STMPE1208S datasheet(15/51 Pages) STMICROELECTRONICS

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STMPE1208S

Capacitive sensors

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5.2

Capacitance compensation

The STMPE1208S is capable to measuring up to 7.2 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"

measure added capacitance 7.2 pF with offset of 6.0 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 10.

Capacitance compensation

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

5.3

Setting of TVR and EVR

The STMPE1208S uses 2 main parameters to control the sensitivity and calibration of the

capacitive sensing system. TVR (touch variance) is a channel-specific value, that specifies

the number of steps the sensed capacitance must be above the internal reference, to be

considered a touch. Generally, this should be set as 4

−10, but it must be bigger than EVR.

The EVR (environment variance) is a shared value that is applied to all the channels. This

specifies the maximum change in capacitance that can be considered due to the shifting of

the environmental factor. Generally, this should be set to 1

−5, but it must be less than TVR.

Environment tracking calibration

On power up, a calibration is executed. The initial calibration takes about 150 clock cycles of

sensor clock for completion. Using 5 kHz sensor clock, this would be 30 mS.

However, if any of the sensors are touched during powering up, calibration is delayed, until

all sensors are untouched. In this case, the time taken for calibration, from the time when all

sensors are untouched is:

2 * calibration interval + 150 * sensor clock period

Increasing capacitance

30.0pF

Reference delay register = 0

Dynamic range = 0 - 7.2 pF

Reference delay register = 6.0 pF

Dynamic range = 6.0 - 13.2 pF

Reference delay = 6.0 pF

S_REF external capacitance = 30 pF

Dynamic range = 36 - 43.2 pF

6.0 pF

7.2 pF

6.0 pF

7.2 pF

CS00043

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