L9803 datasheet(29/126 Pages) STMICROELECTRONICS

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L9803

Clocks, Reset, Interrupts & Power saving modes

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3.7

Power Saving Modes

3.7.1

Introduction

There are three Power Saving modes. The Slow Mode may be selected by setting the

relevant bits in the Miscellaneous register as detailed in Section 3.4. Wait and Halt modes

may be entered using the WFI and HALT instructions.

3.7.2

Slow Mode

In Slow mode, the oscillator frequency can be divided by 4, 8, 16 or 32 rather than by 2. The

CPU and peripherals (except CAN, see Note) are clocked at this lower frequency. Slow

mode is used to reduce power consumption.

Note:

Before entering Slow mode and to guarantee low power operations, the CAN Controller

must be placed by software in STANDBY mode.

3.7.3

Wait Mode

Wait mode places the MCU in a low power consumption mode by stopping the CPU. All

peripherals remain active. During Wait mode, the I bit (CC Register) is cleared, so as to

enable all interrupts. All other registers and memory remain unchanged. The MCU will

remain in Wait mode until an Interrupt or Reset occurs, whereupon the Program Counter

branches to the starting address of the Interrupt or Reset Service Routine.

The MCU will remain in Wait mode until a Reset or an Interrupt (coming from CAN, Timers 1

& 2, EEPROM, ADC, PWM 1 & 2, I/O ports peripherals and Power Bridge) occurs, causing

its wake-up.

Refer to Figure 12 below.

Figure 13.

Wait Mode Flow Chart

WAIT INSTRUCTION

RESET

FETCH RESET VECTOR

OR SERVICE INTERRUPT

INTERRUPT

CPU CLOCK

OSCILLATOR

PERIPH. CLOCK

I-BIT

SET

CPU CLOCK

OSCILLATOR

PERIPH. CLOCK

I-BIT

CLEARED

OFF

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