NXP Semiconductors KL25 Series - Freescale Semiconductor, Inc

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 42-12. 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 we have that an electrode with C = 20 pF, with a current source of I = 16
µA and ΔV = 600 mV, PS = 2 and NSCN = 16 have the following sampling time:
T
cap_samp
2*2*16*20pF*600mV
16µA
48µs
42.4.1.3 TSI reference oscillator
The TSI reference oscillator has the same topology of the TSI electrode oscillator. The
TSI reference oscillator instead of using an external capacitor for the electrode oscillator
has an internal reference capacitor.
The TSI reference oscillator has an independent programmable current source controlled
by the SCANC[REFCHRG].
The reference oscillator frequency is given by the following equation:
F
ref_osc
Iref
2 *Cref
*
ΔV
Figure 42-13. Equation 4: TSI reference oscillator frequency
Where:
Functional description
KL25 Sub-Family Reference Manual, Rev. 3, September 2012
796 Freescale Semiconductor, Inc.