Freescale Semiconductor Evaluation Board MPR031/2 for 2- or 3-pad touch sensor controllers DEMOMPR031 DEMOMPR031 データシート
製品コード
DEMOMPR031
MPR03X
Sensors
16
Freescale Semiconductor
7
Theory of Operation
7.1
Introduction
The MPR03X utilizes the principle that a capacitor holds a fixed amount of charge at a specific electric potential. Both the
implementation and the configuration will be described in this section.
implementation and the configuration will be described in this section.
7.2
Capacitance Measurement
The basic measurement technique used by the MPR03X is to charge up the capacitor C on one electrode input with a DC current
I for a time T (the charge time). Before measurement, the electrode input is grounded, so the electrode voltage starts from 0 V
and charges up with a slope,
I for a time T (the charge time). Before measurement, the electrode input is grounded, so the electrode voltage starts from 0 V
and charges up with a slope,
). All of the other electrodes
are grounded during this measurement. At the end of time T, the electrode voltage is measured with a 10 bit ADC. The voltage
is inversely proportional to capacitance according to
is inversely proportional to capacitance according to
The electrode is then discharged back to ground at the same
rate it was charged.
Equation 1
Equation 2
Figure 21. MPR03X Electrode Measurement Charging Pad Capacitance
When measuring capacitance there are some inherent restrictions due to the methodology used. On the MPR03X the voltage
after charging must be in the range that is shown in
after charging must be in the range that is shown in
.
Figure 22.
C
I
dt
dV
C
T
I
V
Electrode Charge Time
T
V
Electrode
Voltage
Electrode
Charging
Electrode
Discharging
Electrode voltage measured here
2T
Electrode Discharge Time
Valid ADC Values vs. VDD
0
100
200
300
400
500
600
700
800
900
1.71
1.91
2.11
2.31
2.51
2.71
VDD (V)
ADC
Counts
ADChigh
ADCmid
ADClow