40
7679H–CAN–08/08
AT90CAN32/64/128
Notes: 1. These options should only be used when not operating close to the maximum frequency of the
device, and only if frequency stability at start-up is not important for the application. These
options are not suitable for crystals.
2. These options are intended for use with ceramic resonators and will ensure frequency stability
at start-up. They can also be used with crystals when not operating close to the maximum fre-
quency of the device, and if frequency stability at start-up is not important for the application.
5.5 Low-frequency Crystal Oscillator
To use a 32.768 kHz watch crystal as the clock source for the device, the low-frequency crystal
Oscillator must be selected by setting the CKSEL Fuses to “0100”, “0101”, “0110”, or “0111”.
The crystal should be connected as shown in Figure 5-3.
Figure 5-3. Low-frequency Crystal Oscillator Connections
12-22 pF capacitors may be necessary if the parasitic impedance (pads, wires & PCB) is very
low.
Table 5-4. Start-up Times for the Oscillator Clock Selection
CKSEL0 SUT1..0
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC
= 5.0V)
Recommended Usage
0 00 258 CK
(1)
14 CK + 4.1 ms
Ceramic resonator, fast
rising power
0 01 258 CK
(1)
14 CK + 65 ms
Ceramic resonator, slowly
rising power
010 1K CK
(2)
14 CK
Ceramic resonator, BOD
enabled
011 1K CK
(2)
14 CK + 4.1 ms
Ceramic resonator, fast
rising power
100 1K CK
(2)
14 CK + 65 ms
Ceramic resonator, slowly
rising power
1 01 16K CK 14 CK
Crystal Oscillator, BOD
enabled
1 10 16K CK 14 CK + 4.1 ms
Crystal Oscillator, fast
rising power
1 11 16K CK 14 CK + 65 ms
Crystal Oscillator, slowly
rising power
XTAL2
XTAL1
GND
12 - 22 pF
12 - 22 pF
32.768 KHz
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