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ΔV: Hysteresis delta voltage
By this equation, for example, an electrode with C
elec
= 20 pF and a current source of I =
16 µA and ΔV = 600 mV has the following oscillation frequency:
F
elec
16 µA
2 * 20pF
*
600mV
0.67MHz
Figure 51-36. Equation 2: TSI electrode oscillator frequency
The current source is used to accommodate the TSI electrode oscillator frequency with
different electrode capacitance sizes.
51.7.1.2 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. 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 51-37. Equation 3: Electrode sampling time
Where:
PS: prescaler value
NSCN: module counter maximum value
I: constant current
C
elec
: electrode capacitance
ΔV: Hysteresis delta voltage
By this equation, an electrode with C = 20 pF, a current source of I = 16 µA and ΔV =
600 mV, PS = 2, and NSCN = 16 has the following sampling time:
Functional description
K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012
1430
Preliminary
Freescale Semiconductor, Inc.
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