] CoCo's TIMER and 60hz

[Bill Nobel]

Hi Allen.

  Roger and I ran a test last night with my logic analyzer on a real Coco3.  VSYNC on the particular machine I tested reported 59.7 Hz.  The Coco3 was never accurate on software time due to fact the IRQ line is shared with other devices besides VSYNC.  Yes they are queued but arn’t serviced accurately based on time.

Bill Nobel
b_nobel at hotmail.com<mailto:b_nobel at hotmail.com>



On May 29, 2019, at 10:32 AM, Allen Huffman <alsplace at pobox.com<mailto:alsplace at pobox.com>> wrote:

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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