K60P100M100SF2RM Datasheet, PDF (1759/1809 Pages) Freescale Semiconductor, Inc – K60 Sub-Family Reference Manual

Chapter 55 Touch sense input (TSI)
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 configurable number of oscillations of the external electrode oscillator.
The electrode oscillator charges and discharges the pin capacitance with a programmable
5-bit binary current source in order to accommodate different sizes of electrode
capacitance. The electrode oscillator frequency, before being compared to that of the
reference oscillator, goes through a prescaler and modulo 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.
Capacitance Measurement Unit
CAPTRM REFCHRG DELVOL
EXTCHRG DELVOL
PS ^{TSI Reference Oscillator}
TSI
Electrode
Oscillator
Prescaler
Counter
Modulo
Control
CLK
16-bit
EN ^Counter
TSICHnCNT
NSCN
Figure 55-2. TSI capacitance measurement unit block diagram
55.3.2 Electrode scan unit
This section 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 has 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 scan period during low
power mode, so contributing to smaller average power consumption. The TSI, in low
power 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 for
a faster response and more robust touch detection. Apart from the periodical mode, the
K60 Sub-Family Reference Manual, Rev. 6, Nov 2011
Freescale Semiconductor, Inc.
1759