NXP Semiconductors freescale K51 Series Reference Manual

capacitance, while the other according to an internal reference capacitor. The pincapacitance measurement is given by the counted number of periods of the referenceoscillator during a configurable number of oscillations of the external electrode oscillator.The electrode oscillator charges and discharges the pin capacitance with a programmable5-bit binary current source in order to accommodate different sizes of electrodecapacitance. The electrode oscillator frequency, before being compared to that of thereference oscillator, goes through a prescaler and modulo counter to decrease itsfrequency and consecutively increase the measurement resolution and noise robustness.The following figure presents the simplified block diagram of how the electrodecapacitance is measured.TSIElectrodeOscillatorNSCNTSICHnCNTCapacitance Measurement UnitPrescalerCounterModuloControlTSI Reference Oscillator16-bitCounterEXTCHRG DELVOL PSREFCHRG DELVOLElectrodeCapacitanceCAPTRMCLKENFigure 52-2. TSI capacitance measurement unit block diagram52.3.2 Electrode scan unitThis section describes the functionality of the electrode scan unit. It is responsible fortriggering the start of the active electrode scan.The touch sense input module needs to periodically scan all active electrodes todetermine if a touch event has occurred. The electrode scan unit has two independentscan periods, one for TSI active mode and the other for TSI low power mode. Thisindependent control allows the application to configure longer scan period during lowpower mode, so contributing to smaller average power consumption. The TSI, in lowpower mode, has the capability to wake the CPU upon an electrode capacitance change.When the CPU wakes, the TSI enters active mode, and produces a shorter scan period fora faster response and more robust touch detection. Apart from the periodical mode, theChapter 52 Touch sense input (TSI)K51 Sub-Family Reference Manual, Rev. 6, Nov 2011Freescale Semiconductor, Inc. 1495