RE: Commodore joystick ports

From: Scott McDonnell (
Date: 2007-04-09 04:32:44

Yeah, data sheet says VCC/2 or 2.5V, but I believe your real-world
testing more than the datasheet. 

Definitely 256 clock cycles shouldn't be needed to discharge the cap if
it is shorting it to ground, but they are used anyway for whatever
reason the engineers decided to do it that way (reusing the same timer,
maybe?) Now, this is something I am NOT familiar with but are the
contents of the counter available while it is running, or is it latched
into a register after the 512 cycles are up? If it is available to read
while free running, this would definitely increase the usefulness.

Scott McDonnell

-----Original Message-----
From: []
On Behalf Of Jim Brain
Sent: Sunday, April 08, 2007 10:15 PM
Subject: Re: Commodore joystick ports

Scott McDonnell wrote:
> Of the 512 clock cycles, the first 256 cylces are used to completely 
> discharge the cap. The second 256 are used to count the time it takes 
> for the cap to charge to 50% of VCC (or 2.5V)
As my results show, I think it's 2.0v and the 256 cycles are not needed 
to discharge the cap.  It discharges in at most 2uS after being clamped 
to 0volts (which makes sense, the shortest RC time would be when there 
is no R.


Jim Brain, Brain Innovations                                      (X) 
Dabbling in WWW, Embedded Systems, Old CBM computers, and Good Times! 

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