Information

T
cap_samp
2*2*16*20pF*600mV
16µA
48µs
52.7.1.3 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 which can be programmable. The SCANC[CAPTRM]
defines the internal reference capacitor trimming value *.
The TSI reference oscillator share the same voltage hysteresis levels defined with the
SCANC[DELVOL] and 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 52-69. 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
52.7.2 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
Using Equation 2 and Equation 1 follows:
Functional descriptions
K20 Sub-Family Reference Manual, Rev. 6, Nov 2011
1590 Freescale Semiconductor, Inc.