42.4.1 Capacitance measurementThe electrode pin capacitance measurement uses a dual oscillator approach. Thefrequency of the TSI electrode oscillator depends on the external electrode capacitanceand the TSI module configuration. After going to a configurable prescaler, the TSIelectrode oscillator signal goes to the input of the module counter. The time for themodule 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 oscillatorThe TSI electrode oscillator circuit is illustrated in the following figure. A configurableconstant current source is used to charge and discharge the external electrodecapacitance. A buffer hysteresis defines the oscillator delta voltage. The delta voltagedefines the margin of high and low voltage which are the reference input of thecomparator in different time.Figure 42-8. TSI electrode oscillator circuitThe current source applied to the pad capacitance is controlled by theSCANC[EXTCHRG]. The hysteresis delta voltage is defined in the module electricalspecifications present in the device Data Sheet. The figure below shows the voltageamplitude waveform of the electrode capacitance charging and discharging with aprogrammable current.Functional descriptionKL25 Sub-Family Reference Manual, Rev. 3, September 2012794 Freescale Semiconductor, Inc.
