Datasheet

External Pin Descriptions
MC68HC812A4 Data Sheet, Rev. 7
Freescale Semiconductor 175
14.7 External Pin Descriptions
This section provides a description of TXD and RXD, the SCI’s two external pins.
14.7.1 TXD Pin
TXD is the SCI transmitter pin. TXD is available for general-purpose I/O when it is not configured for
transmitter operation.
14.7.2 RXD Pin
RXD is the SCI receiver pin. RXD is available for general-purpose I/O when it is not configured for receiver
operation.
14.8 Modes of Operation
The SCI functions the same in normal, special, and emulation modes.
14.9 Low-Power Options
This section provides a description of the three low-power modes:
• Run mode
• Wait mode
• Stop mode
14.9.1 Run Mode
Clearing the transmitter enable or receiver enable bits (TE or RE) in SCI control register 2 (SCCR2)
reduces power consumption in run mode. SCI registers are still accessible when TE or RE is cleared, but
clocks to the core of the SCI are disabled.
14.9.2 Wait Mode
The SCI remains active in wait mode. Any enabled interrupt request from the SCI can bring the MCU out
of wait mode.
If SCI functions are not required during wait mode, reduce power consumption by disabling the SCI before
executing the WAIT instruction.
14.9.3 Stop Mode
For reduced power consumption, the SCI is inactive in stop mode. The STOP instruction does not affect
SCI register states. SCI operation resumes after an external interrupt.
Exiting stop mode by reset aborts any transmission or reception in progress and resets the SCI.