ST72324BJ2 datasheet(49/188 Pages) STMICROELECTRONICS

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ST72324B

Power saving modes

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8.4

Active Halt and Halt modes

Active Halt and Halt modes are the two lowest power consumption modes of the MCU. They

are both entered by executing the ‘HALT’ instruction. The decision to enter either in Active

Halt or Halt mode is given by the MCC/RTC interrupt enable flag (OIE bit in the MCCSR

8.4.1

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 when the OIE bit of the Main Clock

Controller Status register (MCCSR) is set (see Section 10.2: Main clock controller with real-

time clock and beeper (MCC/RTC) on page 64 for more details on the MCCSR register).

The MCU can exit Active Halt mode on reception of either an MCC/RTC interrupt, a specific

interrupt (see Table 24: Interrupt mapping) or a reset. When exiting Active Halt mode by

means of an interrupt, no 256 or 4096 CPU cycle delay occurs. The CPU resumes operation

by servicing the interrupt or by fetching the reset vector which woke it up (see Figure 27).

When entering Active Halt mode, the I[1:0] bits in the CC register are forced to ‘10b’ to

enable interrupts. Therefore, if an interrupt is pending, the MCU wakes up immediately.

In Active Halt mode, only the main oscillator and its associated counter (MCC/RTC) 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).

The safeguard against staying locked in Active Halt mode is provided by the oscillator

interrupt.

Note:

As soon as the interrupt capability of one of the oscillators is selected (MCCSR.OIE bit set),

entering Active Halt mode while the Watchdog is active does not generate a reset.

This means that the device cannot spend more than a defined delay in this power saving

mode.

Caution:

When exiting Active Halt mode following an interrupt, OIE bit of MCCSR register must not be

cleared before tDELAY after the interrupt occurs (tDELAY = 256 or 4096 tCPU delay depending

on option byte). Otherwise, the ST7 enters Halt mode for the remaining tDELAY period.

Figure 26.

Active Halt timing overview

This delay occurs only if the MCU exits Active Halt mode by means of a reset.

Table 25.

MCC/RTC low power mode selection

MCCSR OIE bit

Power saving mode entered when HALT instruction is executed

Halt mode

Active Halt mode

Halt

Run

256 or 4096 CPU

cycle delay(1)

Reset

interrupt

Halt

instruction

Fetch

vector

Active

[MCCSR.OIE = 1]

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