Datasheet

34
ATmega169A/PA/329A/PA/649A/P/3290A/PA/6490A/P [DATASHEET]
8284E–AVR–02/2013
The Low-frequency XTAL Oscillator must be selected by setting the CKSEL Fuses to “0110” or “0111” as shown in
Table 9-11 on page 34. Start-up times are determined by the SUT Fuses as shown in Table 9-10.
Note: 1. This option should only be used if frequency stability at start-up is not important for the application
9.7 External clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure 9-3. To run the
device on an external clock, the CKSEL Fuses must be programmed to “0000”.
Figure 9-3. External clock drive configuration.
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in Table 9-13.
Table 9-10. Start-up times for the low-frequency XTAL oscillator clock selection.
SUT1..0 Additional delay from reset (V
CC
= 5.0V) Recommended usage
00 4CK Fast rising power or BOD enabled
01 4CK + 4.1ms Slowly rising power
10 4CK + 65ms Stable frequency at start-up
11 Reserved
Table 9-11. Start-up times for the low-frequency XTAL oscillator clock selection.
CKSEL3...
0
Start-up time from
power-down and power-save Recommended usage
0110
(1)
1K CK
0111 32K CK Stable frequency at start-up
Table 9-12. XTAL oscillator clock frequency.
CKSEL3:0 Frequency range
Atmel ATmega169A/169PA/649A/649P 0000 0 - 16MHz
Atmel ATmega329A/329PA/3290A/3290PA/6490A/6490P 0000 0 - 20MHz
NC
EXTERNAL
CLOCK
SIGNAL
XTAL2
XTAL1
GND