Re: Commodore joystick ports

From: john/lori (
Date: 2007-04-08 09:51:26

Scott McDonnell wrote:

> 512 cycles, because of the nyquist rate (at least 2x the max desired
> sample frequency), perhaps?

I always assumed it was for simplicity.  You need an 8 bit counter
for the ADC, they just added a bit as a simple way to control the
timing of the discharge and make sure it lasts long enough to fully

> Yep, this is how I remember it working. I was using a 555 timer and a
> counter to emulate the SID pot inputs. I believe this is method is
> called a "dual slope integrating ADC." RC time constants are very
> predictable.

The SID ADC is a single slope integrator.

 > The only reason I still hold some doubt about this is that the paddles
 > use 5V, the resistor changes the amount of time it takes for the cap to
 > reach 63% of 5V. If you input say 2V, it will never reach 63% of 5V. The
 > resistor never changes the voltage, just the current (or charge rate of
 > the cap.) So the "decision level" you mention below would never really
be satisfied.

Which is the beauty of Jim Brains (or whomever's) scheme.
I think you could probably even rig a 555 one shot so that it was synced
as Jim does.  But even a free running oscillator (be it pwm or pulse 
posistion) should work if it's running fast enough.

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