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clock buffers ?

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  • Chris Swinson
    This may sound dumb, but a 32mhz osc chip claims 4.5V output, but even just loaded on my scope its lucky to get up to 3 volts. As such it is useless as I need
    Message 1 of 5 , Mar 9, 2012
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      This may sound dumb, but a 32mhz osc chip claims 4.5V output, but even just
      loaded on my scope its lucky to get up to 3 volts. As such it is useless as
      I need a true TTL logic one of 4.5volts or better. I tried a buffer (F type)
      and it just made it worse down to 2volts. The only way I got it to work was
      to run a TTL buffer on about 7 volts! Of course it didn't last very long,
      but the only option I see is to use a cmos buffer, and run that on 7volts to
      get a 5volt logic 1 output. Though I am not sure the cmos device will have
      enough current to drive TTL stuff at speed anyway.

      So I wonder if anyone has any suggestions on how to buffer the 32mhz so it
      gives a correct 4.5V logic 1 output ? I know this was being done in the 80's
      as I have a old video chip which outputs 4.5 volt from a 3V crystal, so its
      nothing new, just seems "new" chips just do not seem capable for some reason
      :-\

      Cheers,
      Chris
    • Bert Hickman
      Do you have a 0.01 - 0.1 uF ceramic bypass cap connected between your buffer s Vcc and ground? You may be seeing the effects of power supply bus inductance.
      Message 2 of 5 , Mar 9, 2012
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        Do you have a 0.01 - 0.1 uF ceramic bypass cap connected between your
        buffer's Vcc and ground? You may be seeing the effects of power supply
        bus inductance.

        TTL is "current sinking" logic - an output is at logical zero when
        sinking current at an output Vsat voltage between 0.2 - 0.8 volts. Other
        than charging line and input gate capacitance, a logic "one" does not
        have to source any current. If your output voltage is higher than the
        logical high threshold for TTL logic (around 2.7 volts), your 3V buffer
        output still should be sufficient to drive a TTL input.

        See:
        http://www.allaboutcircuits.com/vol_4/chpt_3/10.html

        Bert
        --
        Bert Hickman
        Stoneridge Engineering
        http://www.capturedlightning.com
        ***********************************************************************
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        Chris Swinson wrote:
        > This may sound dumb, but a 32mhz osc chip claims 4.5V output, but even just
        > loaded on my scope its lucky to get up to 3 volts. As such it is useless as
        > I need a true TTL logic one of 4.5volts or better. I tried a buffer (F
        > type)
        > and it just made it worse down to 2volts. The only way I got it to work was
        > to run a TTL buffer on about 7 volts! Of course it didn't last very long,
        > but the only option I see is to use a cmos buffer, and run that on
        > 7volts to
        > get a 5volt logic 1 output. Though I am not sure the cmos device will have
        > enough current to drive TTL stuff at speed anyway.
        >
        > So I wonder if anyone has any suggestions on how to buffer the 32mhz so it
        > gives a correct 4.5V logic 1 output ? I know this was being done in the
        > 80's
        > as I have a old video chip which outputs 4.5 volt from a 3V crystal, so its
        > nothing new, just seems "new" chips just do not seem capable for some
        > reason
        > :-\
        >
        > Cheers,
        > Chris
        >
      • Chris Swinson
        ... From: Bert Hickman To: Sent: Friday, March 09, 2012 1:49 PM Subject: Re: [usa-tesla] clock buffers
        Message 3 of 5 , Mar 9, 2012
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          ----- Original Message -----
          From: "Bert Hickman" <bert.hickman@...>
          To: <usa-tesla@yahoogroups.com>
          Sent: Friday, March 09, 2012 1:49 PM
          Subject: Re: [usa-tesla] clock buffers ?


          > Do you have a 0.01 - 0.1 uF ceramic bypass cap connected between your
          > buffer's Vcc and ground? You may be seeing the effects of power supply
          > bus inductance.
          >

          Yeah, I have tried 100nF, 200nF and even a few uF direct between the vss and
          gnd pins, i could try some better caps and see if it improves my results.



