K60P100M100SF2RM Datasheet, PDF (1781/1809 Pages) Freescale Semiconductor, Inc – K60 Sub-Family Reference Manual

Chapter 55 Touch sense input (TSI)
TSICHnCNT
Iref
*
PS *NSCN
C _* I ref ref
*
Celec
Figure 55-70. Equation 5: Capacitance result value
In the example where F_{ref_osc} = 10.0MHz and T_{cap_samp} = 48 µs, TSICHnCNT = 480
55.7.3 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
electrode scan unit also allows software triggering of the electrode scans. This feature is
very useful for initialization of the touch application to detect the initial electrode
capacitances. This module generates configurable end-of-scan interrupt to indicate the
application that all electrodes were scanned. If a new electrode scan is started while the
previous one is still in progress, an overrun error flag is generated, TSI continues the
previous scan sequence and the latest trigger is ignored.
55.7.3.1 Active electrodes
The electrode scan unit starts the capacitance measurement of all active electrodes. Each
electrode pin should be activated by writing a 1 to the respective PEN[PEN] 16-bit field.
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 pin that does not have its bit (PEN[PEN]) enabled is not scanned and is
skipped.
Only one electrode pin is functional in the low power mode scan and it’s defined by the
PEN[LPSP]. In low power scan mode, the configuration of PEN[PEN] bits is ignored.
K60 Sub-Family Reference Manual, Rev. 6, Nov 2011
Freescale Semiconductor, Inc.
1781