7.2.5 MCU Status Register – MCUSR
The MCU Status Register provides information on which reset source caused an MCU reset.
Bit
7
6
5
4
3
2
1
0
–
–
–
–
WDRF
R/W
BORF
R/W
EXTRF
R/W
PORF
R/W
MCUSR
Read/Write
Initial Value
R
0
R
0
R
0
R
0
See Bit Description
• Bit 3 – WDRF: Watchdog Reset Flag
This bit is set if a watchdog reset occurs. The bit is reset by a power-on reset, or by writing a logic zero to the flag.
• 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.
7.3
Internal Voltage Reference
ATmega16/32/64/M1/C1 features an internal bandgap reference. This reference is used for brown-out detection, and it can
be used as an input to the analog comparators or the ADC. The VREF 2.56V reference to the ADC, DAC or analog
comparators is generated from the internal bandgap reference.
7.3.1 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 7-4. 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] Fuse).
2. When the bandgap reference is connected to the analog comparator (by setting the ACBG bit in ACSR).
3. When the ADC is enabled.
4. When the DAC is enabled.
Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC or the DAC, the user must always allow
the reference to start up before the output from the analog comparator or ADC or DAC 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.
7.3.2 Voltage Reference Characteristics
Table 7-4. Internal Voltage Reference Characteristics(1)
Parameter
Symbol
VBG
Condition
Min.
Typ.
1.1
40
Max.
Unit
V
Bandgap reference voltage
Bandgap reference start-up time
Bandgap reference current consumption
tBG
µs
IBG
15
µA
Note:
1. Values are guidelines only.
42
ATmega16/32/64/M1/C1 [DATASHEET]
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
