RE: Commodore joystick ports

From: J Ellsworth (
Date: 2007-04-09 08:40:21

The dtv had the 512 cycle charge dump type ADC, but for some reason it
didn't seem to work well when the chip came back.  I was having a conflict
with DC Studios, so I didn't spend any time trying to figure out why it was
not stable.


-----Original Message-----
From: [] On
Behalf Of Jim Brain
Sent: Sunday, April 08, 2007 7:21 PM
Subject: Re: Commodore joystick ports

Scott McDonnell wrote:
> I agree with you about the 512 cycles, now that I read up a bit more.
> Sorry, there was some miscommunication there about the multiplexing.
> There are two POT inputs to the SID and two pots on each control port.
> POTX and POTY of the given control port are connected to the SID at the
> same time. The PORTS are multiplexed, not the POT lines. This is done by
> the CD4066. My mistake in not being clear.
It wasn;t your post I was referring.  Someone (Pasi?) suggested the 512 
might be to allow the internal circuitry to be multiplexed, 256 for 
POTY, 256 for POTX.  However, if that were true, I reasoned you should 
see the drive to zero trigger alternate, and it does not.

> I am fairly certain Jeri just used a modern ADC, since it wouldn't make
> much sense to do it exactly the way the original commodore worked when
> there are better and easier ways now. There is an ADC chip connected to
> the POT on the Hummer. I don't have one in front of me, but it is
> probably one of the Dallas Semi integrating slope ADCs (dual slope, most
> likely.) It isn't important to emulate it exactly, as long as the SID
> registers are filled with the values expected by a game. You have a lot
> more license to do it your way when you recreate the internals. That's
> the secret of emulation: As long as the circuit takes the inputs
> expected and outputs the results as expected, what happens in between
> isn't all that important.
Given the low sample rates involved and the target (paddle input), your 
position is correct.  However, Jeri is known for trying to deal with the 
minutiae.  As well, using a regular ADC would no doubt preclude the use 
of the 1351 mouse, which depends on the 512 cycle period.

> Again, yours and Levente's methods both work as you both have
> demonstrated. Nobody is questioning that, of course. But, I am just
I'm not defending it per se.  I had a hammer (uC) for other reasons in 
my project, so I manufactured the "nail" to make it all work.

It's OK, though.  Someone asked me to delve into more details on how the 
PSXJoy emulates the paddles, and I admit I had forgotten a lot of this 
since late 2004, so it was a good refresher.


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

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