Re: PET / cbm2 timing questions...

From: André Fachat <>
Date: Mon, 20 Dec 2010 22:11:56 +0100
Message-ID: <>
> Given that the system tick is *not* generated from mains (check the
> PSU schematic - if it were, there'd be a diode between the mains-side
> of the rectifiers and the 6502 IRQ line), "tick" is generated by one
> of the VLSI chips (I forget which one right now).  The ROMs are
> different for 60Hz and 50Hz machines, but the VLSI chips still

If you count the CRTC as a "VLSI" chip ;-)

The 8296 has some programmable logic chips (two 16->8 bit) which are
mostly for address decoding. And there is a small one, which is used
for timing generation. But this one only generates the dRAM access
signals and stuff like that - not the "jiffy" clock.

> generate the interrupt pulse for the "jiffy" clock.  Perhaps there's
> something subtly off with the 6520 or 6522 emulation?

I hope not... ;-)

The PIA 6520 does not have a timer. But in the PET the jiffy interrupt is 
created by the CB1 interrupt of PIA1 - might be a cycle or a few off from CB1 level change to interrupt signal output. But my video tests and the
for example the No PETs Allowed demo work on the PET real and in VICE!

The VIA 6522 has a timer, which is not used in the interrupt generation.
But the same VIA emulation is also used in the VIC-20, as well as the
VC1541 emulation - which is well-tested with VIC demos and VC1541 true
drive emulation, I'd think.

> > the same that vice exposes for 1Mhz cpu clock (crtc setup ends up at sth
> like
> > 50.025Hz or so). any ideas? :)
> I would have to guess that it works fine on real hardware but some
> aspect of the emulation isn't strictly conforming.

No (I hope not), the CRTC timing is programmed to get this value. 
PAL defines 313 rasterlines for one half-image (actually 625 per full 
image) with 64us (i.e. 64 cycles) per rasterline. That results in 20032 
cycles per screen - 49.920Hz. The second half-image is defined as 312
raster lines, which makes 19968 cycles - 50.080 Hz. Both together 
are 40000 cycles for two screens. So small derivations are ok. 

I didn't check the mentioned ROM though whether that produces the 
mentioned 50.025Hz though.


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Received on 2010-12-20 22:00:05

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