System Integration Module
MOTOROLA
MC68307 USER’S MANUAL
5-19
5.1.5 Low-Power Sleep Logic
Various options for power-saving are available—turning off unused peripherals, reducing
processor clock speed, disabling the processor altogether, or a combination of these. The
SCR includes a clock divider enable bit (CDEN) which substitutes a divided-down version
of the system clock as the clock input to the EC000 core. The division is by a power-of-2, as
programmed in the CD2–0 bits, allowing a 16MHz system clock to be divided down to
approximately 64kHz. There is also a low-power sleep mode enable (LPEN) bit which stops
the clock to the EC000 processor altogether after using bus arbitration to turn off the external
bus. Prior to this happening, the peripherals which are not required can be disabled or their
clocks stopped by programming the appropriate registers, thus further reducing power
consumption. The MC68307 crystal oscillator is not stopped by these internal modes. For
lowest power operation, the external clock source can be slowed or removed altogether by
the user.
NOTE
Because bus arbitration is used, the external address, data bus-
es and control signals (AS, UDS, LDS, R/W) are tristated and so
should have weak pullups to minimize system power consump-
tion.
So, various options for power-saving are available—turning off unused peripherals,
reducing processor clock speed, disabling the processor altogether, or a combination of
these.
A wakeup from low-power sleep mode can be achieved by causing an interrupt at the
interrupt controller Logic which continues to run throughout the period of processor sleep.
Any interrupt source causes a wakeup of the EC000 core processor followed by processing
of that interrupt when unmasked in the SR.
The wakeup operation involves the internal sleep/wake-up logic restarting the clock to the
EC000 core processor and releasing the bus. Normal processing resumes with all register
contents intact, i.e., the processor continues execution from the address following the low-
power enable instruction sequence. Interrupt exception processing is initiated for the
interrupt which caused the wakeup.
After a wakeup, the reset source bits in the SCR are set to a value which indicates a wakeup
has happened. Normally, these bits show the source of the most recent reset of the core
processor. A sleep/wakeup sequence does not cause ANY reset of the static core processor
or the on-chip peripherals. User software can issue a RESET instruction if any external
peripherals require reset after a period of being in sleep mode.
The on-chip peripherals can initiate a wake-up, for example, the timer can be set to wake-
up after a certain elapsed time, or number of external events, or the UART can cause a
wake-up on receiving serial data.
The clocks provided to the various internal modules can all be gated off, to further reduce