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  1. #1
    Join Date
    Oct 2004
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    Chelmsford, MA
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    576

    Question frequency divider

    I am building a frequency divider based on the 74HC74 designs (used in the softrock 40 for example) . However I only have 74LS74's at my disposal. I built the circuit the same way as the 74HC74's are built (I am only testing at first with one flip-flop... if I see a frequency thats half... I know its working hihi). The only difference I can see between the 74HC74 and 74LS74 is that the HC has set and reset while the LS has preselect and clear... however, with the set and reset high and the preselect and clear also high, the function table looks the same? I am still gettting my original frequency at the output of 28.236 MHz! Function-wise it checks out... the frequency should be divided by two. I should note that I am not use a prescalar or any circuit to square off the sine wave from the oscillator but was hoping that the divider would still work (with the rising edges) and I believe it should be fine... but either I am doing something wrong or I am completely missing something here! (have a feeling I am going to be laughing at myself with this one!)

  2. #2

    Default

    Quote Originally Posted by KB1LQC View Post
    I am building a frequency divider based on the 74HC74 designs (used in the softrock 40 for example) . However I only have 74LS74's at my disposal. I built the circuit the same way as the 74HC74's are built (I am only testing at first with one flip-flop... if I see a frequency thats half... I know its working hihi). The only difference I can see between the 74HC74 and 74LS74 is that the HC has set and reset while the LS has preselect and clear... however, with the set and reset high and the preselect and clear also high, the function table looks the same? I am still gettting my original frequency at the output of 28.236 MHz! Function-wise it checks out... the frequency should be divided by two. I should note that I am not use a prescalar or any circuit to square off the sine wave from the oscillator but was hoping that the divider would still work (with the rising edges) and I believe it should be fine... but either I am doing something wrong or I am completely missing something here! (have a feeling I am going to be laughing at myself with this one!)
    The function tables are the same. However, you are exceeding the typical frequency range for LS chips. They max out at about 25MHz. You could use a schmitt trigger in front of the counter to see if the sin wave is causing problems but I expect a frequency limitation. Try increasing the vcc to about 5.5 volts.
    Last edited by AB8RO; 07-19-2008 at 01:14 AM.

  3. #3
    Join Date
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    Default

    Quote Originally Posted by ab8ro View Post
    The function tables are the same. However, you are exceeding the typical frequency range for LS chips. They max out at about 25MHz. You could use a schmidt trigger in front of the counter to see if the sin wave is causing problems but I expect a frequency limitation. Try increasing the vcc to about 5.5 volts.
    Wow haha, I knew I was missing something, didn't occur to me to check the maximum frequency of the chip! I guess thats what I get for selective reading of the data sheet Maybe I will check what I built with an oscillator around 10 MHz, just to see if its working but it looks like I will be buying some different D Flip-Flops.

  4. #4

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    Quote Originally Posted by KB1LQC View Post
    Wow haha, I knew I was missing something, didn't occur to me to check the maximum frequency of the chip! I guess thats what I get for selective reading of the data sheet Maybe I will check what I built with an oscillator around 10 MHz, just to see if its working but it looks like I will be buying some different D Flip-Flops.
    Remember, there will also be a difference in power consumption, and mixing LS and HCT logic (even within max "speed" limits) can cause loading problem, particularly if you try driving multiple LSTTL devices from a single HCT (or other CMOS TTL) output. the noise margins are also different in most cases.

  5. #5
    Join Date
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    5,859

    Default

    Quote Originally Posted by KB1LQC View Post
    Wow haha, I knew I was missing something, didn't occur to me to check the maximum frequency of the chip! I guess thats what I get for selective reading of the data sheet Maybe I will check what I built with an oscillator around 10 MHz, just to see if its working but it looks like I will be buying some different D Flip-Flops.
    Its been along time since I've done this but I think I had to use 74S74's to get some speed out of that type of setup.

    Try going down in frequency to see if you can get your present setup to work as a check of your hookup.

    73,

    Frank
    [FONT=T93Y][COLOR=#0000ff][B]ACFP

    [/B][/COLOR]
    [/FONT]

  6. #6

    Default Tie 'em High

    Faced with the same problem, and eventually realizing that the set/reset terminals might flap about indeterminately at CMOS impedance levels, I eventually tied the set and rest terminals high. Voila! Total functionality ensued. While it does not seem from the spec that attachment of the set/reset terminals is required, it does seem to be necessary.

    Try this fix. You might be surprised.

    Good Luck.

    73

    John

    G4ALA

  7. #7
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    Beautiful Downtown Colorado. (Montrose, SW corner)
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    Default

    My frequency got divided when I hit 60---

    TOM K8ERV Montrose Colo

  8. #8
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    Jan 2008
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    Quote Originally Posted by K8ERV View Post
    My frequency got divided when I hit 60---

    TOM K8ERV Montrose Colo
    Sorry to hear that! I think they have some pills now that will help!
    [FONT=T93Y][COLOR=#0000ff][B]ACFP

    [/B][/COLOR]
    [/FONT]

  9. #9

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    When my wife hit "40" I said that I was going to trade her in for a couple of 20 year-olds. At the time I had a Baptist "preacher" working for me. He looked me straight in the eye and said, "you're not wired for 220"! He was also a licensed amateur radio operator.

    Glen, K9STH

  10. #10
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    3763 Lyle Avenue, North Pole, AK 99705
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    Default

    Quote Originally Posted by AC0FP View Post
    Its been along time since I've done this but I think I had to use 74S74's to get some speed out of that type of setup.

    Try going down in frequency to see if you can get your present setup to work as a check of your hookup.

    73,

    Frank
    You might also want to try some ECL logic devices. (
    Been so long, I forgot the "family" prefix for these) Plessey used to be the big player in ECL "back in the day".
    Don't know who makes it now.

    eric
    "The more you know, the less you don't know."

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