CRTOUCH Datasheet, PDF (17/68 Pages) Freescale Semiconductor, Inc – Capacitive and Resistive Touch Sensing Application Specific IC.

Functional Description
The maximum distance reported in the Zoom Size register can vary this depends on the proportions of the screen.
2.1.6.4 Rotate gesture
The rotate gesture is reported when a variation of the slope between the fingers is detected. This variation must be at least 10°
for it to be reported in the RTSR of the Status Register 1. The direction of the rotate gesture is reported in the RTSRD in the
Status Register 2. The Rotate Angle register reflects the cumulative angle that has been displaced in that direction. This angle
is reported in radians with 5 fractional bits and 3 integer bits. This determines a limit of 456° of continuous rotation that can be
reported by the CRTouch. The following equation can be used to convert the value of the Rotate Angle register to a value in
degrees.
 = ^---^R----^o---^t--^a----^t--^e---^A_3---2-ⁿ--_-^g---^l---^e---^--^---1---8---0-
Eqn. 6
2.2 Capacitive subsystem
The CRTouch controller supports up to four capacitive electrodes as an extended interface. These electrodes may be detected
independently or they can be arranged in a keypad, rotary or slider control configuration. The capacitive subsystem is enabled
through the Configuration register with the Capacitive Control [1:0] bits according to table 4. Additionally, each of the
electrodes of the CRTouch controller may be individually enabled or disabled in the Electrode Enablers register.
Table 5. Capacitive control configuration bits
Capacitive Control [1:0]
00
01
10
11
Configuration
Capacitive subsystem disabled
Rotary control enabled
Slider control enabled
Keypad control enabled
When enabled, the capacitive electrodes are scanned sequentially, from E0 to E3, at a fixed rate of 7 ms. The capacitive
subsystem of the CRTouch controller has a set of configuration and status registers to control several features.
NOTE
Before the desired control is enabled through the Configuration Register, the desired events
to be detected must be enabled in the Capacitive register; otherwise the control will not be
enabled.
2.2.1 Capacitive touch detection
The touch detection is based on a baseline tracking algorithm and thresholds for touch detection. When the system is enabled,
an initial baseline is calculated and continuous baseline recalibration can be enabled through the DCTracker feature with the
DC tracker enable bit in the capacitive system configuration. This recalibration is executed at a fixed number of scanning
periods, determined by the DC Tracker Rate register value. Since the scanning period is fixed at 7 ms, the recalibration period
is equal to the DC Tracker Rate register value multiplied by 7 ms.
To detect a touch, the capacitance samples taken for each electrode are compared against the baseline. If a number of
consecutive samples exceed a certain threshold then that electrode is considered and reported as touched. These thresholds are
defined by the Ex Sensitivity registers and represent the minimum difference that must exist between a capacitive sample and
CRTouch Data Sheet, Rev. 3
Freescale Semiconductor
17