          > TTL is "current sinking" logic - an output is at logical zero when
          > sinking current at an output Vsat voltage between 0.2 - 0.8 volts. Other
          > than charging line and input gate capacitance, a logic "one" does not
          > have to source any current. If your output voltage is higher than the
          > logical high threshold for TTL logic (around 2.7 volts), your 3V buffer
          > output still should be sufficient to drive a TTL input.
          >
          > See:
          > http://www.allaboutcircuits.com/vol_4/chpt_3/10.html
          >

          Yeah know what you mean, though the circuit I am "editing" seems to need a
          4.5V input for a logic 1. I have half a feeling that they really old TTL
          stuff needed 4.5V as a logic one, I know a few years back I was looking for
          some buffers with good hysteresis thinking there is some out there with 3
          volts, but was lucky to find 200mV at the time. This was over 10 years ago
          all in all, but I think 4.5 was the guaranteed logic 1, maybe old age of the
          chips upset this a little I don't know, though if I was using all new stuff
          it wouldn't be a problem.

          Chris
        • McGalliard, Frederick B
          You tried a lower value pull up resistor? These TTL gates should sink up to 50 ma. ________________________________ From: usa-tesla@yahoogroups.com
          Message 4 of 5 , Mar 9, 2012
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            You tried a lower value pull up resistor? These TTL gates should sink up to 50 ma.


            From: usa-tesla@yahoogroups.com [mailto:usa-tesla@yahoogroups.com] On Behalf Of Chris Swinson
            Sent: Friday, March 09, 2012 1:18 AM
            To: usa-tesla@yahoogroups.com
            Subject: [usa-tesla] clock buffers ?

             

            This may sound dumb, but a 32mhz osc chip claims 4.5V output, but even just
            loaded on my scope its lucky to get up to 3 volts. As such it is useless as
            I need a true TTL logic one of 4.5volts or better. I tried a buffer (F type)
            and it just made it worse down to 2volts. The only way I got it to work was
            to run a TTL buffer on about 7 volts! Of course it didn't last very long,
            but the only option I see is to use a cmos buffer, and run that on 7volts to
            get a 5volt logic 1 output. Though I am not sure the cmos device will have
            enough current to drive TTL stuff at speed anyway.

            So I wonder if anyone has any suggestions on how to buffer the 32mhz so it
            gives a correct 4.5V logic 1 output ? I know this was being done in the 80's
            as I have a old video chip which outputs 4.5 volt from a 3V crystal, so its
            nothing new, just seems "new" chips just do not seem capable for some reason
            :-\

            Cheers,
            Chris

          • Chris Swinson
            I think you only need those with open collector types, a lot of the TTL stuff seems to only be 8mA or there abouts. Chris ... From: McGalliard, Frederick B
            Message 5 of 5 , Mar 9, 2012
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              I think you only need those with "open collector" types, a lot of the TTL stuff seems to only be 8mA or there abouts.
               
              Chris
               
              ----- Original Message -----
              Sent: Friday, March 09, 2012 2:18 PM
              Subject: RE: [usa-tesla] clock buffers ?

              You tried a lower value pull up resistor? These TTL gates should sink up to 50 ma.


              From: usa-tesla@yahoogroups.com [mailto:usa-tesla@yahoogroups.com] On Behalf Of Chris Swinson
              Sent: Friday, March 09, 2012 1:18 AM
              To: usa-tesla@yahoogroups.com
              Subject: [usa-tesla] clock buffers ?

               

              This may sound dumb, but a 32mhz osc chip claims 4.5V output, but even just
              loaded on my scope its lucky to get up to 3 volts. As such it is useless as
              I need a true TTL logic one of 4.5volts or better. I tried a buffer (F type)
              and it just made it worse down to 2volts. The only way I got it to work was
              to run a TTL buffer on about 7 volts! Of course it didn't last very long,
              but the only option I see is to use a cmos buffer, and run that on 7volts to
              get a 5volt logic 1 output. Though I am not sure the cmos device will have
              enough current to drive TTL stuff at speed anyway.

              So I wonder if anyone has any suggestions on how to buffer the 32mhz so it
              gives a correct 4.5V logic 1 output ? I know this was being done in the 80's
              as I have a old video chip which outputs 4.5 volt from a 3V crystal, so its
              nothing new, just seems "new" chips just do not seem capable for some reason
              :-\

              Cheers,
              Chris

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