Re: Strange 8255 behavior

From: Gerrit Heitsch <gerrit_at_laosinh.s.bawue.de>
Date: Sun, 10 Jun 2018 23:06:00 +0200
Message-ID: <4e14236e-db4d-b205-468d-e639e8ab13b0@laosinh.s.bawue.de>
On 06/10/2018 10:50 PM, Segher Boessenkool wrote:
> On Sun, Jun 10, 2018 at 10:17:17PM +0200, Gerrit Heitsch wrote:
>> On 06/10/2018 09:46 PM, Segher Boessenkool wrote:
>>> On Sun, Jun 10, 2018 at 06:42:02PM +0200, Gerrit Heitsch wrote:
>>>> On 06/10/2018 05:36 PM, Segher Boessenkool wrote:
>>>>> http://siliconpr0n.org/map/mos/6526/mz_mit20x/
>>>>>
>>>>> The ports are the low half of the pins (PA on the left, PB on the right).
>>>>>
>>>>> (I have a .xcf if anyone is interested, marked quite a few signals, but
>>>>> I haven't done the port stuff very much.  It's about 400MB).
>>>>
>>>> That looks quite different from the one I posted. Looks like MOS did
>>>> quite a bit of redesign between the NMOS 6526 and the HMOS 8521 (which
>>>> still got labeled 6526). Might explain the little differences in the way
>>>> they behave.
>>>
>>> No, this is an actual 6526r4.  This is an 8521r1:
>>>
>>> http://oms.wmhost.com/misc/MOS_6526A_CIA.jpg
>>>
>>> (and this is an 8520r4, the CIA used in amigas; it has a different TOD
>>> clock, and as you can see it's different from 8521 in other ways, too.
>>> But clearly 8520 and 8521  are more related.  The lineage is almost
>>> certainly 6526 -> 8520 -> 8521:
>>>
>>> http://siliconpr0n.org/map/mos/8520/mz_mit20x/ ).
>>
>> Yes, but the 8521 is a drop in replacement for the 6526 (I have a C64
>> Board with a 8521R0 on U2) and later revisions of that chip have been
>> labeled as '6526' again, probably to avoid confusing the customers. You
>> can tell them apart by the datecode or by the '206A' or '216A' next to
>> the datecode.
> 
> _Almost_ drop-in replacement, yes. 

Well, yes, one of the timer IRQ timings is off by one cycle.


  But the die photos are really easy
> to tell apart (an 8521 does not say "6526" on the die, it says "8521").

That's why I said that MOS did quite a bit of changes when they migrated 
the CIA design from NMOS to HMOS.

  Gerrit
Received on 2018-06-11 00:00:06

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