Table 6. Device Clocking Options Select
Device Clocking Option
CKSEL3..0(1)
1000 - 0101
0100 - 0001
0000
External RC Oscillator
Calibrated Internal RC Oscillator
External Clock
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 “ATmega128
Typical Characteristics” on page 332.
Table 7. Number of Watchdog Oscillator Cycles
Typical Time-out (VCC = 5.0V)
Typical Time-Out (VCC = 3.0V)
Number of Cycles
4K (4,096)
4.1 ms
65 ms
4.3 ms
69 ms
64K (65,536)
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.
38
ATmega128(L)
2467P–AVR–08/07
ATMEL [ ATMEL ]
