Re: 6526A synchronous serial behavior?

From: Nicolas Welte <>
Date: Sat, 24 Sep 2011 16:28:23 +0200
Message-ID: <>
Segher Boessenkool wrote:
>> The interesting question is... Does the 8521 behave like a 6526 or  
>> like a 6526A?
> 6526A means a 6526 that runs at 2MHz.  It has nothing to do with
> die revision.  There seems to be a strong correlation though, most
> later chips were 2MHz and most older were not.

Yes this is very unfortunate. The scene seems to refer to the different 
timing versions with the A suffix, while in reality it is the speed grade. 
I found the change came in late 1986, the earliest "new" version I have has 
a date code of late 1986. This also matches with the arrival of the 
reportedly incompatible C128DCR and Aldi-C64 versions.

I learnt the difference of the CIAs the hard way. A CIA in my 64 broke, and 
I bought a replacement. After that, good old 15 seconds copy wouldn't work 
anymore. I debugged it to find it could be fixed by adjusting a CIA timer 
setting by one cycle ...

Another program that was affected was SMON, a machine language monitor with 
the ability to trace programs step by step, by interrupting it after every 
instruction. With the original timer setting it would not work on new CIAs.


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Received on 2011-09-24 15:00:24

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