Datasheet

37
7674F–AVR–09/09
ATmega164P/324P/644P
7.7 128 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 “0011” as shown in Table 7-12.
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 7-13.
7.8 External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
7-4. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
Figure 7-4. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 7-15.
Table 7-12. 128 kHz Internal Oscillator Operating Modes
Nominal Frequency CKSEL3..0
128 kHz 0011
Table 7-13. Start-up Times for the 128 kHz Internal Oscillator
Power Conditions
Start-up Time from
Power-down and Power-save
Additional Delay from
Reset SUT1..0
BOD enabled 6 CK 14CK 00
Fast rising power 6 CK 14CK + 4 ms 01
Slowly rising power 6 CK 14CK + 64 ms 10
Reserved 11
Table 7-14. Crystal Oscillator Clock Frequency
Nominal Frequency CKSEL3..0
0 - 20 kHz 0000
NC
EXTERNAL
CLOCK
SIGNAL
XTAL2
XTAL1
GND