6522 weirdity

From: Dwight K. Elvey <dwightk.elvey_at_amd.com>
Date: Mon Dec 30 16:07:00 2002

>From: "Sellam Ismail" <foo_at_siconic.com>
>Ok, I've kinda figured out the control lines on the 6522.
>First of all, I've got them triggering a flag in the IFR whenever there
>is a state change on any control pin from a ground to either floating or
>For example, CB1 is plugged into ground. I reset the IFR (so no flags
>are set). I remove the ground from CB1. CB1's flag in the IFR gets set.
>I reset that flag. If I plug CB1 into +5V, I get no flag. If I unplug
>CB1 from +5V, I get no flag. If I plug CB1 back into ground, I get a
>Why are the flags being set only when one of the control lines no longer
>senses ground? I tried playing with the PCR to see if setting whether the
>control pins are active positive edge or active negative edge changes this
>behavior but it doesn't. I can work with this the way it is: I'm
>basically tracking state changes on the control pins, which is fine for
>my purposes since the reader will go from TTL false to TTL true if there
>is an error, and back to TTL false when the error gets cleared. But I
>want to know how the behavior of the control lines is set.
>Now, I seem to have a problem with CA1. If I have CA1 connected to
>ground, it's stable. As soon as I remove it from ground and it's
>floating, CA1's state starts to fluctuate wildly for a little over 2
>seconds. It's flag in the IFR keeps triggering over and over. If, right
>after I remove it from ground and it starts fluctuation, I plug it back
>into ground, it stabalizes (it stops changing states). As soon as I
>remove it again, it fluctuates wildly.
>Is CA1 tied to one of the 16-bit timers? It sure seems like it. The docs
>say something about being able to make one of the control lines trigger
>off a timer. Am I triggering the shift register somehow?

 What is the weird behavior? I haven't seen anything I wouldn't
have expected.

>What in tarnations is going on?
>Sellam Ismail Vintage Computer Festival
>International Man of Intrigue and Danger http://www.vintage.org
> * Old computing resources for business and academia at www.VintageTech.com *
Received on Mon Dec 30 2002 - 16:07:00 GMT

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