CRTOUCH Datasheet, PDF (22/68 Pages) Freescale Semiconductor, Inc – Capacitive and Resistive Touch Sensing Application Specific IC.

Serial Communications
Figure 14. Connections to power CRTouch from internal voltage regulator
Figure 15. Recommended connections to power external circuitry from internal voltage regulator
3 Serial Communications
The CRTouch is a four wire and five wire resistive touch Screen and a four key capacitive touch sensing device. It can interact
with a master device either through UART or I2C interfaces
3.1 I2C interface
The registers inside CRTouch can be accessed through the Inter-Integrated Circuit serial interface (I2C, I²C, or IIC). The I2C
interface provides a method of communication with a number of devices and the CRTouch can act only as a slave device inside
an I2C network, therefore it will respond to read and write operations from a Bus master and can never initiate a communication
within the bus. To enable the I2C interface, COM SEL must be tied high when coming out of reset.
CRTouch operates as an I2C slave that sends and receives data through the SDA line on an I2C bus. A bus master initiates all
data transfers to and from CRTouch, and generates the SCL clock that synchronizes the data transfer.
The CRTouch line operates as an open drain bidirectional signal. A pull-up resistor (typically between 4.7  and 10 K) is
required on CRTouch. For CRTouch, SCL is an input only signal which also requires an external pull-up.
Each transmission is initiated with a START condition generated by the master, followed by the CRTouch slave address (7 bits)
plus one bit that indicates if the transaction is a read or write operation, in the case of write sequences it includes one register
address byte and 1 to N data bytes for read or write transactions, followed by a STOP condition that indicates the end of that
transmission.
CRTouch Data Sheet, Rev. 3
22
Freescale Semiconductor