Re: Commodore joystick ports

From: Daniel O'Shea (
Date: 2007-04-12 10:39:51

Jim, how possible do you think it is that your existing code could also 
be adapted to the VIC-20 or Atari paddle inputs? I'm not exactly sure 
how their timing works, but I do know that they use 1000pF capacitors, 
and 1M rather than 500K potentiometers... would it be as simple as 
listening for the interval between IRQs to know which system is connected?

Jim Brain wrote:
> The IRQ is a falling edge trigger.  It watches the POTX (maybe it's Y, I 
> forget)
> SID clamps POT line low, IRQ triggers in AVR
> AVR puts output line in output mode, set to 0. It starts a counter.
> A previously computed value is stuffed into each POT counter compare 
> register.  The value is 512uS- desired reading.
> As the counter reaches the stored values, the internal counter matches 
> interrupts trigger.
> On trigger, the code sets the appropriate POT line to Hi.
> Then, the process repeats.
> Why the 512- value?
> Well, when SID clamps, you know there is 256 low uS coming up, and then 
> we need to do the PWM stuff.
> SID POT registers are backwards.  Namely, 255=0v, while 0=5v.
> So, the lower the number desired, the larger the PWM duty cycle.
> Adding that all up, it's 256 + 256-value = 512-value.
> Now, a normal PWM would go high at 256uS and then low after that.
> That won't work, because except for 100% duty cycle, we drop to 0 before 
> time 512, missing the trigger.  So, I inverted the PWM phase, having the 
> start be variable, but the end always occurs at time 512.  It also 
> simplified the code, since you now could care less where time 256 is, 
> saving some cycles.
> If you look at the code, you'll find reality is a bit messier than my 
> description.  Namely:
> PSX analog sticks are inverted relative to Atari paddles, so I use 256 + 
> value
> The timer is actually more complicated.  I use a free running 65536 
> counter, and just add the desired delay to the current counter value and 
> store as the compare.  Since I use the counter for the oscillator 
> tuning, I needed the counter to be free running, so I could better see 
> out-of-bounds values.
> The timer counts AVR clocks, not c64 clocks, so everything needs to be *8.
> I actually use 246 as my constant, due to the math delay, and the time 
> if takes to service the counter match IRQ.  246 is an experimentally 
> derived value.
> Jim

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