ATmega48/88/168
6.7
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 6-10.
Table 6-10. 128 kHz Internal Oscillator Operating Modes
Nominal Frequency
CKSEL3..0
128 kHz
0011
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 6-11.
Table 6-11. Start-up Times for the 128 kHz Internal Oscillator
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset
Power Conditions
BOD enabled
SUT1..0
00
6 CK
6 CK
14CK(1)
Fast rising power
14CK + 4 ms
14CK + 64 ms
01
Slowly rising power
6 CK
10
Reserved
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.
6.8
External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
6-4 on page 33. To run the device on an external clock, the CKSEL Fuses must be programmed
to “0000” (see Table 6-12).
Table 6-12. Crystal Oscillator Clock Frequency
Frequency
CKSEL3..0
0 - 20 MHz
0011
Figure 6-4. External Clock Drive Configuration
NC
XTAL2
EXTERNAL
CLOCK
XTAL1
GND
SIGNAL
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 6-13.
33
2545E–AVR–02/05
MICROCHIP [ MICROCHIP ]
