Datasheet

ST72321Rx ST72321ARx ST72321Jx
41/193
8 POWER SAVING MODES
8.1 INTRODUCTION
To give a large measure of flexibility to the applica-
tion in terms of power consumption, four main
power saving modes are implemented in the ST7
(see Figure 23): SLOW, WAIT (SLOW WAIT), AC-
TIVE HALT and HALT.
After a RESET the normal operating mode is se-
lected by default (RUN mode). This mode drives
the device (CPU and embedded peripherals) by
means of a master clock which is based on the
main oscillator frequency divided or multiplied by 2
(f
OSC2
).
From RUN mode, the different power saving
modes may be selected by setting the relevant
register bits or by calling the specific ST7 software
instruction whose action depends on the oscillator
status.
Figure 23. Power Saving Mode Transitions
8.2 SLOW MODE
This mode has two targets:
– To reduce power consumption by decreasing the
internal clock in the device,
– To adapt the internal clock frequency (f
CPU
) to
the available supply voltage.
SLOW mode is controlled by three bits in the
MCCSR register: the SMS bit which enables or
disables Slow mode and two CPx bits which select
the internal slow frequency (f
CPU
).
In this mode, the master clock frequency (f
OSC2
)
can be divided by 2, 4, 8 or 16. The CPU and pe-
ripherals are clocked at this lower frequency
(f
CPU
).
Note: SLOW-WAIT mode is activated when enter-
ing the WAIT mode while the device is already in
SLOW mode.
Figure 24. SLOW Mode Clock Transitions
POWER CONSUMPTION
WAIT
SLOW
RUN
ACTIVE HALT
High
Low
SLOW WAIT
HALT
00 01
SMS
CP1:0
f
CPU
NEW SLOW
NORMAL RUN MODE
MCCSR
FREQUENCY
REQUEST
REQUEST
f
OSC2
f
OSC2
/2 f
OSC2
/4 f
OSC2