CRTOUCH Datasheet, PDF (18/68 Pages) Freescale Semiconductor, Inc – Capacitive and Resistive Touch Sensing Application Specific IC.

Functional Description
the signal baseline for a touch to be detected. To avoid false touch detection due to sporadic noise, the Response Time register
defines how many samples the algorithm will consider to determine a touch or a no touch condition.
After an electrode has been detected as touched, recalibration is not performed for that electrode to avoid recalibrating the
baseline at a non-idle signal level. However, under certain environmental conditions, large and sudden changes in the measured
capacitance may occur which may trigger a touch detection. If this condition persists (excessive humidity over an electrode, for
example), the affected electrode will not be able to detect real touches. For this kind of scenario, the stuck key feature allows
the host to define a touch timeout,after which the touched status is released and the electrode is recalibrated.
The actual status of each individual electrode may be read at the Electrode Status register. Each bit represents the status of one
electrode, reporting a 1 when the electrode is detected as touched and a 0 when the electrode is not touched.
For application customization of these parameters, the CRTouch controller provides a set of registers that allow the analysis of
the capacitance behavior of each electrode under the specific application conditions. The Electrode Baseline registers and
Electrode Instant Delta registers provide this information.
The resolution of the capacitive samples and the calculated baselines can be modified through the Capacitive Resolution
registers. Increasing or decreasing the resolution is useful depending on the thickness of the dielectric used. This allows to
modify the signal (touch) to noise ratio for more flexibility on the touch detection. This value is only effective after a reset, a
reset is then needed to make a change in this register to take effect.
2.2.2 Keypad control
The keypad control supports the reporting of touch or release events for each individual electrode. These events are reported
through an events buffer and each of them is enabled through the Events register. The events buffer is mapped to the Electrodes
FIFO register of the device. Each read of this register returns the oldest previously stored in the buffer, until the buffer is empty
and any subsequent read returns a value of 0xFF. The format of the events in the buffer may be consulted in the registers section
of this document.
Additionally to the touch detection, the keypad control provides an autorepeat feature, to log new touch events into the buffer
at a certain rate after detecting the electrode as touched for a period of time. The Auto-repeat Start register controls the timeout
before it starts logging new events, and the Auto-repeat Rate register controls how often a new event is stored in the buffer.
The keypad control also provides the capability of restricting the number of keys that may be pressed at the same time. This is
done by writing a value different than 0 in the Max Touches register. When the number of simultaneous touches is equal to the
Max Touches register, no new touch events for additional keys are stored in the events buffer until one or more keys are released.
2.2.3 Rotary and slider control
The slider and rotary control types provide support for linear or circular arrangements of electrodes. A rotary may be seen as a
circular slider, where the first and last electrodes are adjacent. In the case of a rotary a simultaneous touch of these two electrodes
is considered valid, where in a slider it is not. This is the only difference between these two types of controls, otherwise all of
their behavior is the same.
Slider and rotary controls are oriented to the detection of motion through the control, so they provide the capability to detect
this motion, report its direction, and the amount of displacement. The control is also capable of reporting when the initial touch
is detected, when the motion ends and when all of the control’s electrodes have been released. All these events are enabled
through the Events register.
Two status registers are provided for these controls, the Static Status register and the Dynamic Status register. The Dynamic
Status register reports if movement is being detected at the moment through the Movement Flag, reports the direction of this
movement in the Direction bit and how many positions were advanced since the last status. The static status reports if the control
is currently touched through the touch flag, the position of the touch, and if an invalid position is detected.
To increase the number of positions detectable in a rotary or slider control, positions “between” the electrodes are also reported.
This means that when two electrodes are touched, the control reports a position different than the position of each of the touched
CRTouch Data Sheet, Rev. 3
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Freescale Semiconductor