Hello:
In my application design, a 8MHz crystal is used and the SCG_SOSCCFG[RANGE] is set to 2b11, therefore the gmXOSC shall be 16mA/V minimum and 47mA/V maximum
And I followed the equation gm_crit = 4 * (ESR + RS) * (2πF)^2 * (C0 + CL)^2 to calculate and compare 5*gm_crit to the datasheet value. Should I use 16mA/V or use 47mA/V ?
I don't know if 16 ~47mA/V means part to part variation. If so, from WCCA perspective, I think I shall guarantee the 5*gm_crit < 16mA/V. However, what confused me is in the document AN5426 page 11. The given calculation example uses 47mA/V as criteria.
Can some one help me to clarify? Thank you!
Hello @Stanley_Xu,
Strictly for WCCA, use the minimum gmXOSC = 16 mA/V. The 16–47 mA/V range reflects combined process (part-to-part), voltage, and temperature variation — any given part will deliver a gmXOSC somewhere in that range. The datasheet criterion gmXOSC > 5 × gm_crit guarantees proper oscillation startup and 5x gm_crit can be considered as very safe, while 3x gm_crit is still safe.
Regarding AN5426: the example uses 47 mA/V (max) for illustration, not as a WCCA.
On the other hand, the datasheet also notes: "RS should be selected carefully to have appropriate oscillation amplitude for both protecting crystal or resonator device and satisfying proper oscillation startup condition."
Regards,
Daniel