ATMEGA88 datasheet(45/349 Pages) ATMEL

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ATmega48/88/168

2545D–AVR–07/04

• Bit 1 – EXTRF: External Reset Flag

This bit is set if an External Reset occurs. The bit is reset by a Power-on Reset, or by

writing a logic zero to the flag.

• Bit 0 – PORF: Power-on Reset Flag

This bit is set if a Power-on Reset occurs. The bit is reset only by writing a logic zero to

the flag.

To make use of the Reset Flags to identify a reset condition, the user should read and

then Reset the MCUSR as early as possible in the program. If the register is cleared

before another reset occurs, the source of the reset can be found by examining the

Reset Flags.

Internal Voltage

Reference

ATmega48/88/168 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 23. 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 [2..0] Fuses).

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.

Table 23. Internal Voltage Reference Characteristics(1)

Symbol

Parameter

Condition

Min

Typ

Max

Units

VBG

Bandgap reference voltage

TBD

1.0

1.1

1.2

tBG

Bandgap reference start-up time

TBD

µs

IBG

Bandgap reference current

consumption

TBD

µA

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