[Coco] Coco3 joystick action survey

[jdaggett at gate.net]

 Robert

Something to consider. 

The pots  used in older joysticks maybe carbon composition type resistors. 
If there is a great deal of inactivity using the joysticks, they can become 
very noisy. Like the pops and scratches when a volume control on an old 
radio is turned. Dirt and just plain wear on the resistors may cause irratic 
ADC conversions. 

just a thought. It might be nothing but worth a look into. 

james


On 10 Mar 2008 at 23:01, Robert Gault wrote:

> I could use some help with this. I've been acting as a beta tester on
> a game for the Coco3. The game has a cursor that is moved around on
> the screen to select items. At the moment, there are two other testers
> and the cursor action on my system seems to be unique in that the
> cursor jitters. That is the pointer at most locations on the screen is
> never stationary but always jiggles slightly. The cursor is completely
> steady on the other systems.
> 
> This has been very frustrating for the programmer because it is
> unlikely that my system is unique and it has been impossible (using
> software) to remove the jitter. What I would like to find out is how
> many Coco3s exhibit jitter in the joystick values vs those that don't.
> I'm asking readers to run the short Basic program below and very
> slowly move joystick (right, left, or both) around the screen. Try to
> find a locations where the value obtained by the JOYSTK(#) command is
> not stable. Please report back here what you see.
> 
> 10 POKE &HFFD9,0       2MHz CPU clock
> 20 WIDTH80         or you can use 40
> 30 FOR I=0 TO 1 STEP 0
> 40 EXEC &HA9DE         the ROM joystick routine
> 50 PRINT PEEK(&H15A),PEEK(&H15B),PEEK(&H15C),PEEK(&H15D)
> 60 NEXT
> 
> The values at $15A and $15B are for the right stick and those at $15C
> and $15D the left. On my system, the values are stable at 1MHz (POKE
> &HFFD8,0) but unstable at many positions at 2MHz. I've tried this with
> two Coco3s serial numbers 1001703 and 1308368. I've used three
> joysticks, two deluxe and one regular. There is jitter for all at 2MHz
> with the ROM routine and worse jitter in the game under development.
> 
> The ROM routine for reading the joystick is slower than that used in
> the game which may be why it works at 1MHz. The main questions are
> whether this is a quality control issue in the Coco3 design and what
> if anything can be done about it.
> 
> Both the game routine and the ROM routine are stuck with the
> resolution of the internal DAC, 64 steps. A joystick being a
> continuously variable resistor can easily be set at values in between
> those of any two DAC steps. Depending on the circuit design
> intermediate values could cause oscillation between the two adjacent
> steps. This could be caused by inadequate ICs or by noise on the power
> supply. I've put a scope on my Cocos and they meet or exceed the
> waveforms in the service manual.
> 
> Hopefully there will lots of response to this request for help. I'd
> like to solve this problem and the programmer wants to solve this more
> than I do.  :)
> 
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