QST104 datasheet(8/45 Pages) STMICROELECTRONICS

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QST touch sensing technology

QST104

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3.3

Faulty and unused keys

Any sensing channel that does not have its sense capacitor (CS) fitted is assumed to be

either faulty or unused. This channel takes no further part in operation unless a Master-

commanded recalibration operation shows it to have an in-range burst count again. Faulty,

unused or disabled keys are still bursted but not processed to avoid modifying the sensitivity

of active keys.

This is important for sensing channels that have an open or short circuit fault across CS.

Such channels would otherwise cause very long acquire bursts, and in consequence would

slow the operation of the entire QST device.

To optimize touch response time and device power consumption, if some keys are not used,

we recommend to try suppressing the ones which belong to the same burst. Bursts which

do not have any keys implemented will then not be processed.

3.4

Detection threshold levels

The key capacitance change induced by the presence of a finger is sensed by the variation

in the number of charge transfer pulses to load the capacitor. The difference in the pulse

count number is compared to a threshold in order to detect the key as pressed or not.

Two different thresholds, one for detection and one for the end of detection, create an

hysteresis in order to prevent erratic behavior.

The default threshold levels and hysteresis values are described inSection 6.5: Capacitive

sensing characteristics on page 33.

3.5

Detection integrator filter

Detect Integrator (DI) filter mechanism works together with spread spectrum operation to

dramatically reduce the effects of noise on key states. The DI mechanism requires a

specified number of measurements that qualify as detections (and these must occur in a

row) or the detection will not be reported.

In a similar manner, the end of a touch (loss of signal) also has to be confirmed over several

measurements. This process acts as a type of “debounce” mechanism against noise.

The default DI value for confirming start of touch and end of touch is described in

Section 6.5: Capacitive sensing characteristics on page 33.

3.6

Self-calibration

On power-up, all keys are self-calibrated to provide reliable operation under almost any

conditions. The calibration phase is used to compute a reference value per key which is then

used by the process determining if a key is touched or not. The reference is an average of 8

single acquisitions. As a result, the calibration time of the system can be simply calculated

using the following formula: tCAL = 8 * Burst_Period. The methodology used to measure the

burst period is described in application note AN2547. For a maximum calibration duration

(tCAL), please refer to Section 6.5: Capacitive sensing characteristics on page 33.

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