Information

scan period during low power mode, so contributing to smaller average power
consumption. The TSI, in low power mode, has the capability to wake up 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.
48.3.3 Touch detection unit
The touch detection unit indicates any change in the low power electrode pin capacitance.
The purpose of this module is to only wake up th 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 pre-
configured 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, so minimizes the CPU usage.
48.4 Modes of operation
The TSI module has three operation modes: disabled, active mode and low power mode.
Table 48-1. 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. 2, Feb 2012
1190 Freescale Semiconductor, Inc.