Re: Reading Joystick Position

[dsf@globalcrossing.net (Dan Foster)]

In article <Nj2dnV9YE8OlOkSgXTWcqw@comcast.com>,
Joel <joelbuckley54.nospam.@hotmail.com> wrote:
>
>"Simon Williams" <DONTemail@luddite.ca> wrote in message
>1118189.CMKYXGAG@news.telusplanet.net">news:1118189.CMKYXGAG@news.telusplanet.net...
>> I'm having some problems figuring this out:
>>
>> In Applesoft PDL(0) and PDL(1) are the X & Y coordinates of the
>> joystick, but I can't figure out the actual memory locations for these.
>> According to most of the information I have available I should be able
>> PEEK somewhere between 49252-49255, but these locations do not return
>> the same values and don't seem to be related at all to the joystick.
>> FWIW the joystick I'm using is a QuickShot which has an Apple setting
>> switch, and works perfectly for all games and programs I've tried (which
>> is why I assume the problem is with me and not the hardware).
>>
>> Any help with this would be much appreciated.
>
>You have the right addresses, but the wrong technique.
>Accessing $C070 (49266) starts a quad timer (all four Paddle inputs).
>The paddles provide the R in an RC time constant.
>The timer outputs show up as the hi bit of those 4 addresses.
>After the timers time out, the hi bit goes from high to low.
>A software timing loop determines the PDL value.
>It's fast (12 microseconds per tic), so you can't duplicate this with Peeks
>from Applesoft.
>Here's the routine.
>$FB1E
>PREAD LDA $C070 start timers, enter with paddle # in X
> LDY #0
> NOP
> NOP
>PREAD2 LDA $C064,X
> BPL RTS2D
> INY
> BNE PREAD2
> DEY ; 255 max
>RTS2D RTS

...and the reason why the timing is crucial is because you *must*
wait while the capacitors in the joystick discharges, or you will
end up reading bogus values. Hence, the NOPs (no-operations) inserted
simply to waste a little time in precisely known amounts of delay.

-Dan