CRTOUCH Datasheet, PDF (14/68 Pages) Freescale Semiconductor, Inc – Capacitive and Resistive Touch Sensing Application Specific IC.

Functional Description
During calibration, the touch screen scan period is fixed at 10 ms, regardless of the value configured in the Sampling Rate
register. For each point, 128 samples are taken, so each point should be held at least for 1.3 s for proper calibration.
The calibration processed may be stopped at anytime by clearing bit6 on the Trigger Events register. Asampling period must
pass before enabling it again for a new calibration process. To discard an established calibration both of the display resolution
registers must be written. It is also possible to calibrate the CRTouch device through the Calibration Values set of registers
without any touches required by the user. This is particularly useful for factory calibration to reduce the time required for this
process. In this case, a system must be manually calibrated to generate the calibration values that can be reproduced to other
devices. It is important to consider that the precision achieved by this method depends on the repeatability of the touch sensor
impedances by the manufacturer. After writing the values to these registers the CRTouch must be reset for the calibration to take
effect.
2.1.5 Data FIFO
The device has three FIFO buffers, one for X coordinates, one for Y coordinates, and one for Pressure values. Each FIFO buffer
stores up to16 samples and new samples can not be stored once the FIFO buffer is full. Each point touched on the screen yields
one X coordinate and one Y coordinate which are store onto its respective FIFO buffer. If pressure is enabled on the device,
then each point yields also a pressure value that is stored onto its respective FIFO buffer.
The three FIFO buffers are synchronized and should be read in sequence. The device has a mechanism avoiding that just one
FIFO buffer is read. If there are samples on the FIFO buffers and just one of them is read several times the device will report
the same value until the other FIFO buffers is read. This means if the pressure is enabled the X FIFO buffer, Y FIFO buffer,
and Pressure FIFO buffer should be read once before trying to read any of them again. If Pressure is disabled then only X FIFO
buffer and Y FIFO buffer should be read.
Setting the RTFIFOEN bit into the FIFO Setup register enables the FIFO buffers. Clearing this bit disables the FIFO buffers.
X, Y, and Pressure FIFO buffers are flushed each time the FIFO Setup register is written. The device returns the value 0xFFFF
if the FIFO buffers are disabled or empty.
If the FIFO buffers are enabled a watermark can be configured to generate an event once the number of samples stored onto the
FIFO buffers are bigger or equal than the watermark. The watermark is disabled by setting a zero value.
Figure 11. FIFO Watermark example
CRTouch Data Sheet, Rev. 3
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Freescale Semiconductor