Re: Commodore joystick ports

From: Pasi Ojala (
Date: 2007-04-07 22:21:47

> basically PWMs 5volts into the charge line, while watching 
> the line for the clamp to 0 volts.

You are right, I reread the POT subsection from the
Programmer's Reference Guide and it seems my recollection
has been spoiled by the little ADC knowledge I have gotten
much later. The SID ADC is much simpler than I remembered.

So, something like this happens:

1) every 512 clock cycles SID will ground the POTX/POTY inputs,
   thus emptying any charge accumulated into the external
2) then an external voltage source (+5V) will start to recharge
   the capacitor(s) through an external series resistor (the paddle
3) when the voltage potential of the capacitor reaches a
   "decision level", the time elapsed will be the conversion result.

Does anyone know if the POTX/POTY are multiplexed to use the same
counter, or why is that 512 cycles instead of 256?

I am also no analog expert by any means, but it still seems
to me that it would be possible to simply use a series resistor
to convert the external voltage level into a current.

When used "normally", the voltage is constant and the resistance
changes, changing the current that charges the capacitor.
If the voltage changes, but the resistance remains the same,
the current that charges the capacitor changes. Both cases
should seem about the same to the capacitor..

/She was not one of those fool women who tossed their brains at a man's
 feet along with their hearts. It was just that sometimes, near him,
 thinking clearly became a trifle difficult. That was all./
	-- Min in The Wheel of Time:"A Crown of Swords"

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