newguy
Joined: 24 Jun 2004
Posts: 1907
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| Crystal drive level sanity check.... |
 Posted: Wed Feb 28, 2007 5:25 pm |
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I've never worried about crystal drive before, but for some reason I started to today. I'm about to start the layout of a new board and I'm wondering if I need a series resistor in the crystal circuit or not. If my calculations are correct, I don't, but I'd like someone to tell me if I made a mistake or not.
Here goes....
Crystal specs: 18.432 MHz AT strip-cut, series resistance at resonance 40 ohms max, max drive level 0.5 milliwatt (500 microwatt).
PIC will be running at 5V, HS oscillator mode.
RMS voltage from OSC2 pin calc is as follows: Worst case (highest V), is that the signal coming from OSC2 pin is a 5V square wave, 50% duty cycle. RMS voltage of a DC offset square wave is
Vrms = square root(Vdc^2 + Vacmax^2) = root(2.5^2 + 2.5^2) = 3.54V
From PIC's data sheet (18F2680 by the way), the maximum leakage current into/out of OSC1 pin is +/- 5uA. Which implies that the input impedance of the OSC1 pin is 1 Mohm.
My reasoning then suggests that a 3.54Vrms voltage source is driving a 40 ohm resistance (the crystal) and a series 1 Mohm resistance (the PIC). Total power dissipation of the entire circuit is then
Ptotal = 3.54V^2/(1,000,040 ohms) = 12.5 microwatts.
The crystal's share of this will be absolutely negligible. Certainly much, much less than its 500 microwatt maximum.
Which brings me to my query: if I did my calculations right (and please correct me if I didn't), why does the datasheet suggest that I may need a series resistor of about 330 ohms in order not to overdrive the crystal when it's not going to happen anyway? I guess that begs another question, are there crystals which have maximum drive levels in the microwatt or less range? [I've looked but haven't been able to find any] |
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