Issue 114 January 2000
Reach Out and Touch
Designing a Resistive Touchscreen

Development Notes

I began this project by first locating the touchscreen-interface chips. I then needed a quick and dirty means of testing their functionality, which I did using a Basic Stamp II device with a serial LCD attached. It was a simple matter to interface the chips to the Stamp, and then use the interactive Stamp-development environment to debug the chip interfaces.
Of course, a $50 Basic Stamp is not a good solution for a production product. The production hardware was designed around a lower cost PIC, the 16C622. I intended to discard the Stamp code and write the production code in C. However, I had heard about a compiler for the Stamp BASIC, and decided to give it a try. The results were good and enabled me to use most of the stamp code with little modification. I’ve since used this approach (successfully) on other projects.
The compiler is the PIC BASIC Pro (PBP). It is available from microEngineering Labs for about $250.

Firmware Description

Firmware for the 4-wire Burr-Brown design is available on the Circuit Cellar web site (the 5-wire firmware is nearly identical). The PIC’s job is to sense a touch event, and if detected, send out a data packet containing the location of the touch. The PIC should continue to send out this information as long as a touch is detected and send a special packet at the end when the touch has gone away.
The PIC sends a five-byte data packet to communicate the x/y touch coordinates to the host processor (see Figure 8). The first byte in the five-byte header always has its most significant bit set to one. The other four bytes always have their most significant bits set to zero to allow the receiver to synchronize to the packet. The first byte is either a $C0 or $80, depending on whether the touch is active ($C0) or a touch-up event has occurred ($80).

The four bytes that follow hold the x and y coordinate values, with 14 bits allowed. Because we digitize to 12-bit resolution, bits 12 and 13 are zero filled.
The firmware was written as a state machine, as shown in Figure 9. On powerup, the code enters State 0, where it initializes the data direction registers and blinks the LED three times. A transition to State 1 follows, where the code continuously scans the touchscreen, looking for a touch.

When a touch is detected, a transition to State 2 occurs, where the controller sends out a five-byte data packet, and turns on the LED. While in State 2, the controller continues to scan the touchscreen and sends out a new data packet each as long as a touch is detected. If a touch is not detected, a transition to State 3 occurs, where a final data packet is sent with the header byte set to $80 indicating a touch-up event, the LED is turned off, and a transition back to State 1 is initiated.
A low-level subroutine called Convert is used to talk directly to the touchscreen controller chip. The routine is similar for both 4- and 5-wire chips. Convert passes a variable called channel, which contains a 0 or 1. This variable controls whether we read the x or y channel of the device. The 12-bit result comes back in a 16-bit word named ADC.
Convert is called by a higher level subroutine named Read_Glass. This routine determines if a touch has occurred and sets a flag called touch to indicate such. A touch is determined to have occurred if the result of a Convert operation is above a certain noise threshold. When Convert is called and no touch has occurred, a value near zero is returned.

Wrapping it Up

Well, there you have it. You’ve seen four different means of interfacing a resistive touchscreen to your system. The Burr-Brown chips offer high accuracy, off-the-shelf solutions to both 4- and 5-wire glass types. My roll-your-own PIC interface is a minimal parts count design and is best suited to low-accuracy applications. The TriTech chip offers a unique solution in that it involves no firmware. Those of you whose favorite programming language is solder will appreciate that.