Information

48.3.1 Electrode capacitance measurement unit
The electrode capacitance measurement unit senses the capacitance of a TSI pin and
outputs a 16-bit result. This module is based in dual oscillator architecture. One oscillator
is connected to the external electrode array and oscillates according to the electrode
capacitance, while the other according to an internal reference capacitor. The pin
capacitance measurement is given by the counted number of periods of the reference
oscillator during a pre-defined number of electrode oscillations.
The electrode oscillator charges and discharges the pin capacitance with a programmable
current source in order to accommodate several different sizes of electrode capacitances.
The electrode oscillator frequency, before being compared to that of the reference
oscillator, goes through a prescaler and module counter to decrease its frequency and
consecutively increase the measurement resolution and noise robustness.
The following figure presents the simplified block diagram of how the electrode
capacitance is measured.
TSI
Electrode
Oscillator
NSCN
TSICHnCNT
Capacitance Measurement Unit
Prescaler
Counter
Modulo
Control
TSI Reference Oscillator
16-bit
Counter
EXTCHRG
DELVOL PS
REFCHRG
DELVOL
Electrode
Capacitance
CAPTRM
CLK
EN
Figure 48-2. TSI capacitance measurement unit block diagram
48.3.2 Electrode scan unit
This session describes the functionality of the electrode scan unit. It is responsible for
triggering the start of the active electrode scan.
The touch sense input module needs to periodically scan all active electrodes to
determine if a touch event has occurred. The electrode scan unit is responsible for
defining two independent scan periods, one for TSI active mode and the other for TSI
low power mode. This independent control allows the application to configure longer
Chapter 48 Touch sense input (TSI)
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
Freescale Semiconductor, Inc. 1187