
R01UH0241EJ0010 Rev.0.10
Page 621 of 730
Aug 05, 2011
R8C/38T-A Group
25. Touch Sensor Control Unit
Under development Preliminary document
Specifications in this document are tentative and subject to change.
The capacitance of the touch electrode is measured by measuring the voltage at CHxA while gradually discharging
the electric charge stored in Cc. Measurement can be executed with the following steps:
Either CHxA0 or CHxA1 is selected for CHxA and Va0 or Va1 is selected for Va by setting registers TSCHSEL0 to
TSCHSEL2.
(1) Charge Cc by connecting the CHxC pin to the voltage supply (VCC).
(2) After charging Cc fully, discharge Cc by connecting pins CHxA and CHxB to the ground level (VSS).
(3) After discharging Cc for a short period of time, keep pins CHxA, CHxB, and CHxC at high-impedance (Hi-
Z), and measure the voltage at the CHxA pin. At this point, as shown in
Figure 25.11, when the voltage
measured at the CHxA pin is Va, and the voltage measured at the CHxC pin is Vc, Va at the time of voltage
measurement is expressed by the following formula (A).
The Time-dependent Variation of Va and Vc is shown in
Figure 25.12.(4) Repeat steps (2) and (3).
(5) Set an input level for the CHxA pin using the VLT0 register (input threshold value control register 0).
Count the number of discharges before Va falls below the input threshold value. Continue counting until the
secondary counter reaches 0.
(6) The count value is comprised of the primary counter value of data 1 and data 2.
Figure 25.12
Time-dependent Variation of Va and Vc
As the finger comes closer to the touch electrode, a change of
Cx is generated and Va is expressed by the
following formula (B).
Va =
Vc .............formula (A)
Cr
Cr + Cx
Charge Cc
Repeat discharge and
high-impedance
Voltage
Time
Vc
Measure Va at high-impedance
Va (=
Vc)
Cr
Cr + Cx
Va =
Vc .................formula (B)
Cr
Cr + Cx +
Cx