What crystal package to use : small 3.2x2.5mm or big HC49/US? One parameter determining the choice is the drive level of the crystal. If the drive level is too high, i.e. >200µW typically, the small crystal can't be used.
The drive level is calculated as follows:
- Vpp = peak-to-peak voltage, measured differentiallly over the crystal
- f = crystal frequency
- CL = load capacitance, see the Pierce oscillator on Wikipedia how you calculate it.
- ESR = equivalent series resistor, from the crystal's datasheet
Calculation of CL
- C1 = C2 = 18pF
- Cs = 9pF
- Ci = 6pF, Co=16pF (datasheet Attiny85)
Calculation of drive level
- f = 8MHz
- ESR=80ohm, from the crystal's datasheet
- Vpp=1.7Vpp, measured with differential probe over the crystal
The smaller crystal package can be used safely.
Another parameter that determines the choice is availability of the crystals. For 8MHz, HC49US crystals have better specs, are more widely used and are cheaper than the 3.2x2.5mm package crystals.