Re: [uCsimm] Re:

From: Tom Walsh (
Date: Fri May 19 2000 - 16:00:00 EDT

Ben Lau wrote:

> Tom Walsh wrote:
> >
> > "Kim, Jeong-Hwan" wrote:
> >
> > > Hi, everyone
> > > Is it possible to attach TFD LCD touch screen to ucsimm board??
> >
> > Probably, have you looked at the MC68EZ328 (DragonBall) user manual?
> > It may give you some info as to the TFT displays, so far, I only know
> > that it can use STD displays.
> >
> > >
> > > Can the ucLinux control the touch screen ???
> >
> > Nope, but you are free to write a driver layer to implement one. :-)
> >
> Is there any touch screen available in the market so that
> we can use it with the ucsimm?
> I am just a student,difficult to deal with the
> manfacturers...

Sure, in a way ... What you do is this: Get one of the "4 wire resistive panels",
the structure of a Resistive Touch Panel is an X and a Y axis, each on its' own pair
of wires. What you do to read the panel is: place a current flowing through one of
the axis pairs, say that we start this with the X axis, then ground one leg of the
Y pair and read the voltage on the other lead of the Y, you now have the Y axis
reading. Reverse the X & Y axis current flow & reading (now do current in Y & read
from X) and you will get the X voltage co-ordinate. You must use an ADC (Analog to
Digital Convertor) to take your readings.

In order to translate your readings into pixel co-ordinates (relative to where you
mounted the Touch Panel over the LCD screen), you must know the votage of each axis
in the respective Maximum and Minimum directions. This is achieved by a simple
calibration sequence asking the user to "press the DOT' on the screen when you first
run your application for the initial time. From this Max / Min information, you are
now able to mathematically determine where you are pressing on the touch panel.

If you have ever seen anyone "calibrate" their Palm Organizer, you will know exactly
what a sample calibration process needs to do; the Palm uses a "Four Wire Resistive
Touch Panel" for reading where the Stylus is touching the panel.

Look at my Motherboard schematic at '' and you
will see the measurement circuit that I have just described, it uses the SPI to
interface to the ADC, plus uses a single port pin from the processor to switch
direction (axis) of the sample. Also note, the addition of an optional interrupt to
the MPU that would tell me that the Stylus is pressing the Touch Panel, this way I
do not have to "poll" the ADC, just wait until the user presses the panel!



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Tom Walsh - WN3L - Embedded Systems Consultant
'', ''
"Windows? No thanks, I have work to do..."

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