From: Jim Brain (brain_at_jbrain.com)
Date: 2004-10-21 23:06:58
> 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 Message was sent through the cbm-hackers mailing list
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