I am designing a custom arduino 2560 board and was looking at surface mount crystals for it.
Looking at Digikey for 16 Mhz crystals they ask you (amongst others) for parameters such as "Frequency Stability", "Frequency Tolerance" and "Load Capacitance".
What range of values are appropriate for the Atmega2560? (the Atmega datasheet does not refer to these values)
Also should the value for the capacitors match the Load Tolerance value for the crystal?
On the Mega there is a 1 Meg resistor across the crystal and a 27 Ohm resistor in series with one of the pins of the crystal that were not on previous Arduinos. Can anyone tell me what these resistors do?
Frequency tolerance and stability only matter if you are doing timekeeping. Even then you will notice that at 16MHz the range of available accuracies isn't huge. Values for "ordinary" crystals are +/-30 or 50 ppm for both stability and tolerance.
Load capacitance isn't critical, though values different from those recommended will have some effect on the frequency. Stray capacitance can be significant, so keep the leads short.
The series and parallel resistors will have a damping effect on the resonant circuit. I assume they are included to lessen the amount of higher harmonics produced, which could generate RF interference with other devices. Normally these resistors are not included, but I guess that the Arduino boards have to be extra careful about RF since they have no shielding. Some Arduino-clones don't have them.
Just about any 16MHz fundamental crystal should do (and I think all the 16MHz crystals you find will be fundamental). Load capacitance isn't critical, but the 2 capacitors connected to the atmega crystal pins should be a little less than double the specified load capacitance. Frequency tolerance and stability is up to you, i.e. how accurate to you need the delay(), millis() and similar functions to be. If you don't need them to be very accurate, you could use a ceramic resonator instead, which is cheaper.
The 1 Mohm resistor isn't needed, it's not shown on the avr data sheet and I'm fairly sure it's built into the chip. A series resistor is normally only needed for low frequency operation e.g. when using a 32768 kHz crystal.
tim7:
The series and parallel resistors will have a damping effect on the resonant circuit. I assume they are included to lessen the amount of higher harmonics produced, which could generate RF interference with other devices. Normally these resistors are not included, but I guess that the Arduino boards have to be extra careful about RF since they have no shielding. Some Arduino-clones don't have them.
The parallel 1M DC feedback resistor is on the chip anyway and won't affect resonance Q appreciably as crystals at resonance are a few tens of ohms. Series resistor not used for 16MHz. The oscillator won't be generating harmonics as its a low-amplitude design and no clipping should be happening (this is done to save power anyway, and anyway harmonics would not be radiated on a well designed board with ground plane - compared to what the output pins can radiate).