Information

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
48.7.1.3 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 48-38. Equation 4: TSI reference oscillator frequency
Where:
C
ref
: Internal reference capacitor
I
ref
: Reference oscillator current source
∆V : Hysteresis delta voltage
Considering C
ref
= 1.0 pF, I
ref
= 12 µA and ∆V = 600 mV, follows
F
ref_osc
12µA
2 *1.0pF
*
600mV
10.0MHz
48.7.2 TSI measurement result
The capacitance measurement result is defined by the number of TSI reference oscillator
periods during the sample time and is stored in the TSICHnCNT register.
TSICHnCNT = T
cap_samp
* F
ref_osc
Chapter 48 Touch sense input (TSI)
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
Freescale Semiconductor, Inc. 1205