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smt crystals — Parallax Forums

smt crystals

Graham StablerGraham Stabler Posts: 2,510
edited 2010-07-21 17:37 in Propeller 1
I have some silly ideas involving the propeller and a small toy helicopter I have dismantled, I want to make a super light board and I'd like to use a little SMT crystal.

The data sheet mentions capacitance and impedance values for crystals, is it OK if the crystal has a lower value of capacitance? What can I get away with.

Anyone running on a 20Mhz crystal?

Graham

Comments

  • RaymanRayman Posts: 14,889
    edited 2010-07-21 16:04
    If you don't need TV output, I bet you could get away with a 100 MHz ceramic SMT oscillator. I think Digikey sells them. They are pretty small and light.
    But, somebody else, I think it was Phil was using 5 MHz watch crystals, which are probably the smallest and lightest option. Not really SMT though...

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  • LeonLeon Posts: 7,620
    edited 2010-07-21 16:10
    The frequency will be slightly off with a crystal with the wrong load capacitance. I haven't found it to be a problem, and I don't think that anybody else has.

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    Leon Heller
    Amateur radio callsign: G1HSM
  • RaymanRayman Posts: 14,889
    edited 2010-07-21 16:39
    Do you really want to run at 20MHz? You can't use the PLL with that fast a crystal...

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  • Bobb FwedBobb Fwed Posts: 1,119
    edited 2010-07-21 17:23
    I have been using an 18pF crystal (parallax suggests 20pF I do believe). The only difference is my crystal runs about 0.05% slow. But that doesn't affect too much, and now that I know about it, I can compensate in software for it. I use this one: ABLS-5.000MHZ-K4T because it has a super wide temperature range (ABLS-5.000MHZ-L4Q-T has wider, but I don't care so much about the low temperatures). But the crystal is not especially small (actually extremely normal sized, but SMT).

    Side note: Early on, I learned the 70C limit was not nearly enough for my applications, and I have even killed a couple 85C crystals in normal operation, but 125C is a lofty goal to hit. Because of the operating conditions of the projects I work on, I have tested the propeller, a few crystals, the FTDI, EEPROMs, and a number of other components operating at above 300F (150C) and they still work just fine! Here is a tip at these temperatures: stay away from electrolytic capacitors :-P.

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    April, 2008: when I discovered the answers to all my micro-computational-botherations!

    Some of my objects:
    MCP3X0X ADC Driver - Programmable Schmitt inputs, frequency reading, and more!
    Simple Propeller-based Database - Making life easier and more readable for all your EEPROM storage needs.
    String Manipulation Library - Don't allow strings to be the bane of the Propeller, bend them to your will!
    Fast Inter-Propeller Comm - Fast communication between two propellers (1.37MB/s @100MHz)!
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-07-21 17:37
    You can get small SMT 10MHz crystals. They're a bit outside the spec for using the PLL, but they work fine, nonetheless. I designed one into the Prop Backpack, and Parallax's Spin Stamp uses them, too.

    -Phil
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