AT90S/LS2333 and AT90S/LS4433
20
Notes:
1. Or external power-on reset.
This table shows the start-up times from reset. From sleep, only the clock counting part of the start-up time is used. The
watchdog oscillator is used for timing the real-time part of the start-up time. The number WDT oscillator cycles used for
each time-out is shown in Table 6.
The frequency of the watchdog oscillator is voltage dependent as shown in the Electrical Characteristics section.
Power-On Reset
A Power-On Reset (POR) pulse is generated by an on-chip detection circuit. The detection level is nominally 2.2V. The
POR is activated whenever V
CC
is below the detection level. The POR circuit can be used to trigger the start-up reset, as
well as detect a failure in supply voltage.
The Power-On Reset (POR) circuit ensures that the device is reset from power-on. Reaching the power-on reset threshold
voltage invokes a delay counter, which determines the delay, for which the device is kept in RESET after V
CC
rise. The
time-out period of the delay counter is a combination of internal RC oscillator cycles and external oscillator cycles, and it
can be defined by the user through the CKSEL fuses. The eight different selections for the delay period are presented in
Table 5. The RESET signal is activated again, without any delay, when the V
CC
decreases below detection level.
Figure 25. MCU Start-Up, RESET Tied to VCC.
Table 5. Reset Delay Selections
CKSEL [2:0]
Start-Up Time, t
TOUT
at V
CC
= 2.7V
Start-Up Time, t
TOUT
at V
CC
= 5.0V
Recommended Usage
000
16 ms + 6 CK
4 ms + 6 CK
External Clock, slowly rising power
001
6 CK
6 CK
External Clock, BOD enabled
010
256 ms + 16K CK
64 ms + 16K CK
Crystal Oscillator
011
16 ms + 16K CK
4 ms + 16K CK
Crystal Oscillator, fast rising power
100
16K CK
16K CK
Crystal Oscillator, BOD enabled
101
256 ms + 1K CK
64 ms + 1K CK
Ceramic Resonator
110
16 ms + 1K CK
4 ms + 1K CK
Ceramic Resonator, fast rising power
111
1K CK
1K CK
Ceramic Resonator, BOD enabled
Table 6. Number of Watchdog Oscillator Cycles
Time-out
Number of cycles
4.0 ms (at V
cc
=5.0V)
4K
64 ms (at V
cc
=5.0V)
64K
VCC
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST