David Ross wrote: > > On Thu, 30 Nov 2000 firstname.lastname@example.org wrote: > > > The 8520 is also in one of my 1571's. My other 1571 uses a 6526 in the > > same socket. > > What's the difference between the 8520 and the 6526? Isn't it something > with the TOD clock? Correct. The 6526 (and 8521) have a BCD counter that is fed by the TOD input through a configurable divider with a factor of 5 or 6, making it count 10ths of seconds on 50 and 60Hz power lines. The 8520 on the other hand does not have the divider and the configure bit for it, and has a straight binary counter instead. It simply counts events on the TOD input directly. Still funny that the technical documents on the 8520 still call it a TOD input (which means Time Of Day). There are several different CIA implementations that can be distinguished either by part number or software behaviour: 6526 (pre 1987) 8521 (1986 only?) 6526 (past 1987) 8520 The two different 6526 have a different interrupt timing that differs by one clock cycle on some events, but I don't know the exact details. The 8521 should have been a replacement for the first 6526, it even has the same IRQ timing, but somehow even Commodore put only one of them into each C64/C128. Some issue must have prevented its use in the other socket. And the 8520 has that different TOD behaviour. Nicolas - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail email@example.com.
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