Rainbow Electronics ATmega3290P_V - Manual (Page 45)

45
ATmega329/3290/649/6490
2552H–AVR–11/06
Internal Voltage
Reference ATmega329/3290/649/6490 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.
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 19. 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 [1..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.
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 Com-
parator 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.
Note: 1. Values are guidelines only. Actual values are TBD.
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 V
CC
levels. By controlling the Watchdog Timer prescaler, the Watchdog Reset
interval can be adjusted as shown in Table 21 on page 47. The WDR – Watchdog Reset
– instruction resets the Watchdog 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 ATmega329/3290/649/6490 resets and executes from the Reset
Vector. For timing details on the Watchdog Reset, refer to Table 21 on page 47.
To prevent unintentional disabling of the Watchdog or unintentional change of time-out
period, two different safety levels are selected by the fuse WDTON as shown in Table
20. Refer to “Timed Sequences for Changing the Configuration of the Watchdog Timer”
on page 48 for details.
Table 19. Internal Voltage Reference Characteristics (1)
Symbol Parameter Condition Min Typ Max Units
V
BG
Bandgap reference voltage V
CC
= 2.7V,
T
A
= 25°C
1.0 1.1 1.2 V
t
BG
Bandgap reference start-up time V
CC
= 2.7V,
T
A
= 25°C
40 70 µs
I
BG
Bandgap reference current
consumption V
CC
= 2.7V,
T
A
= 25°C
15 µA
Table 20. WDT Configuration as a Function of the Fuse Settings of WDTON
WDTON
Safety
Level
WDT Initial
State How to Disable the
WDT How to Change
Time-out
Unprogrammed 1 Disabled Timed sequence Timed sequence
Programmed 2 Enabled Always enabled Timed sequence