NXP Semiconductors MKW01Z128, Reference Manual, Page: 828 / 884

Electrode
Voltage
Time
Electrode Capacitor Charging and
Discharging with constant current
Hysteresis
Voltage Delta
Figure 35-3. 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 = V have the following oscillation frequency:
Figure 35-4. TSI electrode oscillator frequency
The current source is used to accommodate the TSI electrode oscillator frequency with
different electrode capacitance sizes.
35.4.1.2 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.
The pin capacitance sampling time is given by the time the module counter takes to go
from 0 to its maximum value, defined by NSCN. The electrode sample time is expressed
by the following equation:
T
cap_samp
PS * NSCN
F
elec
Chapter 35 Touch Sensing Input (TSI)
MKW01Z128 MCU Reference Manual, Rev. 3, 04/2016
Freescale Semiconductor, Inc. 697