114
8331B–AVR–03/12
Atmel AVR XMEGA AU
Figure 9-1.
Reset system overview.
9.3
Reset Sequence
A reset request from any reset source will immediately reset the device and keep it in reset as
long as the request is active. When all reset requests are released, the device will go through
three stages before the device starts running again:
• Reset counter delay
• Oscillator startup
• Oscillator calibration
If another reset requests occurs during this process, the reset sequence will start over again.
9.3.1
Reset Counter
The reset counter can delay reset release with a programmable period from when all reset
requests are released. The reset delay is timed from the 1kHz output of the ultra low power
(ULP) internal oscillator, and in addition 24 System clock
(clk
SYS
) cycles are counted before reset
is released. The reset delay is set by the STARTUPTIME fuse bits. The selectable delays are
shown in
.
MCU Stat
u
s
Register (MCUSR)
Bro
w
n-o
u
t
Reset
BODLE
V
EL [2:0]
Delay Co
u
nters
TIMEOUT
W
DRF
BORF
EXTRF
PORF
ULP
Oscillator
SPIKE
FILTER
P
u
ll-
u
p Resistor
JTRF
W
atchdog
Reset
SUT[1:0]
Po
w
er-on Reset
Soft
w
are
Reset
External
Reset
PDI
Reset
Table 9-1.
Reset delay.
SUT[1:0]
Number of 1kHz ULP Oscillator Clock Cycles
Recommended Usage
00
64K Clk
ULP
+ 24 Clk
SYS
Stable frequency at startup
01
4K Clk
ULP
+ 24 Clk
SYS
Slowly
rising
power
10
Reserved
-
11
24 Clk
SYS
Fast rising power or BOD enabled