35
8271D–AVR–05/11
ATmega48A/PA/88A/PA/168A/PA/328/P
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 9-13.
Note: 1. Note that the 128kHz oscillator is a very low power clock source, and is not designed for 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 + 4.1ms 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. 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. 128kHz Internal Oscillator Operating Modes
Nominal Frequency
(1)
CKSEL3...0
128kHz 0011
Table 9-14. 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 6 CK 14CK
(1)
00
Fast rising power 6 CK 14CK + 4ms 01
Slowly rising power 6 CK 14CK + 64ms 10
Reserved 11
Table 9-15. Crystal Oscillator Clock Frequency
Frequency CKSEL3...0
0 - 20MHz 0000
PB7
EXTERNAL
CLOCK
SIGNAL
XTAL2
XTAL1
GND
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