Electronic Design

Haptic Feedback Chips Make Virtual-Button Applications On Handheld Devices A Snap

V irtual pushbutton switches on handheld screens may work well, but users also want haptic feedback telling them their button-push was effective. Haptic feedback enthusiasts fondly remember the “buckling spring” technology of the IBM Model M keyboard, but time moves on.

Designers who want to include haptic feedback for virtual buttons on a flat piece of glass today should look at Maxim’s MAX11810 and MAX11811 (25-MHz SPI or 400-kHz I2C). These touch-interface systems integrate four-wire touchscreen controllers with positional and pressure sensing, plus haptic-output drivers and infrared-based proximity sensing.

Haptic feedback, which works as well with permanently dedicated touch sensors associated with permanent “button” locations as with on-screen buttons, is being added not just to cell phones but also to MP3 players, portable media players, digital photo frames, multifunction printers, point-ofsale and financial terminals, barcode scanners, card readers, automobile systems, and industrial-control equipment.

Modern handheld devices use dc motors that spin eccentric masses or piezo devices to generate haptic feedback. Yet new designs use the more versatile piezo linear resonant actuator (LRA). Working with the LRA, the Maxim devices can generate more than 50,000 different haptic patterns thanks to a built-in haptic waveform generator, accessed by means of programmable registers. They also offload the haptic feedback functions from the application processor or system microcontroller, taking some processing load off the controller and reducing latency.

For example, their on-chip processing can validate touch events before they are sent to the application processor. Also, a dynamic-aperture feature provides position hysteresis (avoiding touches around a previous touch), which reduces latency in the touch response. And working autonomously from the applications processor, an on-chip FIFO buffers data transfer between the devices and application processor.

Taken together, these features enhance continuous input applications such as on-screen drawing and writing, where a direct connection between the touch-sensor and application processor would result in many missed data points, impacting resolution. Meanwhile, the infrared sensing de-sensitizes the handheld device’s touchscreen capability in cell-phone applications when the instrument is held up next to the user’s ear. Activating functions like redial with the user’s nose in those circumstances is not a desirable addition to the feature set.

The chips also reduce the parts count in the bill of materials. For instance, they incorporate their own H-bridge drivers for dc-motor vibrators as well as drivers for the external high-voltage amp necessary for piezo actuators. Both chips, as well as two versions without the position hysteresis and on-chip FIFO, operate on supply voltages from 1.7 to 3.6 V.

The full-featured MAX11810 and MAX11811 are available in a 2.1- by 2.1-mm, 20-pin thin quad flat no-lead (TQFN) package and 16-pin wafer-level packaging (WLP) and cost $1.81 and $176 each in 1000-unit lots. DON TUITE


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