ST72345C4 datasheet(70/247 Pages) STMICROELECTRONICS

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Power-saving modes

ST72344xx, ST2345xx

70/247

Doc ID 12321 Rev 6

9.6

Auto-wakeup from Halt mode

Auto-wakeup from Halt (AWUFH) mode is similar to Halt mode with the addition of an

internal RC oscillator for wake-up. Compared to Active-Halt mode, AWUFH has lower power

consumption because the main clock is not kept running, but there is no accurate real-time

clock available.

It is entered by executing the HALT instruction when the AWUEN bit in the AWUCSR

Main clock controller with real-time clock and beeper (MCC/RTC) for more details).

Figure 32.

AWUFH mode block diagram

As soon as Halt mode is entered, and if the AWUEN bit has been set in the AWUCSR

a fixed divider and a programmable prescaler controlled by the AWUPR register. The output

of this prescaler provides 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 startup delay, the CPU resumes operation 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 internally fAWU_RC to the ICAP2 input of the 16-bit timer A, allowing 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 reset (see Section 9.4: Halt mode).

●

When entering AWUFH mode, the I[1:0] bits in the CC register are forced to 10b to

enable interrupts. 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, including 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 oscillator like the AWU oscillator).

●

The compatibility of the Watchdog operation with the 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 generate a

Watchdog reset.

AWU RC

AWUFH

fAWU_RC

AWUFH

oscillator

prescaler

interrupt

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divider

to Timer input capture

/1 .. 255

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