QST104 datasheet(9/45 Pages) STMICROELECTRONICS

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QST104

QST touch sensing technology

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3.7

Fast positive recalibration

The device autorecalibrates a key when its signal reflects a decrease in capacitance higher

than a fixed threshold (PosRecalTh) for a defined number of acquisitions (PoseRecalI).

3.8

Forced key recalibration

A recalibration of the device may be issued at any time by sending to the QST device the

appropriate I2C command or by tying the RESET pin to ground.

It is possible to recalibrate independently any individual key using an I2C command.

3.9

Max On-Duration

The device can time out and automatically recalibrate each key independently after a fixed

duration of continuous touch detection. This prevents the keys from becoming ‘stuck on’ due

to foreign objects or other sudden influences. This is known as the Max On-Duration feature.

After recalibration, the key will continue to operate normally, even if partially or fully

obstructed. Max On-Duration works independently per channel: a timeout on one channel

has no effect on another channel.

Infinite timeout is useful in applications where a prolonged detection can occur and where

the output must reflect the detection no matter how long. In infinite timeout mode, the

designer should take care to ensure that drift in CS, CX, and VDD do not cause the device to

remain “stuck on” inadvertently even when the touching object is removed from the sense

field. Timeout durations are not accurate and can vary substantially depending on VDD and

temperature values, and should not be relied upon for critical functions.

3.10

Drift compensation

Signal drift can occur because of changes in CX, CS, and VDD over time. Depending on the

CS type and quality, the signal may vary substantially with temperature and veiling. If keys

are subject to extremes of temperature or humidity, the signal can also drift. It is crucial that

drift be compensated, otherwise false detections, non detections, and sensitivity shifts will

follow.

Drift compensation slowly corrects the reference level of each key while no detection is in

effect. The rate of reference adjustment must be performed slowly or else legitimate

detections can also be ignored. The device compensates drift on each channel

independently using a maximum compensation rate to the reference level.

Once a touch is sensed, the drift compensation mechanism ceases since the signal is

legitimately high, and therefore should not cause the reference level to change.

The signal drift compensation is “asymmetric”: the reference level compensates drift in one

direction faster than it does in the other. Specifically, it compensates faster for increasing

signals than for decreasing signals. Decreasing signals should not be compensated for

quickly, since an approaching finger could be compensated for partially or entirely while

approaching the sense electrode. However, an obstruction over the sense pad, for which the

sensor has already made full allowance, could suddenly be removed leaving the sensor with

an artificially elevated reference level and thus become insensitive to touch. In this latter

case, the sensor will compensate for the object's removal very quickly, usually in only a few

seconds.

QST104 Datasheet(PDF) 9 Page - STMicroelectronics

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