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Ceramic Resonator for 18-2 oscillator

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  • bill rowe
    with avrs i m used to using 3 pin ceramic resonators like this http://www.digikey.ca/product-detail/en/ZTT-2.00MG/XC1806-ND/440131 Can I use one with the 1802,
    Message 1 of 4 , Aug 5, 2013
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      with avrs i'm used to using 3 pin ceramic resonators like this http://www.digikey.ca/product-detail/en/ZTT-2.00MG/XC1806-ND/440131

      Can I use one with the 1802, just wiring it between clock and crystal with the 3rd pin to ground?
    • Lee Hart
      ... The resonator will work, but you also need two resistors. I am now using this very same part (only the 1.8 MHz version) on my rev.F Membership Cards. The
      Message 2 of 4 , Aug 5, 2013
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        bill rowe wrote:
        > with avrs i'm used to using 3 pin ceramic resonators like
        > this http://www.digikey.ca/product-detail/en/ZTT-2.00MG/XC1806-ND/440131
        >
        > Can I use one with the 1802, just wiring it between clock and crystal
        > with the 3rd pin to ground?

        The resonator will work, but you also need two resistors. I am now using
        this very same part (only the 1.8 MHz version) on my rev.F Membership
        Cards. The resonator gives you a stable 1.8 MHz oscillator, very close
        to the "standard" 1.79 MHz used with the 1802 and 1861. Here is the
        circuit needed (view with a fixed width font like Courier):

        ..... inverter (inside 1802, or external)
        ... _________|\o______________oscillator output
        .. | ....... |/ ..........|
        .. |____/\/\_______/\/\___|
        .. | ... R1 .... | . R2
        .. | .... _ .... |
        .. |____|| ||____|
        . _|_ . ||_|| . _|_
        . ___ ... X1 .. ___
        .. | C1 ..... C2 |
        . gnd ......... gnd

        X1 is the resonator, and C1 and C2 are its built-in capacitors (about 30pf).

        R1 is a high-value (like 1 to 10 megohms). It is needed to bias the
        inverter's input into its linear region.

        R2 is chosen to act as a low-pass filter with C1 and C2. Resonators have
        a much lower "Q" than crystals. Without R2, it can oscillate at an
        unstable frequency *above* the resonator frequency, set by the RC time
        constant of R2, C1, C2, and the stray capacitance across the resonator
        and R1. If R2 is too large, then it can oscillate at a *lower* frequency
        than the resonator, again due to the RC time constants.

        On the Membership Card, I used a single 1 meg trimpot to provide both R1
        and R2. The inverter is a 4093 schmitt-trigger gate.

        - With the trimpot at one end (R1=1meg, R2=0) it runs as an RC
        oscillator at 2-2.5 MHz.
        - With the trimpot at the other end (R1=0, R2=1meg), it runs as
        an RC oscillator at a very low frequency like 10-20 KHz.
        - With the trimpot set for about R2=100-300 ohms, it "locks in"
        on the resonator's frequency and runs at 1.79 MHz.

        --
        Failure is only the opportunity to begin again more intelligently.
        -- Henry Ford
        --
        Lee A. Hart, http://www.sunrise-ev.com/MembershipCard.htm
      • bill rowe
        thanks lee. So I would need like 1M and 220 ohms? that would be an RC of 130 ns which seems odd. To: cosmacelf@yahoogroups.com From: leeahart@earthlink.net
        Message 3 of 4 , Aug 5, 2013
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          thanks lee. So I would need like 1M and 220 ohms?  that would be an RC of 130 ns which seems odd.



          To: cosmacelf@yahoogroups.com
          From: leeahart@...
          Date: Mon, 5 Aug 2013 13:03:13 -0500
          Subject: Re: [cosmacelf] Ceramic Resonator for 18-2 oscillator

           
          bill rowe wrote:
          > with avrs i'm used to using 3 pin ceramic resonators like
          > this http://www.digikey.ca/product-detail/en/ZTT-2.00MG/XC1806-ND/440131
          >
          > Can I use one with the 1802, just wiring it between clock and crystal
          > with the 3rd pin to ground?

