Datasheet

38
8272A–AVR–01/10
164A/164PA/324A/324PA/644A/644PA/1284/1284P
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 “0011” as shown in Table 8-12.
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 8-13.
8.8 External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
8-4. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
Figure 8-4. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 8-15.
Table 8-12. 128 kHz Internal Oscillator Operating Modes
(2)
Nominal Frequency CKSEL3..0
128 kHz 0011
Table 8-13. 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
NC
EXTERNAL
CLOCK
SIGNAL
XTAL2
XTAL1
GND