Atmel ATmega645, Application Note

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Atmel AVR543
3
8423A-AVR-11/11
Table 2-3. ESR recommendation for 32.768kHz crystals for Atmel
ATmega645A/6450A/649A/6490A.
Crystal CL [pF] Max ESR
(1)
[k Ω ]
6.5 75
9.0 65
12.5 30
Note: 1. The values stated are for an oscillator allowance safety margin of 5. Since the
oscillator’s transconductance is temperature compensated one can use a safety
margin of 4, thus giving a max ESR of 90, 80 and 40k
Ω respectively.
For examples of crystals that comply with the requirements, see Appendix A .
The startup times are increased as shown in Table 2-4 .
Table 2-4. Startup times with 32.768kHz crystals.
Crystal CL [pF]
Startup time
(1)
[ms]
Atmel ATmega645/6450/649/6490
Startup time
(1)
[ms]
ATmega645A/6450A/649A/6490A
6.5 - 600
9.0 300 700
12.5 400 1700
Note: 1. Crystals usually need ~3000ms before they are completely stable with any
oscillator design. The time stated is before the crystal is running with a sufficient
amplitude and frequency stability.