Thread: Mk14 vdu
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Old 14th Nov 2020, 2:46 pm   #903
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 5,304
Default Re: Mk14 vdu

I'm not in a position to rebuild my OrtonView yest so I have been trying a few extra experiments based on this statement by Karen regarding FW692:-

Originally Posted by Karen_O
I've got one more trick to try - I've widened the margins for NENIN release during the active display region. This will mean the SC/MP running a bit slower but worth it if it works.
Once again using my tone generator program which generates 888Hz on Flag 0 when no VDU is enabled, I have the following results when OrtonView is enabled.

FW692 / PIC16F877 = 832Hz
FW523 / PIC16F877 = 832Hz

So no actual difference in the slowdown between those two firmware versions. I also ran Charset under the second combination above, still with 200pF capacitors on A8-A11, and I don't get any corruption there either. In real terms the capacitors appear to make more difference than the difference in the firmware.

The above figure (832Hz) struck me as slower than I remembered, so I tried FW523 / PIC16F877A, the combination I originally made the speed comparison with, and interestingly the frequency rises to 851Hz with that combination, suggesting that something, somewhere, takes less time on the 877A than it does on the plain 877. That would explain why the graphics shudder problem proved so intractable, when the timing of everything is so dependent on exact cycle counts. It also suggests that the problem might still be worth fixing on the 877A for a reward of somewhat greater system speed.
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