42.4.1 Capacitance measurement
The electrode pin capacitance measurement uses a dual oscillator approach. The
frequency of the TSI electrode oscillator depends on the external electrode capacitance
and the TSI module configuration. After going to a configurable prescaler, the TSI
electrode oscillator signal goes to the input of the module counter. The time for the
module counter to reach its module value is measured using the TSI reference oscillator.
The measured electrode capacitance is directly proportional to the time.
42.4.1.1 TSI electrode oscillator
The TSI electrode oscillator circuit is illustrated in the following figure. A configurable
constant current source is used to charge and discharge the external electrode
capacitance. A buffer hysteresis defines the oscillator delta voltage. The delta voltage
defines the margin of high and low voltage which are the reference input of the
comparator in different time.
Figure 42-8. TSI electrode oscillator circuit
The current source applied to the pad capacitance is controlled by the
SCANC[EXTCHRG]. The hysteresis delta voltage is defined in the module electrical
specifications present in the device Data Sheet. The figure below shows the voltage
amplitude waveform of the electrode capacitance charging and discharging with a
programmable current.
Functional description
KL25 Sub-Family Reference Manual, Rev. 3, September 2012
794 Freescale Semiconductor, Inc.
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