Information

Using Equation 2 and Equation 1 follows:
TSICHnCNT
Iref * PS *NSCN
Cref
*
Ielec
* Celec
Figure 48-39. Equation 5: Capacitance result value
In the example where F
ref_osc
= 10.0MHz and T
cap_samp
= 48 µs, TSICHnCNT = 480
48.7.3 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
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.
48.7.3.1 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.
Once 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 does not have its enable bit (PEN[PEN]) are not scanned and are
skipped.
Functional descriptions
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
1208 Freescale Semiconductor, Inc.