Touch technology has changed from a touch point to a 2D touch area. The most common 2D touch applications are PC navigation touch pads and LCD touch screens. MAX® IIZ CPLDs are ideal for touch-pad and touch-screen solutions and for integrating capacitive touch switches into products.

Single and Two-Point Touch Solutions

Single touch point solutions have been available for years. Single point solutions are traditionally implemented with resistive or capacitive 4-wire Iridium-Tin-Oxide (ITO) screen overlays. These touch screen solutions are limited to decoding a single point at a time and two or more touches are incorrectly interpreted. In addition, the solutions are analog and require sophisticated analog electronics for accurate decoding. MAX IIZ devices offer an all digital alternative, using a digital resistive ITO touch screen.

The Altera® digital touch reference design decodes all single and two-point touch patterns and can decode many 3, 4, and 5 point patterns. The digital touch-screen controller reference design converts the 30-pin touch screen output to industry standard serial interface output. MAX IIZ devices allow easy modification of the design to accommodate proprietary serial or parallel interfaces. The MAX IIZ Digital Touch Screen Development board (available in Q1 '09) includes a MAX IIZ controller card, 5.7” Digital SmartTouch screen, and a 4” digital touch pad.

Multipoint Touch Solutions

A multitouch screen or pad takes the flexibility of a user interface to a new level. The Altera multitouch reference design can interpret an unlimited number of simultaneous touch points. The reference design works with up to a 14-cm x 16-cm capacitive touch pad or capacitive touch screen. The 2D multitouch reference design is based on a MAX IIZ EPM240Z CPLD and an Analog Devices AD7142 integrated capacitance-to-digital converter (CDC).

The AD7142 device, which senses the capacitance variant of the ITO sensor board, has only 14 capacitance sensor channels. In our reference design, the MAX IIZ CPLD expands the AD7142 device's capability to handle a two-dimensional ITO glass or film (see Figure 1). The application processor uses I2C (see Figure 2) to access the CDC register file of the AD7142 device and sets the MAX IIZ CPLD to drive the SRC signal to the appropriate axle.

The reference design uses SPI or I2C to access the CDC register file of the AD7142 device and sets the MAX IIZ CPLD to select which column of the touch sensor is being actively scanned. The MAX IIZ CPLD also generates an interrupt signal when the touch screen first senses a touch after a long pause. The MAX IIZ CPLD enhances the ability of the AD7142 to sense a 1 x 14 array of capacitive touch points to a 16 x 14 array of sense points.

Figure 1. Multitouch Diagram

Figure 2. Touch Points

The Altera reference design offering provisions for power-down or sleep modes. The first level of sleep is obtained by the application processor reducing the sample rate. The application processor can also sample subset horizontal and vertical traces, and use the accuracy of the AD7142 device to interpolate touches between active traces. The maximum power savings is achieved when the application processor is powered down, and the MAX IIZ CPLD implements a very power-efficient capacitance detection system to sense when the screen is touched. Once the CPLD detects a touch, it can wake the processor using an interrupt signal. Once awake, the system can properly read the touch location.