• 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.
8.1
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.
8.1.1
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 8-4. To save power, the reference is not always turned on. The
reference is on during the following situations:
1. When the BOD is enabled (by programming the BODLEVEL [2..0] Fuses).
2. When the bandgap reference is connected to the Analog Comparator (by setting the
ACBG bit in ACSR).
3. 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 8-4.
Internal Voltage Reference Characteristics
Symbol Parameter
Condition
Min(1)
Typ(1)
Max(1) Units
VCC=2.7
TA=25°C
VBG
tBG
IBG
Bandgap reference voltage
1.0
1.1
1.2
70
V
VCC=2.7
TA=25°C
Bandgap reference start-up time
40
10
µs
µA
VCC=2.7
Bandgap reference current consumption
TA=25°C
Note:
1. Values are guidelines only. Actual values are TBD.
48
ATmega48/88/168
2545E–AVR–02/05
MICROCHIP [ MICROCHIP ]
