CRTOUCH Datasheet, PDF (20/68 Pages) Freescale Semiconductor, Inc – Capacitive and Resistive Touch Sensing Application Specific IC.

Functional Description
Figure 13. CRTouch wakeup using I2C communication
• Use the wakeup signal to bring the part into normal run mode before starting any I2C communication. It is important
to return the wakeup signal into its idle state (high) after the communication is finished, otherwise the part will remain
into normal run mode increasing the overall system power consumption.
When the wakeup pin is used to return the part to run mode, the system will remain in this state for 50 s. If a new
communication starts with a Start condition followed by the device slave address and a valid command for the CRTouch while
in normal run mode, the part will remain in run state. It will return to sleep mode only after the communication remains idle for
1ms after receiving a stop condition. When the part wakes by receiving its slave address through I2C, it remains in run mode
for 1ms waiting for a new communication to start. It goes back to sleep mode after 1 ms of inactivity on the I2C bus.
2.3.2.3 UART communication in sleep mode
When UART communication is used for CRTouch, a start bit will transition the part from sleep mode to run mode. When the
part is in sleep mode the internal clock used for the UART communication is off. Upon reception of a start bit the internal clock
is re-started, the part returns into normal run mode and the communication can be resumed normally.
In the majority of the cases the byte that wokeup the part is properly received and stored. The exception to this rule occurs when
the internal circuitry is transitioning from normal run mode (either because of a previous communication, wakeup pin use, or
screen X and Y coordinates calculation) into sleep mode. If the UART start bit is received at the exact moment the internal
circuitry starts the transition, the initial byte (start of frame) will be lost. This will result in a complete frame loss when it occurs.
There are three alternatives when enabling Sleep mode using UART:
• Send the start of frame byte twice in each frame.
• Asserting the wakeup pin for at least 10 s before sending the start of frame. It is important to return the wakeup signal
into its idle state (high) after the communication is finished, otherwise the part will remain in normal run mode
increasing the overall system power consumption.
• Implementing a timeout on the host side to retry the command if there is no response received within the next 1ms of
sending the command.
2.3.3 Shutdown mode
Shutdown is enabled through the SHUTDOWN bit in the Configuration register. This mode sends the part into the lowest power
consumption state. In this mode, all the resistive touchscreen scanning, serial communication and any other internal activity are
stopped. This mode is intended for when a device is in a standby or hibernating state and wishes to reduce power consumption
to its minimum.
There are three ways to come out of Shutdown mode:
• Asserting the wakeup signal for more than 10 s
• Using the reset pin
• Optionally enabling a capacitive electrode as wakeup source and performing a touch on it.
In all cases the part will recover the latest value for the configuration registers and will resume will resume normal operating
mode. The part will use either normal run mode or Sleep mode based on the latest configuration for the SLEEPEN bit before
going into Shutdown.
CRTouch Data Sheet, Rev. 3
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Freescale Semiconductor