ATmega325P datasheet(39/364 Pages) ATMEL

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8023F–AVR–07/09

ATmega325P/3250P

9.3

BOD Disable

When the Brown-out Detector (BOD) is enabled by BODLEVEL fuses, Table 25-3 on page 272,

the BOD is actively monitoring the power supply voltage during a sleep period. To save power, it

is possible to disable the BOD by software for some of the sleep modes, see Table 9-1 on page

38. The sleep mode power consumption will then be at the same level as when BOD is globally

disabled by fuses. If BOD is disabled in software, the BOD function is turned off immediately

after entering the sleep mode. Upon wake-up from sleep, BOD is automatically enabled again.

This ensures safe operation in case the V

CC level has dropped during the sleep period.

When the BOD has been disabled, the wake-up time from sleep mode will be approximately 60

µs to ensure that the BOD is working correctly before the MCU continues executing code.

BOD disable is controlled by bit 6, BODS (BOD Sleep) in the control register MCUCR, see

”MCUCR – MCU Control Register” on page 43. Writing this bit to one turns off the BOD in rele-

vant sleep modes, while a zero in this bit keeps BOD active. Default setting keeps BOD active,

i.e. BODS set to zero.

Writing to the BODS bit is controlled by a timed sequence and an enable bit, see ”MCUCR –

MCU Control Register” on page 43.

9.4

Idle Mode

When the SM2:0 bits are written to 000, the SLEEP instruction makes the MCU enter Idle mode,

stopping the CPU but allowing the SPI, USART, Analog Comparator, ADC, USI, Timer/Coun-

ters, Watchdog, and the interrupt system to continue operating. This sleep mode basically halts

clk

CPU and clkFLASH, while allowing the other clocks to run.

Idle mode enables the MCU to wake up from external triggered interrupts as well as internal

ones like the Timer Overflow and USART Transmit Complete interrupts. If wake-up from the

Analog Comparator interrupt is not required, the Analog Comparator can be powered down by

setting the ACD bit in the Analog Comparator Control and Status Register – ACSR. This will

reduce power consumption in Idle mode. If the ADC is enabled, a conversion starts automati-

cally when this mode is entered.

9.5

ADC Noise Reduction Mode

When the SM2:0 bits are written to 001, the SLEEP instruction makes the MCU enter ADC

Noise Reduction mode, stopping the CPU but allowing the ADC, the external interrupts, the USI

start condition detection, Timer/Counter2 and the Watchdog to continue operating (if enabled).

This sleep mode basically halts clk

I/O, clkCPU, and clkFLASH, while allowing the other clocks to run.

This improves the noise environment for the ADC, enabling higher resolution measurements. If

the ADC is enabled, a conversion starts automatically when this mode is entered. Apart form the

ADC Conversion Complete interrupt, only an External Reset, a Watchdog Reset, a Brown-out

Reset, USI start condition interrupt, a Timer/Counter2 interrupt, an SPM/EEPROM ready inter-

rupt, an external level interrupt on INT0 or a pin change interrupt can wake up the MCU from

ADC Noise Reduction mode.

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