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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.
51.3.3 Touch detection unit
The touch detection unit indicates any change in the low-power electrode pin
capacitance. The purpose of this module is to wake up only the CPU from low-power
modes in the event of a electrode capacitance change. So, if there is no capacitance
change in the electrode, the MCU stays in low-power mode indefinitely, while keeping
the electrode monitoring, ensuring minimal power consumption.
This module compares the pin capacitance value in the result register with a
preconfigured low and high threshold. If the capacitance result register value is outside
the ranges defined by upper and lower threshold, the touch detection unit generates an
out-of-range flag indicating a pin capacitance change.
The upper and lower threshold values are configurable, allowing the application to select
the magnitude of the capacitance change to trigger the out-of-range flag. With the
threshold values programmed properly, the application noise level does not cause
frequent CPU interrupts, minimizing the CPU usage.
51.4 Modes of operation
The TSI module has three operation modes: disabled, active mode, and low-power mode.
Table 51-1. TSI Module funtionality in MCU operation modes
MCU operation mode TSI clock sources TSI operation mode
when TSIEN = 1
Functional electrode
pins
Required STPE state
Run LPOCLK, MSGIRCLK,
OSCERCLK
Active mode All Don’t care
Wait LPOCLK, MSGIRCLK,
OSCERCLK
Active mode All Don’t care
Table continues on the next page...
Modes of operation
K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012
1414
Preliminary
Freescale Semiconductor, Inc.
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