Re: Commodore joystick ports

From: Jim Brain (
Date: 2004-11-01 07:48:31

>> An AVR clocked at 8Mhz should be suitable for jitterless operation if 
>> I'm right...
> It is, if that is ALL you are doing, or if you can accomplish what you 
> need in the 256 "wait" cycles, but that's not always possible.

As an update, I decided to try this.  Instead of having both sides of 
the interface handles by different timer IRQs, I combined them, as the 
transfer of 1 byte of data from the originating system is clocks by a 
20uS clock, which is 160uS/byte.  Thus, when I see the falling edge, I 
transfer a byte.  The transfer and the parse of the incoming byte is 
taken care of before the charge cycle starts, so the jitter I was seeing 
went completely away.

However, even with no jitter, the LSB still fluctuates.  It even 
fluctuates when I comment out all the code except the POT emulation code 
(and set the POTs to 127).  So, the SID must not be capable of 1LSB 
resolution.  Makes sense, as they were just paddles, who cares about the 


Jim Brain, Brain Innovations                      
Dabbling in WWW, Embedded Systems, Old CBM computers, and Good Times!

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