STMPE1208S datasheet(16/51 Pages) STMICROELECTRONICS

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Capacitive sensors

STMPE1208S

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The STMPE1208S maintains 2 parameters for each TOUCH channels: TVR and

CALIBRATED IMPEDANCE. CALIBRATED IMPEDANCE is an internal reference of which, if

the currently measured IMPEDANCE exceeds the CALIBRATED IMPEDANCE by a

magnitude of TVR, it is considered a TOUCH.

If the IMPEDANCE is more than the CALIBRATED IMPEDANCE, but the magnitude does

not exceed CALIBRATED IMPEDANCE by TVR, it is not considered a TOUCH. In this case,

2 scenarios are possible:

1. Environmental changes has caused the IMPEDANCE to increase

2. Finger is near the sensing pad, but not near enough

In case 1, the change in IMPEDANCE is expected to be small, as environmental changes

are normally gradual. A value "EVR" is maintained to specify the maximum IMPEDANCE

change that is still considered an environmental change.

ETC WAIT register state a period of time of which, all TOUCH inputs must remain "NO

TOUCH" for the next calibration to be carried out.

CAL INTERVAL states the period of time between successive calibrations when there are

prolonged NO TOUCH condition.

5.3.1

3-stage data filtering system

The output from the calibration unit is an instantaneous "TOUCH" or "NO TOUCH" status.

This output is directed to the filtering stage where the 2 stage noise filtering and 1 stage

data filtering is applied to the touch status.

Table 6.

Calibration action under different scenarios

Scenario

Touch sensing and calibration action

IMP>CALIBRATED IMP + TVR

TOUCH,

no calibration

IMP>CALIBRATED IMP + EVR

NO TOUCH,

no calibration

IMP<CALIBRATED IMP + TVR

IMP<CALIBRATED IMP + EVR

NO TOUCH,

new CALIBRATED IMP = previous

IMP>CALIBRATED IMP

CALIBRATED IMP + change in IMP

IMP<CALIBRATED IMP

NO TOUCH,

new CALIBRATED IMP = new IMP

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