Rainbow Electronics ATmega8515L - Manual (Page 47)

47
ATmega8515(L)
2512A–AVR–04/02
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 interval can be adjusted as shown in Table 21 on page 49. 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 ATmega8515 resets and executes from the Reset Vector. For tim-
ing details on the Watchdog Reset, refer to page 46.
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 “Timed
Sequences for Changing the Configuration of the Watchdog Timer” on page 50 for
details.
Table 19. Internal Voltage Reference Characteristics
Symbol Parameter Min Typ Max Units
V
BG
Bandgap reference voltage 1.15 1.23 1.35 V
t
BG
Bandgap reference start-up time 40 70 µs
I
BG
Bandgap reference current consumption 10 µA