2545M–AVR–09/07

ATmega48/88/168

9.8.3 Brown-out Detector

If the Brown-out Detector is not needed by the application, this module should be turned off. If

the Brown-out Detector is enabled by the BODLEVEL Fuses, it will be enabled in all sleep

modes, and hence, always consume power. In the deeper sleep modes, this will contribute sig-

nificantly to the total current consumption. Refer to “Brown-out Detection” on page 48 for details

on how to configure the Brown-out Detector.

9.8.4 Internal Voltage Reference

The Internal Voltage Reference will be enabled when needed by the Brown-out Detection, the

Analog Comparator or the ADC. If these modules are disabled as described in the sections

above, the internal voltage reference will be disabled and it will not be consuming power. When

turned on again, the user must allow the reference to start up before the output is used. If the

reference is kept on in sleep mode, the output can be used immediately. Refer to “Internal Volt-

age Reference” on page 49 for details on the start-up time.

9.8.5 Watchdog Timer

If the Watchdog Timer is not needed in the application, the module should be turned off. If the

Watchdog Timer is enabled, it will be enabled in all sleep modes and hence always consume

power. In the deeper sleep modes, this will contribute significantly to the total current consump-

tion. Refer to “Watchdog Timer” on page 50 for details on how to configure the Watchdog Timer.

9.8.6 Port Pins

When entering a sleep mode, all port pins should be configured to use minimum power. The

most important is then to ensure that no pins drive resistive loads. In sleep modes where both

the I/O clock (clk

I/O

) and the ADC clock (clk

ADC

) are stopped, the input buffers of the device will

be disabled. This ensures that no power is consumed by the input logic when not needed. In

some cases, the input logic is needed for detecting wake-up conditions, and it will then be

enabled. Refer to the section “Digital Input Enable and Sleep Modes” on page 76 for details on

which pins are enabled. If the input buffer is enabled and the input signal is left floating or have

an analog signal level close to V

/2, the input buffer will use excessive power.

For analog input pins, the digital input buffer should be disabled at all times. An analog signal

level close to V

/2 on an input pin can cause significant current even in active mode. Digital

input buffers can be disabled by writing to the Digital Input Disable Registers (DIDR1 and

DIDR0). Refer to “DIDR1 – Digital Input Disable Register 1” on page 244 and “DIDR0 – Digital

Input Disable Register 0” on page 260 for details.

9.8.7 On-chip Debug System

If the On-chip debug system is enabled by the DWEN Fuse and the chip enters sleep mode, the

main clock source is enabled and hence always consumes power. In the deeper sleep modes,

this will contribute significantly to the total current consumption.

2545M–AVR–09/07

ATmega48/88/168

9.9 Register Description

9.9.1 SMCR – Sleep Mode Control Register

The Sleep Mode Control Register contains control bits for power management.

• Bits 7..4 Res: Reserved Bits

These bits are unused bits in the ATmega48/88/168, and will always read as zero.

• Bits 3..1 – SM2..0: Sleep Mode Select Bits 2, 1, and 0

These bits select between the five available sleep modes as shown in Table 9-2.

Note: 1. Standby mode is only recommended for use with external crystals or resonators.

• Bit 0 – SE: Sleep Enable

The SE bit must be written to logic one to make the MCU enter the sleep mode when the SLEEP

instruction is executed. To avoid the MCU entering the sleep mode unless it is the programmer’s

purpose, it is recommended to write the Sleep Enable (SE) bit to one just before the execution of

the SLEEP instruction and to clear it immediately after waking up.

Bit 76543210

0x33 (0x53) – – – – SM2 SM1 SM0 SE SMCR

Read/Write R R R R R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Table 9-2. Sleep Mode Select

SM2 SM1 SM0 Sleep Mode

000Idle

0 0 1 ADC Noise Reduction

0 1 0 Power-down

0 1 1 Power-save

100Reserved

101Reserved

1 1 0 Standby

(1)

111Reserved

2545M–AVR–09/07

ATmega48/88/168

9.9.2 PRR – Power Reduction Register

• Bit 7 - PRTWI: Power Reduction TWI

Writing a logic one to this bit shuts down the TWI by stopping the clock to the module. When

waking up the TWI again, the TWI should be re initialized to ensure proper operation.

• Bit 6 - PRTIM2: Power Reduction Timer/Counter2

Writing a logic one to this bit shuts down the Timer/Counter2 module in synchronous mode (AS2

is 0). When the Timer/Counter2 is enabled, operation will continue like before the shutdown.

• Bit 5 - PRTIM0: Power Reduction Timer/Counter0

Writing a logic one to this bit shuts down the Timer/Counter0 module. When the Timer/Counter0

is enabled, operation will continue like before the shutdown.

• Bit 4 - Res: Reserved bit

This bit is reserved in ATmega48/88/168 and will always read as zero.

• Bit 3 - PRTIM1: Power Reduction Timer/Counter1

Writing a logic one to this bit shuts down the Timer/Counter1 module. When the Timer/Counter1

is enabled, operation will continue like before the shutdown.

• Bit 2 - PRSPI: Power Reduction Serial Peripheral Interface

If using debugWIRE On-chip Debug System, this bit should not be written to one.

Writing a logic one to this bit shuts down the Serial Peripheral Interface by stopping the clock to

the module. When waking up the SPI again, the SPI should be re initialized to ensure proper

operation.

• Bit 1 - PRUSART0: Power Reduction USART0

Writing a logic one to this bit shuts down the USART by stopping the clock to the module. When

waking up the USART again, the USART should be re initialized to ensure proper operation.

• Bit 0 - PRADC: Power Reduction ADC

Writing a logic one to this bit shuts down the ADC. The ADC must be disabled before shut down.

The analog comparator cannot use the ADC input MUX when the ADC is shut down.

Bit 76543210

(0x64) PRTWI PRTIM2 PRTIM0 – PRTIM1 PRSPI PRUSART0 PRADC PRR

Read/Write R/W R/W R/W R R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Part #	ATMEGA48-20AU
Description	MCU 8BIT ATMEGA RISC 4KB FLASH 3.3V/5V 32TQFP - Trays
Category	IC
Availability	Out of Stock
Qty	0

ATMEGA48-20AU

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