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2490R–AVR–02/2013
ATmega64(L)
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 MCUCSR 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
ATmega64 features an internal bandgap reference. This reference is used for Brown-out Detec-
tion, and it can be used as an input to the Analog Comparator or the ADC. The 2.56V reference
to the ADC is generated from the internal bandgap reference.
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 20. To save power, the reference is not always turned on. The ref-
erence is on during the following situations:
1.
When the BOD is enabled (by programming the BODEN 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.
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.
Watchdog Timer
The Watchdog Timer is clocked from a separate On-chip Oscillator which runs at 1 Mhz. This is
the typical value at V
CC = 5V. See characterization data for typical values at other VCC levels. By
controlling the Watchdog Timer prescaler, the Watchdog Reset interval can be adjusted as
Timer. The Watchdog Timer is also reset when it is disabled and when a Chip Reset occurs.
Eight different clock cycle periods can be selected to determine the reset period. If the reset
period expires without another Watchdog Reset, the ATmega64 resets and executes from the
Reset Vector. For timing details on the Watchdog Reset, refer to
page 55.To prevent unintentional disabling of the Watchdog or unintentional change of Time-out period,
three different safety levels are selected by the fuses M103C and WDTON as shown in
Table21. Safety level 0 corresponds to the setting in ATmega103. There is no restriction on enabling
Table 20. Internal Voltage Reference Characteristics
Symbol
Parameter
Min
Typ
Max
Units
VBG
Bandgap reference voltage
1.15
1.23
1.35
V
tBG
Bandgap reference start-up time
40
70
s
IBG
Bandgap reference current consumption
10
A