K60P100M100SF2RM Datasheet, PDF (1779/1809 Pages) Freescale Semiconductor, Inc – K60 Sub-Family Reference Manual

Chapter 55 Touch sense input (TSI)
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 will have the following oscillation frequency:
F_elec
16 µA
2 * 20pF _* 600mV
0.67MHz
Figure 55-67. Equation 2: TSI electrode oscillator frequency
The current source and hysteresis delta voltage are used to accommodate the TSI
electrode oscillator frequency with different electrode capacitance sizes.
55.7.1.2 Electrode oscillator and counter control
The TSI oscillator frequency signal goes through a prescaler defined by the GENCS[PS]
and then enters a counter. The bit field GENCS[NSCN] defines the number of scans for
each external electrode.
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 55-68. Equation 3: Electrode sampling time
Where:
PS: prescaler value
NSCN: number of scan
I: constant current
C_elec: electrode capacitance
ΔV: Hysteresis delta voltage
By this equation, an electrode with C = 20 pF, with a current source of I = 16 µA and ΔV
= 600 mV, PS = 2 and NSCN = 16 will have the following sampling time:
K60 Sub-Family Reference Manual, Rev. 6, Nov 2011
Freescale Semiconductor, Inc.
1779