AT90USB646 datasheet(62/456 Pages) ATMEL

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7593L–AVR–09/12

AT90USB64/128

• Bit 2 – BORF: Brown-out Reset Flag

This bit is set if a Brown-out Reset occurs. The bit is reset by a Power-on Reset, or by writing a

logic zero to the flag.

• 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.

9.7

Internal voltage reference

Atmel AT90USB64/128 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.

9.7.1

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 9-4. 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] 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 Comparator 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.

Table 9-4.

Internal voltage reference characteristics.

Symbol

Parameter

Condition

Min.

Typ.

Max.

Units

VBG

Bandgap reference voltage

1.0

1.1

1.2

tBG

Bandgap reference start-up time

µs

IBG

Bandgap reference current

consumption

µA

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