Commodore 64 (was "Re: Teaching kids about computers...")

Ensor wrote: Wow, never seen anything like that. The ones on the right and middle are all I know as PSUs. Ya learn something new every day. Same for the SID. (And, I suspect the VIC-I paddle inputs on the VIC-20). Years later, the C64 Mouse used the paddles inputs to represent X and Y digital coordinates via a very ingenious method, IMHO. The SID ADC works in 512 cycle periods, clamping the internal cap to ground for 256 cycles, and then unclamping, watching how many cycles it takes to charge the cap. The faster the charge, the lower the number. The mouse ASIC would start up feeding +5v to the paddle line through some small resistance, watch for the line to drop to GND, and then start timing, turning off the 5V. At count 256, if would then start counting to a certain position, and then bring the line to 5v. The position was time period 256+desired value, as I recall. The value was ((6 bits of counter mod 64) * 2). On reception, you stripped bit 0 and bit 7, and you had the counter. A few years ago, based on this reverse engineering some CBM folks had done a few years back to create a mouse interface for the 64, I implemented some AVR code to do it all in the AVR, and support the full range of ADC values. Commodore's solution had a fixed 1MHz crystal, so bit 0 jittered, as the timing was just a bit off (PAL units run a bit below 1MHz, while US ones run a bit over.). The AVR code switches into internal oscillator mode and then trims the oscillator trim register in real time to sync with the 512 cycle clamp event. Jim