Information

F
elec
I
2 * C
elec
*
ΔV
Figure 48-35. 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 48-36. Equation 2: TSI electrode oscillator frequency
The current source is used to accommodate the TSI electrode oscillator frequency with
different electrode capacitance sizes.
48.7.1.2 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 module counter. The bit field GENCS[NSCN] defines the maximum
count value of the module counter.
The pin capacitance sampling time is given by the time the module counter takes to go
from zero to its maximum value, defined by NSCN. The electrode sample time is
expressed by the following equation:
T
cap_samp
PS * NSCN
F
elec
Using Equation 1.
T
cap_samp
2 * PS * NSCN * Celec * ΔV
I
Figure 48-37. Equation 3: Electrode sampling time
Where:
PS: prescaler value
NSCN: module counter maximum value
Functional descriptions
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
1204 Freescale Semiconductor, Inc.