ST7LITE20F2 datasheet(59/166 Pages) STMICROELECTRONICS

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Power saving modes

Doc ID 8349 Rev 5

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9.6

Auto-wakeup from HALT mode

Auto Wake Up From Halt (AWUF) 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, AWUF has lower power consumption (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.

Figure 28. AWUF 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 AWUF 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 AWUF mode behavior is the same as normal Halt mode:

●

The MCU can exit AWUF mode by means of any interrupt with exit from Halt capability

or a reset (see Section 9.4: HALT mode).

●

When entering AWUF 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 AWUF 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 Watchdog operation with AWUF 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

(ei0 source)

oscillator

prescaler/1 .. 255

AWUFH interrupt

/64

divider

to Timer input capture

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