Re: Help with SX64 User Port?

From: Gerrit Heitsch <>
Date: Thu, 22 Jun 2017 20:01:29 +0200
Message-ID: <>
On 06/22/2017 07:36 PM, wrote:
>> On 2015-10-13, at 07:48, Gerrit Heitsch <> wrote:
>> At the end of 1986, Commodore changed the process from NMOS to HMOS-II. For a short while, some CIAs were labeled '8521R0' but then they went back to the '6526' label. Usually they were also fast enough for get the 'A' for 2 MHz operation. The HMOS-II-CIA also changed a bit timingwise, that's where the legend originated that the 6526A behaves differently than the 6526.
> They DO behave differently. For example my old test cart shows CIA related failure when using the "6526A".

Does it still show that when you use a 6526A-1 made in 1985? I expect 
not. So the definig characteristic is not the 'A' but the datecode. If 
the CIA was made at the end of 1986 or later, it's HMOS-II and 
internally a 8521. Since HMOS-II is faster, they all tested good for 2 
MHz and go the 'A' stamped on them. Some of them were even good for 3 
MHz and got a 'B'.

  Do we know how exactly the differ timing-wise? I guess this is related 
to what Marko wrote yesterday?

It's something about the timer IRQ happening one cycle earlier or later 
than the NMOS version.


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Received on 2017-06-22 18:02:41

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