MC68HC908GR8 Datasheet, PDF (174/286 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Serial Communications Interface (SCI)
18.5 Low-Power Modes
The WAIT and STOP instructions put the MCU in low power-consumption standby modes.
18.5.1 Wait Mode
The SCI module remains active after the execution of a WAIT instruction. Any enabled CPU interrupt
request from the SCI module can bring the MCU out of wait mode.
If SCI module functions are not required during wait mode, reduce power consumption by disabling the
module before executing the WAIT instruction.
Refer to Chapter 3 Low-Power Modes for information on exiting wait mode.
18.5.2 Stop Mode
The SCI module is inactive in stop mode. The STOP instruction does not affect SCI register states. SCI
module operation resumes when the MCU exits stop mode.
Because the internal clock is inactive during stop mode, entering stop mode during an SCI transmission
or reception results in invalid data.
Refer to Chapter 3 Low-Power Modes for information on exiting stop mode.
18.6 SCI During Break Module Interrupts
The system integration module (SIM) controls whether status bits in other modules can be cleared during
the break state. The BCFE bit in the SIM break flag control register (SBFCR) enables software to clear
status bits during the break state.
To allow software to clear status bits during a break interrupt, write a 1 to the BCFE bit. If a status bit is
cleared during the break state, it remains cleared when the MCU exits the break state.
To protect status bits during the break state, write a 0 to the BCFE bit. With BCFE at 0 (its default state),
software can read and write I/O registers during the break state without affecting status bits. Some status
bits have a 2-step read/write clearing procedure. If software does the first step on such a bit before the
break, the bit cannot change during the break state as long as BCFE is at 0. After the break, doing the
second step clears the status bit.
18.7 I/O Signals
Port E shares two of its pins with the SCI module. The two SCI I/O pins are:
• PE2/TxD — Transmit data
• PE1/RxD — Receive data
18.7.1 PE2/TxD (Transmit Data)
The PE2/TxD pin is the serial data output from the SCI transmitter. The SCI shares the PE2/TxD pin with
port E. When the SCI is enabled, the PE2/TxD pin is an output regardless of the state of the DDRE0 bit
in data direction register E (DDRE).
MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7
174
Freescale Semiconductor