PET / cbm2 timing questions...

From: Groepaz <>
Date: Mon, 20 Dec 2010 19:36:50 +0100
Message-Id: <>

i am trying to fix some timing related stuff in vice, and some related 
questions popped up :) bear with me, i don't know the pet (let alone cbm2 
machines) much =)

- whats the exact cpu clock? is it really 1.0MHz ? i realise that according to 
the schematics it is, however when i lower the clock to 0.999600Mhz, suddenly 
the "8296d diagnostics" program no more reports the 50Hz interrupt as bad (if 
editor rom 901474-04 (50Hz) is used) - which seems odd :)
- what exactly is the difference between a 50Hz and a 60Hz PET? as far as i 
understood, the difference is only the editor rom, which would set up the crtc 
for either 50 or 60 Hz. or in other words, a 50hz PET can be converted to a 
60Hz PET just by changing the editor rom. correct? this would also mean that 
above mentioned "8296d diagnostics" program must fail on a 60Hz machine, since 
the irq will be 60Hz there, correct? (infact it will also report the editor 
rom as bad if not the above mentioned one is used)
- said irq is (more or less) directly triggered by the power line frequency, 
thus always 50 or 60 hz on the respective machines, no matter of crtc setup - 
correct? (not sure on this one =))

as a side node, above makes me wonder a bit on how accurate the timing of the 
50Hz interrupt really is (or could be even, since the power line frequency 
isnt exactly stable) - ie why that diagnostic test can work properly (on a 
real machine) without considering a certain error, which would be well within 
the same that vice exposes for 1Mhz cpu clock (crtc setup ends up at sth like 
50.025Hz or so). any ideas? :)

similar questions on cbm2 modells:

- would b500 line machines use the same cpu clock as c64s ? i assume yes here, 
- for 6x0/7x0 machines, is the exact cpu clock really 2Mhz? a comment in the 
vice source suggests it is, but actually a lower clock is used to work around 
some ieee timing problem. again a bit odd :)
- for 6x0/7x0 machines, is again the only difference regarding 50/60Hz 
machines the roms?


The weirder you are going to behave, the more normal you should look... When I 
see a kid with three or four rings in his nose, I know there is absolutely 
nothing extraordinary about that person. 
<P.J. O'Rourke>

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Received on 2010-12-20 19:00:04

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