AN2633 datasheet(14/48 Pages) STMICROELECTRONICS

14 / 48 page

Power supply and clocks

AN2633

14/48

Figure 3.

Low power mode state diagram

1.4.1

Normal Run mode

This is the default run mode. The CPU executes instructions and any or all of the on-chip

peripherals are in active state. You can turn-on or turn-off the clock of any of the peripherals

writing to Peripheral clock gating register 0 (SCU_PCGR0) or Peripheral clock gating

of the various clocks in order to optimize power usage while operating in normal run mode.

1.4.2

Special Interrupt Run mode

Special Interrupt mode FIQ

The special interrupt mode using FIQ causes the CPU to temporarily operate at full speed

(fMSTR as clock frequency) while servicing one or more interrupts and return back to normal

run mode with the speed selected by the clock RCLKDIV divider (see Figure 2) when the

interrupt routine is complete. You enable/disable this mode using the CPU_INTR bit in the

Power management register (SCU_PWRMNG).

Special Interrupt mode IRQ

The special interrupt mode using IRQ causes the CPU to operate at full speed (fMSTR as

clock frequency) when the IRQ service routine reads the vector address register in the VIC

and jumps then to the specified interrupt with the speed selected by the RCLKDIV clock

divider. You enable/disable this mode using the CPU_INTR bit in the Power management

Workarounds

To operate at full speed when servicing the IRQ interrupts, you have to configure the desired

clock frequency at the beginning of the interrupt routine. Then, you have to switch the

system clock back to the operating frequency after disabling the interrupt.

1.4.3

Idle mode

Idle mode is entered under software control, by writing the value ‘001b’ to the

PWR_MODE[2:0] bits in the Power management register (SCU_PWRMNG). In this mode,

Normal

Run

mode

Sleep

mode

Idle

mode

Special

Run mode

Interrupt

Power up reset

Set Idle

Mode

Interrupt

Reset

Interrupt

Return from

Interrupt

Set sleep

mode

Wake-up

RTC alarm

Reset

RTC alarm

Wake-up

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