NXP Semiconductors K32 L2A Series, Reference Manual, Page: 1275 / 1379

Electrode
Voltage
Time
Electrode Capacitor Charging and
Discharging with constant current
Hysteresis
Voltage Delta
Figure 49-3. TSI electrode oscillator chart
The oscillator frequency is give by the following equation
F
elec
I
2 * C
elec
*
Δ
V
Equation 4. TSI electrode oscillator frequency
Where:
I: constant current
C
elec
: electrode capacitance
Δ
V: Hysteresis delta voltage
So by this equation, for example, an electrode with C
elec
= 20 pF, with a current source of
I = 16 µA and
Δ
V = 600 m V have the following oscillation frequency:
F
elec
16 µA
2 * 20pF
*
600mV 0.67MHz
Equation 5. TSI electrode oscillator frequency
The current source is used to accommodate the TSI electrode oscillator frequency with
different electrode capacitance sizes.
49.4.1.2 Electrode oscillator and counter module control
The TSI oscillator frequency signal goes through a prescaler defined by the GENCS[PS]
and then enters in a modulus counter. GENCS[NSCN] defines the maximum count value
of the modulus counter.
Chapter 49 Touch Sensing Input (TSI)
K32 L2A Reference Manual, Rev. 2, 01/2020
NXP Semiconductors 1275