50
4317K–AVR–03/2013
AT90PWM2/3/2B/3B
Figure 9-6.
Watchdog Reset During Operation
9.0.7
MCU Status Register – MCUSR
The MCU Status Register provides information on which reset source caused an MCU reset.
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.
9.1
Internal Voltage Reference
AT90PWM2/2B/3/3B features an internal bandgap reference (1.1V). This reference is used for
Brown-out Detection. A 2.56V voltage reference is generated thanks to the bandgap
, it can be used as
a voltage reference for the DAC and/or the ADC, and can also be used as analog input for the
analog comparators. In order to use the internal Vref, it is necessary to configure it thanks to the
REFS1 and REFS0 bits in the ADMUX register and to set an analog feature which requires it.
9.1.1
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 9-4. To save power, the reference is not always turned on. The
reference is on during the following situations:
CK
CC
Bit
7
65
43
21
0
–
WDRF
BORF
EXTRF
PORF
MCUSR
Read/Write
R
R/W
Initial Value
0
See Bit Description