NXP Semiconductors KL25 Series - Freescale Semiconductor, Inc

Electrode
Voltage
Time
Electrode Capacitor Charging and
Discharging with constant current
Hysteresis
Voltage Delta
Figure 42-9. TSI electrode oscillator chart
The oscillator frequency is give by the following equation
F
elec
I
2 * C
elec
*
ΔV
Figure 42-10. Equation 1: TSI electrode oscillator frequency
Where:
I: constant current
C
elec
: electrode capacitance
ΔV: 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 ΔV = 600 mV have the following oscillation frequency:
F
elec
16 µA
2 * 20pF
*
600mV
0.67MHz
Figure 42-11. Equation 2: TSI electrode oscillator frequency
The current source is used to accommodate the TSI electrode oscillator frequency with
different electrode capacitance sizes.
42.4.1.2 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. The bit field GENCS[NSCN] defines the maximum
count value of the modulus counter.
Chapter 42 Touch Sensing Input (TSI)
KL25 Sub-Family Reference Manual, Rev. 3, September 2012
Freescale Semiconductor, Inc. 795