38
2467S–AVR–07/09
ATmega128
Note: 1. For all fuses “1” means unprogrammed while “0” means programmed.
The various choices for each clocking option is given in the following sections. When the CPU
wakes up from Power-down or Power-save, the selected clock source is used to time the start-
up, ensuring stable Oscillator operation before instruction execution starts. When the CPU starts
from reset, there is as an additional delay allowing the power to reach a stable level before com-
mencing normal operation. The Watchdog Oscillator is used for timing this real-time part of the
start-up time. The number of WDT Oscillator cycles used for each time-out is shown in Table 7.
The frequency of the Watchdog Oscillator is voltage dependent as shown in the “Typical Char-
acteristics” on page 333.
Default Clock
Source
The device is shipped with CKSEL = “0001” and SUT = “10”. The default clock source setting is
therefore the Internal RC Oscillator with longest startup time. This default setting ensures that all
users can make their desired clock source setting using an In-System or Parallel Programmer.
External RC Oscillator 1000 - 0101
Calibrated Internal RC Oscillator 0100 - 0001
External Clock 0000
Table 7. Number of Watchdog Oscillator Cycles
Typical Time-out (V
CC
= 5.0V) Typical Time-Out (V
CC
= 3.0V) Number of Cycles
4.1 ms 4.3 ms 4K (4,096)
65 ms 69 ms 64K (65,536)
Table 6. Device Clocking Options Select
Device Clocking Option CKSEL3..0
(1)
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