Datasheet

34
2545T–AVR–05/11
ATmega48/88/168
Notes: 1. The device is shipped with this option selected.
2. If 8MHz frequency exceeds the specification of the device (depends on V
CC
), 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 9-9.
Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to
14CK + 4.1ms to ensure programming mode can be entered.
2. The device is shipped with this option selected.
9.7 128kHz internal oscillator
The 128kHz internal oscillator is a low power oscillator providing a clock of 128kHz. 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 9-10.
Note: 1. Note that the 128kHz 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 9-11.
Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to
14CK + 4.1ms to ensure programming mode can be entered.
Table 9-8. Internal calibrated RC oscillator operating modes
(1)(2)
.
Frequency range (MHz) CKSEL3..0
7.3 - 8.1 0010
Table 9-9. Start-up times for the internal calibrated RC Oscillator clock selection.
Power conditions
Start-up time from
power-down and power-save
Additional delay from
reset (V
CC
= 5.0V) SUT1..0
BOD enabled 6CK 14CK
(1)
00
Fast rising power 6CK 14CK + 4.1ms 01
Slowly rising power 6CK 14CK + 65ms
(2)
10
Reserved 11
Table 9-10. 128kHz internal oscillator operating modes.
Nominal frequency CKSEL3..0
128kHz 0011
Table 9-11. Start-up times for the 128kHz internal oscillator.
Power conditions
Start-up time from
power-down and power-save
Additional delay from
reset SUT1..0
BOD enabled 6CK 14CK
(1)
00
Fast rising power 6CK 14CK + 4ms 01
Slowly rising power 6CK 14CK + 64ms 10
Reserved 11