Re: 6526A synchronous serial behavior?

From: Nicolas Welte <no_spam_at_x1541.de>
Date: Sat, 24 Sep 2011 16:28:23 +0200
Message-ID: <4E7DE907.1090009@x1541.de>
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.

Nicolas



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