multi-touch in X

Christopher Heiny clheiny at
Thu Jul 12 23:59:01 CEST 2007

On Thursday 12 July 2007, ground control picked up the following 
transmission from Mark:
> All the touch screens I have ever worked with give an x and y value
> of distance from a corner.  These have been 4-pin resistive touch
> screens.  If thats the case then the controller would have to be able
> to switch which pins are drive and which read the data in order to
> get two points.  (and the screens electrical path would have to be
> free of diodes).
> Which if we can do: a 2-touch touchscreen would be a piece of cake.
> Then we would just have to process the distance between touches and
> determine if its a big finger or two separate locations.  Also this
> would allow the touches to be picked up by the middle of the finger
> which im guessing is how the iphone gets good finger keyboard
> accuracy.

Capacitive touch screens, such as that in the iPhone or Synaptics 
ClearPad product line, generally work somewhat differently.  Resolution 
is much better (both in "dots per mm" and repeatability) than a 
resistive screen can manage.

[Those of you in the western Pennsylvania area (OK, not many) may wish 
to go check out SDG Systems work at Western PA Linux User Group meeting 
this Saturday.]

That said, it doesn't require a huge amount of hardware rework to 
integrate a ClearPad.  I2C is already there.  About the only thing 
missing on the current GTA01 boards is an available GPIO to route an 
attention interrupt.  The ClearPad can still be polled for data, but 
using an ATTN line is more efficient.

> On 7/12/07, Tim Newsom <cephdon at> wrote:
> > On Thu, 12 Jul 2007 13:07, Joe Pfeiffer wrote:
> > > It strikes me as highly unlikely -- that would require hardware
> > > changes.
> >
> > Getting the packages into gta02 does not require any hardware
> > change. Making use of that functionality most likely will. I am
> > sure that's what you meant, but I wanted to state it explicitly. 
> > Adding software is easy... Adding new functionality like this is
> > much harder when its not supported by hardware.
> >
> > --Tim

More information about the openmoko-devel mailing list