Electronic – Crystal drive level

crystal

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*(pifreqVdd*(Cl+Co))2

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.

Related Solutions

Electronic – crystal, resonator, crystal resonator, crystal oscillator

Crystal, or quartz crystal. Crystal resonator is not at all incorrect but they have been called just crystals for over 50 years to no harm...
Ceramic resonator; maybe just resonator, but certainly not crystal or oscillator.
Basically yes. (The middle terminal will be ground). However there are also ceramic filters with this configuration, so be careful and double check the datasheet. (Especially if they are marked with common radio IF frequencies, 455 kHz or 10.7 MHz!)
I don't know; most likely no.2 in different packaging, I have never seen quartz crystals packaged this way.
Yes.
No, though connected to the right pins on an IC it might be. A crystal oscillator is either:

a. something like package (1) with 4 pins; it takes 3.3V or 5V in, and puts out a clean signal (making it dead easy to use), or

b. a circuit involving these components, possibly a resistor, and some kind of amplifier (inside the IC, or a separate transistor) or a misused logic gate, to drive the crystal. In the latter case you can usually substitute a ceramic resonator, but you may have to change the capacitors and a gain setting resistor (or drive limiting resistor) to make it work at all, or work reliably.

Electrical – Crystal marking

This is an Abracon crystal. Code is as follows:

                                      ATUHZYR
A = Abracon identifier ---------------'||||||
T = First "ten" digit of frequency ----'|||||
U = First "unit" digit of frequency ----'||||
H = First "tenth" digit of frequency ----'|||
Z = Month letter (A to L) ----------------'||
Y = Last digit of year --------------------'|
R = Traceability code ( character A to Z ) -'

Example:

4.0000MHz February 2003 would be coded A040B3C (as in your picture).

Source: Abracon.

Best Answer

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Electronic – crystal, resonator, crystal resonator, crystal oscillator

Electrical – Crystal marking

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