Datasheet

574

32133D–11/2011

UC3D

The capacitive sensor charge length, discharge length, and settle length can be determined for

each acquisition type using the CHLEN, DILEN, and SELEN fields in Configuration Registers 0

and 1. The lengths are specified in terms of prescaler clocks.

27.6.3 Capacitive Count Acquisition

For the QTouch group A, and QTouch group B types of acquisition, the module acquires count

values from the sensors, buffers them, and makes them available for reading in the ACOUNT

27.6.4 Autonomous QTouch and DMATouch

For autonomous QTouch and DMATouch, a complete detection algorithm is implemented within

the CAT module. The additional parameters needed to control the detection algorithm must be

specified by the user in the ATCFG2 and ATCFG3 registers.

Autonomous QTouch and DMATouch sensitivity and out-of-touch sensitivity can be adjusted

with the SENSE and OUTSENS fields, respectively, in ATCFG2. Each field accepts values from

one to 255 where 255 is the least sensitive setting. The value in the OUTSENS field should be

smaller than the value in the SENSE field.

To avoid false positives a detect integration filtering technique can be used. The number of suc-

cessive detects required is specified in the FILTER field of the ATCFG2 register.

To compensate for changes in capacitance the CAT can recalibrate the autonomous QTouch

sensor periodically. The timing of this calibration is done with the NDRIFT and PDRIFT fields in

the Configuration register, ATCFG3. It is recommended that the PDRIFT value is smaller than

the NDRIFT value.

The autonomous QTouch sensor and DMATouch sensors will also recalibrate if the count value

goes too far positive beyond a threshold. This positive recalibration threshold is specified by the

PTHR field in the ATCFG3 register.

The following block diagram shows the sequence of acquisition and processing operations used

by the CAT module. The AISR written bit is internal and not visible in the user interface.