Information

48.7.1.1 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 48-33. TSI electrode oscillator circuit
The current source applied to the pad capacitance is controlled by the
SCANC[EXTCHRG]. The hysteresis delta voltage is defined inn the module electrical
specifications present in the device DataSheet.. The figure below shows the voltage
amplitude waveform of the electrode capacitance charging and discharging with a
programmable current.
Electrode
Voltage
Time
Electrode Capacitor Charging and
Discharging with constant current
Hysteresis
Voltage Delta
Figure 48-34. TSI electrode oscillator chart
The oscillator frequency is give by the following equation
Chapter 48 Touch sense input (TSI)
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
Freescale Semiconductor, Inc. 1205