Information

51.7.3 Electrode scan unit
This section describes the functionality of the electrode scan unit, which is responsible
for triggering the start of the active electrode scan.
The TSI module needs to periodically scan all active electrodes to determine whether 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 scan period
during low-power mode, thereby 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 a shorter scan
period can provide a faster response time and more robust touch detection. Apart from
the periodical mode, the electrode scan unit also allows software triggering of the
electrode scans. This feature is very useful for initialization of the touch application for
detecting the initial electrode capacitances. This module generates configurable end-of-
scan interrupt to indicate the application that all electrodes were scanned. In the event
starting a new electrode scan while a previous one is still in progress an overrun error flag
is generated.
51.7.3.1 Active electrodes
The electrode scan unit is responsible to start the capacitance measurement of all active
electrodes. Each electrode pin should be activated by writing a 1 to the respective
PEN[PEN] bit.
After an electrode scan is triggered, the electrode scan unit, controls the scanning of all
the active electrodes sequentially. It starts the scanning of the electrode pin TSI_IN[0]
and goes sequentially, scanning until it reaches the electrode pin TSI_IN[15]. The
electrode pins that do not have their enable bit (PEN[PEN]) are not scanned and are
skipped.
Only one electrode pin is functional in the low-power mode scan and it is defined by the
field PEN[LPSP]. In low-power scan mode, the configuration of PEN[PEN] bits are
ignored.
Functional description
K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012
1432
Preliminary
Freescale Semiconductor, Inc.
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