Datasheet

33
2545F–AVR–06/05
ATmega48/88/168
7.7 128 kHz Internal Oscillator
The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz. The fre-
quency is nominal at 3V and 25°C. This clock may be select as the system clock by
programming the CKSEL Fuses to “11” as shown in Table 7-10.
Note: 1. The frequency is preliminary value. Actual value is TBD.
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 7-11.
Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to
14CK + 4.1 ms to ensure programming mode can be entered.
7.8 External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
7-4 on page 33. To run the device on an external clock, the CKSEL Fuses must be programmed
to “0000” (see Table 7-12).
Figure 7-4. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 7-13.
Table 7-10. 128 kHz Internal Oscillator Operating Modes
Nominal Frequency CKSEL3..0
128 kHz 0011
Table 7-11. Start-up Times for the 128 kHz Internal Oscillator
Power Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset SUT1..0
BOD enabled 6 CK 14CK
(1)
00
Fast rising power 6 CK 14CK + 4 ms 01
Slowly rising power 6 CK 14CK + 64 ms 10
Reserved 11
Table 7-12. Crystal Oscillator Clock Frequency
Frequency CKSEL3..0
0 - 20 MHz 0011
NC
EXTERNAL
CLOCK
SIGNAL
XTAL2
XTAL1
GND