ST7L15 datasheet(39/138 Pages) STMICROELECTRONICS

39 / 138 page

ST7L15, ST7L19

39/138

POWER SAVING MODES (cont’d)

Figure 25. ACTIVE HALT Mode Flowchart

Notes:

1. This delay occurs only if the MCU exits ACTIVE HALT

mode by means of a RESET.

2. Peripherals clocked with an external clock source can

still be active.

3. Only the RTC1 interrupt and some specific interrupts

can exit the MCU from ACTIVE HALT mode. Refer to

Table 5, “Interrupt Mapping,” on page 32 for more details.

4. Before servicing an interrupt, the CC register is pushed

on the stack. The I bit of the CC register is set during the

interrupt routine and cleared when the CC register is

popped.

9.6 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). Compared to ACTIVE HALT

mode, AWUFH has lower power consumption (the

main clock is not kept running but there is no accu-

rate realtime clock available).

It is entered by executing the HALT instruction

when the AWUEN bit in the AWUCSR register has

been set.

Figure 26. 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 input capture of the 12-bit Auto-Re-

load timer, allowing the fAWU_RC to be measured

using the main oscillator clock as a reference time-

base.

HALT INSTRUCTION

RESET

INTERRUPT3)

CPU

OSCILLATOR

PERIPHERALS2)

IBIT

OFF

FETCH RESET VECTOR

OR SERVICE INTERRUPT

CPU

OSCILLATOR

PERIPHERALS2)

IBIT

OFF

X4)

CPU

OSCILLATOR

PERIPHERALS

IBIT

X4)

256 OR 4096 CPU CLOCK

DELAY

(Active Halt enabled)

(AWUCSR.AWUEN=0)

CYCLE

AWU RC

AWUFH

fAWU_RC

AWUFH

(ei0 source)

oscillator

prescaler/1 .. 255

interrupt

/64

divider

to Timer input capture

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