STMPE821_09 Datasheet, PDF (8/55 Pages) STMicroelectronics – 8-channel capacitive touch key controller

STMPE821 functional overview
STMPE821
1.4
Calibration algorithm
The STMPE821 maintains 2 parameters for each TOUCH channel: 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.
Table 3.
Calibration action under different scenarios
Scenario
Touch sensing and calibration action
IMP>CALIBRATED IMP + TVR
IMP<CALIBRATED IMP + TVR
IMP>CALIBRATED IMP + EVR
IMP<CALIBRATED IMP + TVR
IMP<CALIBRATED IMP + EVR
IMP>CALIBRATED IMP
IMP>CALIBRATED IMP
IMP<CALIBRATED IMP
TOUCH,
no calibration
NO TOUCH,
no calibration
NO TOUCH,
new CALIBRATED IMP = previous CALIBRATED
IMP + change in IMP
CALIBRATED IMP + change in IMP
NO TOUCH,
new CALIBRATED IMP = new IMP
‘IMP’ and ‘CALIBRATED IMP’ used in this table is not the direct register read-out.
IMP = 127 - impedance register read-out
CALIBRATED IMP = 127 - calibrated impedance register read out.
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.
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