Applications and Design Examples
MOTOROLA
MC68328 USER’S MANUAL 11/10/97
16-15
APPLICATIONS
AND
14
DESIGN
EXAMPLES
PRELIMINARY
16.3 POWER-SAVING TIPS
You can save power on the MC68328 by following these tips:
Keep the core shut down as much as possible by putting it in doze mode. Set the STOP
bit in the power control register during any peiod of inactivity.
Shut down the PLL to enter sleep mode. Set the DISPLL bit in the PLL control register
to conserve power when the processor is inactive for an extended period of time.
Avoid the following pull-up resistor conditions to save the most power:
t When an internal pull-up is enabled on an input with an external device driving it
low.
t When an internal pull-up is enabled on an I/O with the internal output device driving
it low.
Drive the outputs to their high state before entering sleep mode. The outputs have an
internal pull-up that will not consume excess current when the pin is driven high.
However, some external devices may require that the signal from the MC68328 port be
low to save the most power.
Disable the pull-ups on inputs that are always driven externally. Inputs that are
continuously driven from an external device do not require an internal pull-up resistor.
You can save power by disabling the internal pull-up resistor on the MC68328 I/O ports
that are configured as inputs and always driven to a high or low state by the external
device.
Disable the I/O port internal pull-ups one by one to isolate excess current drain in sleep
mode.
Beware of outputs sourcing current for externally powered-down devices. When the
MC68328 is powered down in sleep or doze mode, the output pins remain in the state
they were in before going to sleep. This may cause an external device with its power
turned off to try to power itself from the MC68328 output pins that are connected to it.
In this case, it may be best to make sure the MC68328 output pin is in the low state and
that internal pull-up is disabled.
Open inputs will cause excess current drain. JTAG inputs are often forgotten.
Inputs that are not being driven rail to rail consume excess current.