Re: 6526A synchronous serial behavior?

On Sep 22, 2011, at 12:57 PM, Wolfgang Moser wrote:

> Nate Lawson schrieb:
>> On Sep 21, 2011, at 3:22 PM, Segher Boessenkool wrote:
>> 
>> It's 500 Kbit/sec inbound to the 1571. Some devices receive it just
>> fine, others don't. I'm going to try reducing the clock rate slowly
>> and see where it becomes reliable. If it truly is synchronous with
> 
> if it is different behavior from drive to drive then this let's me think
> on different CIA revisions. 6526 vs. 6526A vs. 8520, you know even VICE
> does support two different 6526 types, the "old" and the "new" one which
> have some off-by-one clock cycle differences. I don't know about the
> exact details, but since the shift registers may be the worst
> studied/tested functional blocks of the CIA, this could be a reason.
> 
> Maybe you want to have a look for a piece of CIA type detection code and
> check, if there is a relation beweeten "unreliable" 1571 drives and a
> certain CIA type.


I would think all 1571 drives have a 6526A because they have to run at 2 MHz. The 6526 can only do 1 MHz. As for the 8520, it would be strange to move to the HMOS line in only this chip.

Also, you have to remember the 1571CR, where the 6526 was merged into a single chip with the WDC logic. I have no idea how that will perform for 500 Kbit/sec serial.

-Nate


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