STMPE321QTR datasheet(7/40 Pages) STMICROELECTRONICS

7 / 40 page

STMPE321

Capacitance compensation

Doc ID 15791 Rev 3

7/40

2.1

Calibration algorithm

The STMPE321 maintains 2 parameters for each touch channel: TVR and CALIBRATED

IMPEDANCE. CALIBRATED IMPEDANCE is an internal reference which, if the currently

measured IMPEDANCE exceeds the CALIBRATED IMPEDANCE by a magnitude of TVR, is

considered a “TOUCH”.

If the IMPEDANCE is higher 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 have 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.

‘IMP’ and ‘CALIBRATED IMP’ used in this table is not the direct register read-out.

IMP = 127 - impedance register readout

CALIBRATED IMP = 127 - calibrated impedance register readout.

The ETC WAIT register states a period of time for which all TOUCH inputs must remain "NO

TOUCH" for the next calibration to be carried out.

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

are prolonged NO TOUCH conditions.

Table 3.

Calibration action under different scenarios

Scenario

Touch sensing and calibration action

IMP>CALIBRATED IMP + TVR

TOUCH,

no calibration

IMP<CALIBRATED IMP + TVR

IMP>CALIBRATED IMP + EVR

NO TOUCH,

no calibration

IMP<CALIBRATED IMP + TVR

IMP<CALIBRATED IMP + EVR

IMP>CALIBRATED IMP

NO TOUCH,

new CALIBRATED IMP = previous CALIBRATED

IMP + change in IMP

IMP>CALIBRATED IMP

CALIBRATED IMP + change in IMP

IMP<CALIBRATED IMP

NO TOUCH,

new CALIBRATED IMP = new IMP

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