          The resonator will work, but you also need two resistors. I am now using
          this very same part (only the 1.8 MHz version) on my rev.F Membership
          Cards. The resonator gives you a stable 1.8 MHz oscillator, very close
          to the "standard" 1.79 MHz used with the 1802 and 1861. Here is the
          circuit needed (view with a fixed width font like Courier):

          ..... inverter (inside 1802, or external)
          ... _________|\o______________oscillator output
          .. | ....... |/ ..........|
          .. |____/\/\_______/\/\___|
          .. | ... R1 .... | . R2
          .. | .... _ .... |
          .. |____|| ||____|
          . _|_ . ||_|| . _|_
          . ___ ... X1 .. ___
          .. | C1 ..... C2 |
          . gnd ......... gnd

          X1 is the resonator, and C1 and C2 are its built-in capacitors (about 30pf).

          R1 is a high-value (like 1 to 10 megohms). It is needed to bias the
          inverter's input into its linear region.

          R2 is chosen to act as a low-pass filter with C1 and C2. Resonators have
          a much lower "Q" than crystals. Without R2, it can oscillate at an
          unstable frequency *above* the resonator frequency, set by the RC time
          constant of R2, C1, C2, and the stray capacitance across the resonator
          and R1. If R2 is too large, then it can oscillate at a *lower* frequency
          than the resonator, again due to the RC time constants.

          On the Membership Card, I used a single 1 meg trimpot to provide both R1
          and R2. The inverter is a 4093 schmitt-trigger gate.

          - With the trimpot at one end (R1=1meg, R2=0) it runs as an RC
          oscillator at 2-2.5 MHz.
          - With the trimpot at the other end (R1=0, R2=1meg), it runs as
          an RC oscillator at a very low frequency like 10-20 KHz.
          - With the trimpot set for about R2=100-300 ohms, it "locks in"
          on the resonator's frequency and runs at 1.79 MHz.

          --
          Failure is only the opportunity to begin again more intelligently.
          -- Henry Ford
          --
          Lee A. Hart, http://www.sunrise-ev.com/MembershipCard.htm

        • Lee Hart
          ... The value of R2 includes both the external resistor and the internal output resistance of the inverter. The capacitance also includes the stray capacitance
          Message 4 of 4 , Aug 5, 2013
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            bill rowe wrote:
            > thanks lee. So I would need like 1M and 220 ohms? that would be an RC
            > of 130 ns which seems odd.

            The value of R2 includes both the external resistor and the internal
            output resistance of the inverter. The capacitance also includes the
            stray capacitance to ground, besides that inside the resonator itself.
            So it is rather hard to predict.

            I was using a 4093 as the oscillator inverter. With a selection of
            different brands of 4093's, I got

            R2=0 2.0 to 3.4 MHz at 5v
            1.4 to 2.3 MHz at 4v
            0.5 to 1.2 MHz at 3v

            Notice that 1.8 MHz (my resonator's frequency) is *inside* these limits.
            If the oscillator just happened to be close to 1.8 MHz, it "locks in" on
            this frequency, and wants to stay there. For example, if I powered it up
            at 5v, it might start oscillating at 2.5 MHz. As I lowered the supply
            voltage, the frequency drops until it gets to 2 MHz, when it suddenly
            "jumps" to 1.8 MHz. Once at this frequency, it stays there even if I
            raise the supply voltage back to 5v.

            Next, I studied the range of values for R2 that would make it start at
            1.8 MHz and stay there.

            5v supply R2 = 500 to 2.2k
            4v supply R2 = 300 to 1.5k
            3.6v supply R2 = 0 to 1k

            The actual value for R2 isn't quite that wide; it varies perhaps 2:1 for
            each chip. For example at 5v, one chip might need R2 = 500-1k, another
            needs 1k-2k, etc.

            The internal inverter inside the 1802 probably has a higher output
            resistance than the 4093, because it is optimized to be used as an
            oscillator. So, you'll have to experiment. But I'm quite sure you can
            find a value for R2 that will work with all 1802's at 5v.
            --
            Failure is only the opportunity to begin again more intelligently.
            -- Henry Ford
            --
            Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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