ST72361-Auto
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POWER SAVING MODES (Cont’d)
8.6 AUTO WAKE-UP FROM HALT MODE
Auto Wake-Up From Halt (AWUFH) mode is simi-
lar to Halt mode with the addition of an internal RC
oscillator for wake-up. Compared to ACTIVE
HALT mode, AWUFH has lower power consump-
tion because the main clock is not kept running,
but there is no accurate realtime clock available.
It is entered by executing the HALT instruction
when the AWUEN bit in the AWUCSR register has
been set and the OIE bit in the MCCSR register is
tails).
Figure 29. AWUFH Mode Block Diagram
As soon as HALT mode is entered, and if the
AWUEN bit has been set in the AWUCSR register,
the AWU RC oscillator provides a clock signal
(fAWU_RC). Its frequency is divided by a fixed divid-
er and a programmable prescaler controlled by the
AWUPR register. The output of this prescaler pro-
vides the delay time. When the delay has elapsed
the AWUF flag is set by hardware and an interrupt
wakes up the MCU from Halt mode. At the same
time the main oscillator is immediately turned on
and a 256 or 4096 cycle delay is used to stabilize
it. After this start-up delay, the CPU resumes oper-
ation by servicing the AWUFH interrupt. The AWU
flag and its associated interrupt are cleared by
software reading the AWUCSR register.
To compensate for any frequency dispersion of
the AWU RC oscillator, it can be calibrated by
measuring the clock frequency fAWU_RC and then
calculating the right prescaler value. Measurement
mode is enabled by setting the AWUM bit in the
AWUCSR register in Run mode. This connects
fAWU_RC to the ICAP1 input of the 16-bit timer, al-
lowing the fAWU_RC to be measured using the main
oscillator clock as a reference timebase.
Similarities with Halt mode
The following AWUFH mode behavior is the same
as normal Halt mode:
– The MCU can exit AWUFH mode by means of
any interrupt with exit from Halt capability or a re-
– When entering AWUFH mode, the I[1:0] bits in
the CC register are forced to 10b to enable inter-
rupts. Therefore, if an interrupt is pending, the
MCU wakes up immediately.
– In AWUFH mode, the main oscillator is turned off
causing all internal processing to be stopped, in-
cluding the operation of the on-chip peripherals.
None of the peripherals are clocked except those
which get their clock supply from another clock
generator (such as an external or auxiliary oscil-
lator like the AWU oscillator).
– The compatibility of Watchdog operation with
AWUFH mode is configured by the WDGHALT
option bit in the option byte. Depending on this
setting, the HALT instruction when executed
while the Watchdog system is enabled, can gen-
erate a Watchdog RESET.
Figure 30. AWUF Halt Timing Diagram
AWU RC
AWUFH
fAWU_RC
AWUFH
(ei0 source)
oscillator
prescaler
interrupt
/64
divider
to Timer input capture
/1 .. 255
AWUFH interrupt
fCPU
RUN MODE
HALT MODE
256 or 4096 tCPU
RUN MODE
fAWU_RC
Clear
by software
tAWU