ST72561 datasheet(44/262 Pages) STMICROELECTRONICS

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ST72561

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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

cleared (see Section 10.2 on page 61 for more de-

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 behaviour 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-

set (see Section 8.4 "HALT MODE").

– 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

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