STMPE821 datasheet(7/55 Pages) STMICROELECTRONICS

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STMPE821

Capacitive compensation

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Capacitive compensation

The STMPE821 is capable to measuring up to 5.0 pF in capacitance difference between the

reference point (Zref) and the individual channels. In the case where the PCB connection

between the sensor pads and the device is too long, the "REFERENCE DELAY" register is

able to shift the reference by up to 5.0 pF, allowing the TOUCH channels to measure added

capacitance 5.0 pF with offset of 5.0 pF, as shown in following diagram.

In case this is still not enough to compensate for the capacitance on sensor lines (due to

very long sensor trace), an external capacitor of up to 30 pF can be connected at the A_Ref

pin. This allows to further shift up the dynamic range of the capacitance measurement.

Figure 4.

Capacitance compensation

The sensed capacitance is accessible to host through the "IMPEDANCE" registers.

2.1

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.

Increasing capacitance

30.0pF

Reference delay register = 0

Dynamic range = 0 - 5.0 pF

Reference delay register = 5.0 pF

Dynamic range = 5.0 - 10.0 pF

Reference delay = 5.0 pF

ARef = 30 pF

Dynamic range = 35.0 - 40.0 pF

5.0 pF

CS00052

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