ATmega48/88/168
Table 8-8.
Internal Calibrated RC Oscillator Operating Modes(1)(2)
Frequency Range (MHz)
CKSEL3..0
7.3 - 8.1
0010
Notes: 1. The device is shipped with this option selected.
2. If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8
Fuse can be programmed in order to divide the internal frequency by 8.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 8-9 on page 35.
Table 8-9.
Start-up times for the internal calibrated RC Oscillator clock selection
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (VCC = 5.0V)
Power Conditions
BOD enabled
SUT1..0
00
6 CK
6 CK
14CK(1)
Fast rising power
Slowly rising power
14CK + 4.1 ms
14CK + 65 ms(2)
01
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.
2. The device is shipped with this option selected.
8.7
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 8-10.
Table 8-10. 128 kHz Internal Oscillator Operating Modes
Nominal Frequency
CKSEL3..0
128 kHz
0011
Note:
1. Note that the 128 kHz oscillator is a very low power clock source, and is not designed for a
high accuracy.
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 8-11.
Table 8-11. 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.
35
2545M–AVR–09/07
ATMEL [ ATMEL ]
