CRTOUCH Datasheet, PDF (10/68 Pages) Freescale Semiconductor, Inc – Capacitive and Resistive Touch Sensing Application Specific IC.

Functional Description
2.1.3 Resistive events
The CRTouch has a TouchPending signal that can assert on configurable events in the system to alert the host of a new event
that requires attention. This signal can be used by the host to avoid time based polling of the status of the device and therefore
unnecessary use of the communication bus. Whenever an event is detected in the system, it will be reflected in its associated bit
in Status Register 1. All event bits of this register remain latched and are cleared until the register is read. This avoids losing
any detected event if the host cannot read the status of the device for several sampling periods. Each event can also be enabled
to assert the TouchPending signal through the Triggers Enable register. By setting the bits in this register, when an event is
recorded in the status register the TouchPending signal is asserted, and remains asserted until the status is read.
The most basic events that are related to the touchscreen are the resistive new sample and the touchscreen release events. The
resistive new sample is triggererd when a new sample of the screen coordinates is available, which means that it is produced at
the touch detection and subsequently at every scan period. The release event is produced only when the screen transitions from
touched to not touched. In this case the last valid coordinates value remains in the Coordinate registers.
Additional to the events status, the Status Register 1 reflects the state of the screen through its most significant bits, screen
touched and two touch bits. These bits are updated with each sampling period and unlike the events bits, they are not latched
and are not cleared with a register read, so their value always reflects the state of the screen at the last scanning sequence.
When the screen transitions to the touched state, the Resistive Touch Screen Touched and the Resistive New Sample Event bits
in the Resistive Touch Status Register 1 are set. With every new coordinate sample, the Resistive Touch Screen Touched and
the Resistive New Sample Event bits will be set after the coordinate values are available in the X and Y coordinates registers.
When the screen is released, the Resistive New Sample Event bit will be set, indicating the end of the scanning sequence, but
the Resistive Touch Screen Touched bit will now be cleared, reflecting the released state of the screen. After this condition is
met, no more touchscreen events will be reported in the status register or the TouchPending pin until a new touch is detected in
the screen.
The TouchPending signal is active low and is capable of driving an LED if user feedback is desired.
2.1.4 Calibration process
The resistive touch screen should be calibrated to get an accurate performance. This inlcudes X and Y coordinate and gesture
detection. The calibration process consists of a series of points touched by the user to give the device references on the screen
orientation, offsets, and linearity. Two different processes may be executed: one, for only single touch calibration, the other for
single and two touch calibration. In both cases, three different points need to be touched for single touch calibration. It is
recommended that these points be touched with a stylus to increase the precision achieved. In the case of two touch calibration,
two pairs of points need to be touched. As expected the two touch gestures are performed with fingers, these pairs of points
must also be pressed with fingers.
The three single touch calibration points are the following: The first coordinate should be at 10% of X and 10% of Y axis. The
second coordinate should be at 50% of X and 90% of Y axis. The third coordinate should be at 90% of X and 50% of Y axis.
The two pairs of two touch coordinates are the following and may be pressed in any order: The first pair is at 50% on the X axis
and 10% and 90% on Y axis; the second pair is 50% on Y axis and 10% and 90% on X axis.
These percentages correspond to the graphic display screen resolution. The Horizontal Display Resolution and Vertical Display
Resolution must be configured with the corresponding display resolution before executing the calibration process. Note that
both registers must be written for their new values to take effect. When the CRTouch is calibrated with the correct resolution,
the X and Y coordinates calculated by the device directly correspond to the display pixels and need no additional processing by
the graphics controller. Usually the user application displays an image showing the calibration points one at a time that must be
touched to calibrate the screen.
For increased precision, the CRTouch performs a validation of the touched points during calibration to determine if the values
are within the expected range. When the validation is not positive, a calibration error is signaled in the Status Error register. This
validation mechanism does not prevent the device from being calibrated. It serves as information to the host processor to
determine if the calibration was executed correctly. Other methods of validation can be to display an additional point in the
display and verify that the coordinate reported by the CRTouch corresponds to the displayed point. The built-in validation
mechanism is accurate only within a 5° diphase between the display and the touch panels.
CRTouch Data Sheet, Rev. 3
10
Freescale Semiconductor