ATMEGA325 datasheet(44/353 Pages) ATMEL

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ATmega325/3250/645/6450

2570J–AVR–11/06

Internal Voltage

Reference

ATmega325/3250/645/6450 features an internal bandgap reference. This reference is

used for Brown-out Detection, and it can be used as an input to the Analog Comparator

or the ADC.

Voltage Reference Enable

Signals and Start-up Time

The voltage reference has a start-up time that may influence the way it should be used.

The start-up time is given in Table 19. To save power, the reference is not always turned

on. The reference is on during the following situations:

When the BOD is enabled (by programming the BODLEVEL [1..0] Fuse).

When the bandgap reference is connected to the Analog Comparator (by setting

the ACBG bit in ACSR).

When the ADC is enabled.

Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the

user must always allow the reference to start up before the output from the Analog Com-

parator or ADC is used. To reduce power consumption in Power-down mode, the user

can avoid the three conditions above to ensure that the reference is turned off before

entering Power-down mode.

Note:

1. Values are guidelines only. Actual values are TBD.

Watchdog Timer

The Watchdog Timer is clocked from a separate On-chip Oscillator which runs at

1 MHz. This is the typical value at V

CC = 5V. See characterization data for typical values

at other V

CC levels. By controlling the Watchdog Timer prescaler, the Watchdog Reset

interval can be adjusted as shown in Table 21 on page 46. The WDR – Watchdog Reset

– instruction resets the Watchdog Timer. The Watchdog Timer is also reset when it is

disabled and when a Chip Reset occurs. Eight different clock cycle periods can be

selected to determine the reset period. If the reset period expires without another

Watchdog Reset, the ATmega325/3250/645/6450 resets and executes from the Reset

Vector. For timing details on the Watchdog Reset, refer to Table 21 on page 46.

To prevent unintentional disabling of the Watchdog or unintentional change of time-out

period, two different safety levels are selected by the fuse WDTON as shown in Table

20. Refer to “Timed Sequences for Changing the Configuration of the Watchdog Timer”

on page 47 for details.

Table 19. Internal Voltage Reference Characteristics

(1)

Symbol

Parameter

Condition

Min

Typ

Max

Units

VBG

Bandgap reference voltage

CC = 2.7V,

TA = 25°C

1.0

1.1

1.2

tBG

Bandgap reference start-up time

VCC = 2.7V,

A = 25°C

µs

IBG

Bandgap reference current

consumption

VCC = 2.7V,

TA = 25°C

µA

Table 20. WDT Configuration as a Function of the Fuse Settings of WDTON

WDTON

Safety

Level

WDT Initial

State

How to Disable the

WDT

How to Change

Time-out

Unprogrammed

Disabled

Timed sequence

Programmed

Enabled

Always enabled

Timed sequence

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