Fwd: Re: Openmoko Bug #1194: Touchscreen Jitter

Joerg Reisenweber joerg at openmoko.org
Tue Jun 17 23:12:19 CEST 2008

Dunno whether that's you, Andy...
anyway, this very strongly suggests the jitter is associated to one PLANE of 
the ts only (like I guessed quite some time ago). This means: either 
the "long-axis" X+,Y- rails on powered pane are on the "inside"(near LCD). 
Touchpoint contacts to the outer plane, and there some sort of hum, EMI, RF, 
or god knows what is introduced to the relatively highZ "target"plane from 
environment and is spoiling the A/D for this axis.
OR outer and inner plane are reversed (means (+) and (-) rails are on the 
short parts of outer plane), and the interference comes from our 
LED-power-converter (or the LCD itself - though I can't imagine this) 
coupling to the inner plane that goes to A/D-in.

Once more I like to ask whether one of the coder wizards can setup a 
simple "sample at 500k/s and dump to some file"-type debug helper tiny 
program, so we might analyze for the real waveform we see on "long axis".

We may need this to improve hardware circuitry (filters!) of future 
resistive-ts designs, as well as to know what we have to tune the ts-lib to 
cope with for recent devices.

A *GOOD* scope probe printout _maybe_ is even better, but this simple 
A/D-thingy will serve perfectly for a first approach (a least we know for 
*sure* this is what the A/D-converter (and thus tslib) got)

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