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91
8210C–AVR–09/11
Atmel AVR XMEGA D
•
Enabled:
In this mode, the V
CC
level is continuously monitored, and a drop in V
CC
below V
BOT
for a period of t
BOD
will give a brownout reset
•
Sampled:
In this mode, the BOD circuit will sample the V
CC
level with a period identical to
that of the 1kHz output from the ultra low power (ULP) internal oscillator. Between each
sample, the BOD is turned off. This mode will reduce the power consumption compared to
the enabled mode, but a fall in the V
CC
level between two positive edges of the 1kHz ULP
oscillator output will not be detected. If a brownout is detected in this mode, the BOD circuit is
set in enabled mode to ensure that the device is kept in reset until V
CC
is above V
BOT
again
The BODACT fuse determines the BOD setting for active mode and idle mode, while the
BODPD fuse determines the brownout detection setting for all sleep modes, except idle mode.
8.4.3
External Reset
The external reset circuit is connected to the external RESET pin. The external reset will trigger
when the RESET pin is driven below the RESET pin threshold voltage, V
RST
, for longer than the
minimum pulse period, t
EXT
. The reset will be held as long as the pin is kept low. The RESET pin
includes an internal pull-up resistor.
Figure 8-5.
External reset characteristics.
For external reset characterization data consult the device datasheet.
8.4.4
Watchdog Reset
The watchdog timer (WDT) is a system function for monitoring correct program operation. If the
WDT is not reset from the software within a programmable timout period, a watchdog reset will
be given. The watchdog reset is active for one to two clock cycles of the 2MHz internal oscillator.
Table 8-3.
BOD setting fuse decoding.
BODACT[1:0]/ BODPD[1:0]
Mode
00
Reserved
01
Sampled
10
Enabled
11
Disabled
CC
t
EXT