ST7DALIF2 datasheet(54/171 Pages) STMICROELECTRONICS

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Power saving modes

ST7DALIF2

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11.4.1

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

external interference or by an unforeseen logical condition.

●

For the same reason, reinitialize the level sensitiveness of each external interrupt as a

precautionary 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 memory. For example, avoid defining a constant in program memory with

the value 0x8E.

●

As the HALT instruction clears the interrupt mask in the CC register to allow interrupts,

the user may choose to clear all pending interrupt bits before executing 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).

11.5

Active-halt mode

Active-halt mode is the lowest power consumption mode of the MCU with a real time clock

available. It is entered by executing the ‘HALT’ instruction. The decision to enter either in

Active-halt or Halt mode is given by the LTCSR/ATCSR register status as shown in the

following table:

The MCU can exit Active-halt mode on reception of a specific interrupt (see Table 15:

Interrupt mapping on page 47) or a RESET.

●

When exiting Active-halt mode by means of a RESET, a 256 or 4096 CPU cycle delay

occurs. After the start up delay, the CPU resumes operation by fetching the reset vector

which woke it up (see Figure 26).

●

When exiting Active-halt mode by means of an interrupt, the CPU immediately resumes

operation by servicing the interrupt vector which woke it up (see Figure 26).

When entering Active-halt mode, the I bit in the CC register is cleared to enable interrupts.

Therefore, if an interrupt is pending, the MCU wakes up immediately (see Note 3).

In Active-halt mode, only the main oscillator and the selected timer counter (LT/AT) are

running to keep a wake-up time base. All other peripherals are not clocked except those

which get their clock supply from another clock generator (such as external or auxiliary

oscillator).

Table 21.

Active-Halt control

LTCSR1 TB1IE bit

ATCSR OVFIE bit

ATCSRCK1 bit

ATCSRCK0 bit

Meaning

Active-halt mode disabled

Active-halt mode enabled

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