Re: Commodore joystick ports

From: Hársfalvi Levente (
Date: 2004-10-24 22:04:23


Jim Brain wrote:

> Many apologies. The last_name first_name thing is hard to discern from 
> context.

Yep, I should probably write my lastname (in the e-mail address) in 

> I tried that last night, but if the resistance is < 3k, the 
> uController's ability to source is more powerful than the SID's ability 
> to sink, so no falling edge is seen, so the unit does not work.  
> However, at 3k, I can get down to "4".

Or, you could drive the POT line using 2 portbits, one with and another 
without (or a small) series resistor. The code could turn on the second 
output for, say, the first 1 or 2 SID cycles of the acive period, 
effectively shorting the charging time down to 1 or 2 cycles.

Just another thing that I should mention: the 1351 document describes 
only 1 LSB jitter. When I built my board and after I fixed the delay 
line code, I noticed similar results: the "noise" fell down to no more 
than 1 LSB. (If you noticed more jitter, then there's probably still 
some possibility to enhance your code -- no more than 1 LSB of jitter is 

(One idea: whilst interrupts are always "cycle exact" on the PIC, they 
aren't on the Atmel. ...Though, as the Atmel's core runs at a much 
higher frequency, this shouldn't explain the +-3 cycles noise. Noisy 
analog environment, and / or more than one enabled IRQ sources in the 
mcu, these are that I could suspect at first).

> My protocol idea assumes you don't scan the keyboard rows during the 
> time you request raw data.

O.K. No, I didn't mean that it interfered with the keyboard scan 
process; I meant, it could interfere with the user who's currently 
typing, and by that, is shorting some lines that are currently used in 
the communication. (Or it's not possible, and I'm simply wrong in that).

Best regards,


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