Datasheet

35
8025M–AVR–6/11
ATmega48P/88P/168P
9.7 128 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 9-13.
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 9-14.
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.
9.8 External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
9-4 on page 35. To run the device on an external clock, the CKSEL Fuses must be programmed
to “0000” (see Table 9-15).
Figure 9-4. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 9-16.
Table 9-13. 128 kHz Internal Oscillator Operating Modes
Nominal Frequency
(1)
CKSEL[3:0]
128 kHz 0011
Table 9-14. Start-up Times for the 128 kHz Internal Oscillator
Power Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset SUT[1:0]
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 9-15. Crystal Oscillator Clock Frequency
Frequency CKSEL[3:0]
0 - 20 MHz 0000
NC
EXTERNAL
CLOCK
SIGNAL
XTAL2
XTAL1
GND