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What's the difference between 4 pin and 2 pin clock crystals?

Discussion in 'Electronic Basics' started by Kasterborus, May 15, 2008.

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  1. Kasterborus

    Kasterborus Guest

    I have a circuit requiring a 25MHz clock signal, all the components I
    can find are 4 pin 25Mhz crystals - how do these differ from the 2 pin
    crystals in my spec?

  2. Rich Webb

    Rich Webb Guest

    Links to what you have found and what you're looking for would help.

    *Probably* what you're calling "4 pin crystals" are canned
    oscillators. These take in Vcc and ground and then output a clock
    signal. Using one relieves you of the need to match the load
    capacitors to the "bare" (2 pin) crystal. They also typically have
    enough drive capacity for additional clocked logic, if needed, where
    many microcontroller crystal oscillators may not.

    4 pin oscillator

    2 pin crystal
  3. Kasterborus

    Kasterborus Guest

  4. Rich Webb

    Rich Webb Guest

    Yes. The whole assembly of [74AC04 + 1 Mohm + crystal + caps] is
    conceptually what's packaged inside of a canned (4-pin) oscillator.
  5. If you go with the four pin part you can dump the 74ac04, the 1M
    resistor, the Xtal and the two 15pf caps.

    Just add power and ground and send the output to pin7 of the ir drive.
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