ST7DALIF2 datasheet(56/171 Pages) STMICROELECTRONICS

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Power saving modes

ST7DALIF2

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11.6

Auto wakeup from Halt mode

Auto Wake Up From Halt (AWUFH) mode is similar to Halt mode with the addition of a

specific internal RC oscillator for wake-up (Auto Wake Up from Halt Oscillator). Compared to

Active-halt mode, AWUFH has lower power consumption (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

Figure 27.

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 start-up 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 fAWU_RC to the input capture of the 12-bit Auto-Reload timer, 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 11.4: Halt mode).

●

When entering AWUFH mode, the I bit in the CC register is forced to 0 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

AWU RC

AWUFH

fAWU_RC

AWUFH

(ei0 source)

oscillator

prescaler/1 .. 255

interrupt

/64

divider

to Timer input capture

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