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
42.4.2 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:
TSICHnCNT
Iref * PS *NSCN
Cref
*
Ielec
* Celec
Figure 42-14. Equation 5: Capacitance result value
In the example where F
ref_osc
= 10.0MHz and T
cap_samp
= 48 µs, TSICHnCNT = 480
42.4.3 Enable TSI module
The TSI module can be fully functional in run, wait and low power modes. The
TSI_GENCS[TSIEN] bit must be set to enable the TSI module in run and wait mode.
When TSI_GENCS[STPE] bit is set, it allows the TSI module to work in low power
mode.
42.4.4 Software and hardware trigger
The TSI module allows a software or hardware trigger to start a scan. When a software
trigger is applied ( TSI_GENCS[STM] bit clear), the TSI_GENCS[SWTS] bit must be
written "1" to start the scan electrode channel that is identified by TSI_DATA[TSICH].
When a hardware trigger is applied ( TSI_GENCS[STM] bit set), the TSI will not start
scanning until the hardware trigger arrives. The hardware trigger is different depending
on the MCU configuration. Generally, it could be an event that RTC overflows. See chip
configuration section for details.
Chapter 42 Touch Sensing Input (TSI)
KL25 Sub-Family Reference Manual, Rev. 3, September 2012
Freescale Semiconductor, Inc. 797
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