From: J Ellsworth (c1_at_syndicomm.com)
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.
Jeri
-----Original Message-----
From: owner-cbm-hackers@ling.gu.se [mailto:owner-cbm-hackers@ling.gu.se] On
Behalf Of Jim Brain
Sent: Sunday, April 08, 2007 7:21 PM
To: cbm-hackers@ling.gu.se
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
--
Jim Brain, Brain Innovations (X)
brain@jbrain.com
Dabbling in WWW, Embedded Systems, Old CBM computers, and Good Times!
Home: http://www.jbrain.com
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