Author Topic: dsPIC33: FRC vs. XTAL Primary Osc  (Read 4955 times)

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Offline A-sic Enginerd

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dsPIC33: FRC vs. XTAL Primary Osc
« on: July 13, 2011, 04:51:05 pm »
Any trade-offs?
Please, I get how to select between them and all the clocking options there and blah blah blah. No need to waste your typing time giving the rundown on how to set things up from that perspective.

My question is limited strictly to whether there's any advantage / disadvantage of going with internal vs. external osc.

Am planning on feeding it through to the PLL (unless someone convinces me of a good reason not to), and application is pretty basic. Vanilla system control. This part is actually way overkill for this application, but want to get more familiar with it for an up-coming project. Nothing where super tight freq control is mission critical or anything like that.

Thanks guys.
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Offline ejeffrey

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Re: dsPIC33: FRC vs. XTAL Primary Osc
« Reply #1 on: July 13, 2011, 05:46:45 pm »
Internal oscillator is cheaper and easier to set up, and external crystal is more accurate, stable, and can be any frequency you want.  I guess the external crystal takes more power, but I don't know.  Crystals are more shock/vibration sensitive.

You would use the external crystal when you needed a communications interface to be at a specific frequency or if you wanted a stable clock for the ADC sample rate.
 

Offline A-sic Enginerd

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Re: dsPIC33: FRC vs. XTAL Primary Osc
« Reply #2 on: July 13, 2011, 06:50:15 pm »
Internal oscillator is cheaper and easier to set up, and external crystal is more accurate, stable, and can be any frequency you want.  I guess the external crystal takes more power, but I don't know.  Crystals are more shock/vibration sensitive.

You would use the external crystal when you needed a communications interface to be at a specific frequency or if you wanted a stable clock for the ADC sample rate.

Got it. So internal is probably OK for generic don't give a crap applications. Jitter and stability can be a concern. External can give me precision and long term stability. Sounds reasonable.

Thanks!
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Online NiHaoMike

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Re: dsPIC33: FRC vs. XTAL Primary Osc
« Reply #3 on: July 19, 2011, 03:46:30 am »
FWIW, my friend Brittany Benzaia is using the FRC (with PLL I assume) in the dsPIC30F4011 for a hybrid A/C controller (high speed SRM compressor drive plus some other A/C related control) and it was obviously more than accurate enough for that job.

However, my friend Megan Cornner used 4 dsPICs (don't know which ones) in a model airplane and she couldn't get the propellers to operate balanced enough until she put a crystal oscillator module in the hub (which has an Arduino as the flight computer) and ran a coax to each dsPIC. (I assume the actual problem was noise messing up the oscillators or an overlooked configuration mistake since a few percent difference should not cause any flight problems.)
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