I need some help. I'm very new to this but I want to build something using a SAMD21 Cortex M0 MCU.
Most of the eval boards and the Arduino Zero are using a 32.768khz .crystal so I figure I will do the same.
I'm having difficult understanding the datasheet and the various ways of calculating the load capacitance.
The datasheet:
http://www.atmel.com/images/atmel-42181-sam-d21_datasheet.pdf
page 1093 states these characteristics:
Cl - 12.5 pf max
Cshunt - 0.1pf typ.
Cxin32 parasitic load - 3.1 pf typ
Cxout32 parasitic load - 3.2 pf typ
Crystal ESR (Cl - 12.5pf) - 141kohm max
On page 1089, the datasheet gives the method to calculate the external load cap (Clext)
Clext = 2(Cl - Cstray - CShunt)
where based on the figure on page 1092, Cshunt is the crystal's internal shunt capacitance and Cstray is the cap between the Xin and Xout pins.
Seems simple enough but here are my questions:
Why does the datasheet show a Cshunt typical value if it is a function of the crystal? Is this the recommended value to target when selecting the crystal? Or is this an internal shunt value that isn't represented in the formula?
What are the value of the Cxin32/Cxout32 parasitic load cap used for? Are they the internal cap (to ground) of the pins? Should they figure into the calculation of Clext?
Finally, any recommendation on crystals and load caps for this specific use?
I tried to verify how to calculate the Cl values from various implementations but they often don't give the part number of the crystal used so it's hard to know what load cap the crystal requires.
Thanks in advance.