ST7LITEU05 datasheet(43/115 Pages) STMICROELECTRONICS

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POWER SAVING MODES (Cont’d)

9.4.2.1 HALT Mode Recommendations

– Make sure that an external event is available to

wake up the microcontroller from Halt mode.

– When using an external interrupt to wake up the

microcontroller, reinitialize the corresponding I/O

as “Input Pull-up with Interrupt” before executing

the HALT instruction. The main reason for this is

that the I/O may be wrongly configured due to ex-

ternal interference or by an unforeseen logical

condition.

– For the same reason, reinitialize the level sensi-

tiveness of each external interrupt as a precau-

tionary measure.

– The opcode for the HALT instruction is 0x8E. To

avoid an unexpected HALT instruction due to a

program counter failure, it is advised to clear all

occurrences of the data value 0x8E from memo-

ry. For example, avoid defining a constant in

ROM with the value 0x8E.

– As the HALT instruction clears the I bit in the CC

to clear all pending interrupt bits before execut-

ing the HALT instruction. This avoids entering

other peripheral interrupt routines after executing

the external interrupt routine corresponding to

the wake-up event (reset or external interrupt).

9.5 AUTO WAKE UP FROM HALT MODE

Auto Wake Up From Halt (AWUFH) mode is simi-

lar to Halt mode with the addition of a specific in-

ternal RC oscillator for wake-up (Auto Wake-Up

from Halt oscillator) which replaces the main clock

which was active before entering HALT mode.

Compared to ACTIVE-HALT mode, AWUFH 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 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 following actions are performed:

– the AWUF flag is set by hardware,

– an interrupt wakes-up the MCU from Halt mode,

– the main oscillator is immediately turned on and

the 256 or 512 CPU cycle delay is used to stabi-

lize it.

After this start-up delay, the CPU resumes opera-

tion 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 8-bit lite timer,

allowing the fAWU_RC to be measured using the

main oscillator clock as a reference timebase.

AWU RC

AWUFH

fAWU_RC

AWUFH

(ei0 source)

oscillator

prescaler/1 .. 255

interrupt

/64

divider

to 8-bit Timer input capture

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