50
ATmega8515(L)
2512K–AVR–01/10
Internal Voltage
Reference
ATmega8515 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.
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 BODEN Fuse).
2.
When the bandgap reference is connected to the Analog Comparator (by setting
the ACBG bit in ACSR).
Thus, when the BOD is not enabled, after setting the ACBG bit, the user must always
allow the reference to start up before the output from the Analog Comparator is used. To
reduce power consumption in Power-down mode, the user can avoid the two 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 frequency at V
CC = 5V. See characterization data for typical
values at other V
CC levels. By controlling the Watchdog Timer prescaler, the 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 ATmega8515 resets and executes from the Reset Vector. For tim-
ing details on the Watchdog Reset, refer to
page 49.
To prevent unintentional disabling of the Watchdog or unintentional change of time-out
period, three different safety levels are selected by the Fuses S8515C and WDTON as
shown in
Table 20. Safety level 0 corresponds to the setting in AT90S4414/8515. There
is no restriction on enabling the WDT in any of the safety levels. Refer to
“Timeddetails.
Table 19. 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