Rainbow Electronics ATmega64C1 - Manual (Page 49)

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7647A–AVR–02/08
ATmega32/64/M1/C1
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
ATmega32/64/M1/C1 features an internal bandgap reference. This reference is used for Brown-
out DetectionDetection, and it can be used as an input to the Analog Comparators or the ADC.
The V
REF
2.56V reference to the ADC, DAC or Analog Comparators is generated from the inter-
nal 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 Compar-
ator 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
Note: 1. Values are guidelines only.
Table 7-4. Internal Voltage Reference Characteristics (1)
Symbol Parameter Condition Min. Typ. Max. Units
V
BG
Bandgap reference voltage 1.1 V
t
BG
Bandgap reference start-up time 40 µs
I
BG
Bandgap reference current
consumption
15 µA