From: Jim Brain (brain_at_jbrain.com)
Date: 2004-10-21 23:00:48
> more, it appears that it bangs the 6 bit position data digitally (why it
> would do this on the POT port, I don't understand...) Is this what you
> getting from this?:
> http://www.funet.fi/pub/cbm/documents/projects/interfaces/mouse/Mouse.html
Yes. It does that to "feed" 1uS chunks of 5V to the capacitor used by SID
to determine the pot position (using the idea that you can detect voltage
by charging a cap to that voltage and measuring how long it takes to
discharge.) The more 1uS chunks of 5V I feed, the more charge the cap
has, the higher voltage is recorded. Primitive, but I was amazed that it
works pretty well.
> I would think it would make more sense to simply emulate a paddle. In
> which
> case, you could use a transistor to create a current source on each pot
> port
> which would emulate a paddle. It is a little early for me this morning to
> think clearly, but I will look at this later tonight and try to help more
> if
> someone doesn't already answer you.
Because I loathed this bit-bang the POT line approach, I tried a bunch of
other ideas first.
My initial idea was to use PWM through a low pass RC filter to create the
correct voltage, but that did not work at all. I'm still not sure why
this simplistic approach did not work.
I then thought of using a digital pot, but it looks like the POT lines
need a 500k pot swing to go from 0 to 255, and I couldn't find a part that
swung that far.
I then looked at serial DACs, as Maxim has some low cost dual SPI DAC
parts, but Could not determine if that would work, so abandoned the idea.
I did not try a current approach (Analog is my weak subject), but wouldn't
I need some sort of analog input to the transistor in order to bias it to
a certain level?
Jim
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