R01UH0823EJ0100 Rev.1.00
Page 1513 of 1823
Jul 31, 2019
RX23W Group
43. Capacitive Touch Sensing Unit (CTSU)
43.3
Operation
43.3.1
Principles of Measurement Operation
shows the measurement circuit.
Figure 43.4
Measurement Circuit (m = 2, 3, 4, 7, 8, 12, 13, 22, 23, 27, 30, 35)
The electrostatic capacitance measurement operation principles of the CTSU current frequency conversion method are
explained using
(1) The electrostatic capacitance of the electrode is charged by turning SW1 on and SW2 off (
).
(2) The charged capacitance is discharged by turning SW1 off and SW2 on (
Current flows to the switched capacitor filter by switching between charging and discharging in steps (1) and (2). At this
time, the value of electrostatic capacitance varies depending on whether a finger is in close proximity, so the flowing
current changes. A clock is generated by supplying the control current, which is proportional to the amount of the current
flowing through the switched capacitor filter, from the circuit that generates the TSCAP power supply to the ICO. The
counter is used to measure the clock frequency which changes depending on whether a finger is in close proximity, and
the value read from the counter is used by software to determine contact with a finger (
Figure 43.5
Charging Operation (m = 2, 3, 4, 7, 8, 12, 13, 22, 23, 27, 30, 35)
SW1
SW2
ICO
Switched
capacitor filter
R = 1/(fC)
Counter
Sensor drive pulse
LPF
Reference
electric
potential
-
+
VCC
TSCAP
Sensor
(electrode)
Control
current
Power
supply
TSm
SW1
SW2
ICO
Switched
capacitor filter
i = fCV
Counter
Sensor drive pulse
LPF
-
+
VCC
TSCAP
Sensor
(electrode)
Control
current
Reference
electric
potential
Power
supply
TSm