QST108 datasheet(10/51 Pages) STMICROELECTRONICS

10 / 51 page

QST touch sensing technology

QST108

10/51

Figure 3 shows an example of detection with DI=2 and EDI=2 meaning 3 consecutive

samples are necessary to trigger the key detection or end of detection

Figure 3.

Detection signals

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 6.6: Capacitive sensing characteristics on page 35.

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 (PosRecalI).

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.

Time

Reference count

Reference + EofDeTh

Reference + DeTh

Hysteresis

Key Detection signal

= Sampling point

QST108 Datasheet(PDF) 10 Page - STMicroelectronics