David Montero

crystal drive level

Discussion created by David Montero on Sep 21, 2016

Dear all

I´m working on a new design with MPC5744 (LQFP) microcontroller. As far as I know, the crystal unit drive level could be obtained from the following equation:

P=2*Rl*(pi*freq*Vdd*(Cl+Co))2

Taking in to account MPC5744 evaluation board, we have following values:

Rlà crystal series resistance 50R

Freqà40 MHz

VDDà1,7V (from datasheet)

Clà8 pF (stray capacitance 4 pF)

Coà6 pF (shunt capacitance)

Applying those values in the equation I obtain that the crystal unit requires 894uW Drive level. I don’t understand how the  eval board crystal unit could work, as it has a 500uW drive capability.

I think that I´m doing something wrong but I don´t know what.

Thanks in advance.

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