Issue
114 January 2000
Reach
Out and Touch
Designing
a Resistive Touchscreen
Taking
the 5-wire route
Burr-Brown
has the ADS7845 device for a 5-wire touchscreen interface.
Like its cousin the ADS7843, it connects to your microprocessor
via a simple serial interface. It too uses a 12-bit
ADC. The pinouts of the chips are nearly identical.
The 5-wire device uses one of the two spare analog inputs
available on the 4-wire device to accommodate the fifth
wire input.
Another
way to interface to a 5-wire touchscreen is to do it
all with a PIC. As shown in Figure
6, this method results in a low parts count. The
thing that makes this design easy is the fact that we
can control the four corners of the bottom plane with
the PIC’s digital drivers and run the sense-plane
wire directly into the PIC’s on-chip ADC. Thus
eliminating the need for fancy analog multiplexing using
external FETs.
|
(Click
here to enlarge)
|
Figure
6—Four of the PIC’s digital lines are used to provide
x and y voltage gradients. A single analog input
is used to determine the contact point by measuring
the pick-off voltage in both the x and the y planes.
|
As
you can see, we used four PIC I/O lines (RB2–5)
to connect to the four corners, labeled UL (upper left),
LL (lower left), UR (upper right), and LR (lower right).
To generate a left-to-right voltage gradient, the PIC
sets UL and LL to a low (0 V) and sets UR and LR high
(5 V). It then performs an ADC conversion, reading AN0.
The
presence of a voltage greater than a few counts indicates
a touch. The bleed resistor R5 in Figure 6 pulls the
ADC input low, so we have no problem knowing that a
touch has occurred.
To
generate a top-to-bottom voltage gradient, the PIC simply
sets UL and UR to high and LR and LL to low. Note that
LL is always low and UR is always high. Although we
could hardwire them to ground and +5 V respectively,
it’s better to allow the PIC to do this to preserve
balanced levels on all four corners.
Once
the PIC has secured readings for the x and y
directions, it can adjust for offset and scale factors,
and determine the x and y positions. The
PIC I used was a PIC16C71 with an 8-bit ADC, which works
for applications where positional accuracy is not important.
Newer members of the PIC family have better accuracy
and would improve this design.
Look
Ma, No Processor
If
you want a simple interface to a 4-wire glass and you
can live with a predefined output format, the TriTech
TR88L811 chip makes it possible to go from the glass
to a serial bitstream with only one chip. If you have
an extra serial channel available and only need 10 bits
of positional accuracy, then this may be the way to
go.
The TR88L811 is designed for standalone applications
and requires only a 1.8432-MHz crystal and an RS-232–level
shifter to form a complete interface that you can attach
to a spare PC COM port.
Figure
7 shows an example circuit that steals its power from
the PC’s COM port. The TriTech device scans the
touchscreen continuously and sends a serial data packet
out of its TxD pin when a touch is detected. The data
packet, sent at 19,200 bps, contains five bytes—a
header and two bytes each of x and y position.
The position is resolved to 10 bits, which is adequate
for most applications.
|
(Click
here to enlarge)
|
Figure
7—The processor in this interface is actually the
Tritec TR88L811, a dedicated 4-wire touchscreen
interface device. It handles all of the touchscreen
scanning, touch detection, and message formatting
chores. The 3-volt device was originally developed
for the PDA industry. |
If
a touch is maintained, the chip will send data out at
a rate of approximately 200 coordinate pairs per second.
The main advantage of this device is that it requires
no firmware. As long as you can live with the 10-bit
resolution and can work with its output format, then
it’s a turnkey solution.
You
can buy the raw touchscreen glass from several manufacturers.
I’ve had a good experience dealing with the Bergquist
Company and I’ve also been successful interfacing
to glass made by Elo and Microtouch.
