Atmel AVR042, Примечание к применению

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AVR Hardware Design Considerations [APPLICATION NOTE]
Atmel-2521M-AVR-Hardware-Design-Considerations_ApplicationNote_092014
11
One should be aware that if selecting a different clock source than is actually applied, the AVR might not run
since different oscillator circuits are activated internally in the AVR dependent on the selected clock option. Since
the fuses are not cleared by a memory erase, it can cause problems if incorrect settings are chosen
4.2 General About Crystals and Ceramic Resonators
The typical type of crystal used for the AVR is the AT-cut parallel resonant crystal. The ceramic resonator is very
similar to the AT-cut parallel resonant crystal, but is so to say a low cost, low quality version of the crystal. The
ceramic resonator has a lower Q-value, which is both an advantage and disadvantage. Due to the lower Q-value
the oscillator frequency of the ceramic resonator can more easily be “tuned” to a desired frequency, but is also
more sensitive to temperature and load changes, causing undesired frequency variations. The advantage of the
ceramic resonators is that it has a faster start-up than crystals.
In general there will not be distinguished between crystals and ceramic resonators in this section and the term
“resonator” will thus refer to both devices.
Table 4-1. (Style - tt.table title)
Ceramic Resonator Quartz Crystal
Aging ±3000ppm ±10ppm
Frequency tolerance ±2000 - 5000ppm ±20ppm
Frequency temperature characteristics ±20 - 50ppm/˚C ±0.5ppm/˚C
Frequency pullability ±100 - 350ppm/pF ±15ppm/pF
Oscillator rise time 0.01ms - 0.5ms 1ms - 10ms
Quality factor (Qm) 100 - 5000 10
3
– 5 x 10
5
The parallel resonator is intended used in circuits, which contains reactive components, usual capacitors. Such
circuits depend on the combination of the reactive components and the resonator to accomplish the phase shift
necessary to start and maintain the oscillation at a given frequency. Basic oscillator circuits used for parallel
resonators are illustrated in Figure 4-1 . The part of the circuit above the dashed line, represent the oscillator
circuit internally in the AVR.
Figure 4-1. Basic Inverter Circuits Equivalent to the Oscillator Circuits in AVRs
R
f
C
L2
C
L1
Xtal
XTAL1 XTAL2
(A)
Clo
R
f
C
L2
C
L1
Xtal
XTAL1/
TOSC1
XTAL2/
TOSC2
R
b
(B)
Oscillator circuit for crystals and ceramic resonators faster than 400kHz.
2. Circuit for low frequency crystals (32.768kHz) (not on all Atmel AVR).
Simplifying the description of the Atmel AVR built-in oscillator circuits they can be understood as the inverter
based oscillator circuits illustrated in Figure 4-1 . The circuit used with resonators of frequencies above 400kHz is
depicted in (A). When using this circuit, capacitive load must be applied externally. The oscillator circuit seen in