Electronic – Crystal drive level


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:

Taking in to account MPC5744 evaluation board, we have following values:
Rl= crystal series resistance 50R
Freq=40 MHz
VDD=1,7V (from datasheet)
Cl=8 pF (stray capacitance 4 pF)
Co=6 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.

Best Answer

Let me correct your language a bit. The maximum drive capability of the crystal in the eval board is 500uW. It requires typically 50uW to work.

Your calculation indeed indicates that it is possible the crystal is being overdriven. That could result in drift or other problems. However note that the 50\$\Omega\$ is a maximum figure and the voltage across the crystal is probably less than the supply voltage (you would probably need an active FET oscilloscope probe to measure it directly with any accuracy).

Chances are in practice the crystal is operating within proper bounds, but you should certainly take care to use clock monitor features if it's a safety-critical application. There is always this sort of unsatisfactory guarantee of proper operation (drive level and starting) with crystals unless the manufacturer recommends a specific supplier and model (and stands behind their recommendation).

You can use an external oscillator module or a resonator if you want better guarantees.