Combining TTL and CMOS

Discussion in 'The Projects Forum' started by RyanS, Aug 15, 2007.

  1. RyanS

    Thread Starter New Member

    Aug 15, 2007
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    I recently pulled out a couple old electronics learning kits of mine that came with CMOS 4000 series logic chips. I thought it might be cool to (try to) build a simple 4-bit processor with the combined parts.

    I don't have enough parts with just the CMOS chips, and I don't really feel like spending the extra time and money to get more, but I have a couple TTL 7404 inverter chips and a bunch of 74S37 nand chips.

    If I ran everything at 5 volts, would the CMOS work with the TTL, or would it ker-splode? If it would work, is there anything that I might have to take into account such as a difference in the number of inputs I could drive with one output, etc?

    Any help would be appreciated.:)
     
  2. Salgat

    Active Member

    Dec 23, 2006
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    Should work fine. CMOS, if I remember right, has a slightly higher voltage for TRUE logic(3.8V?), but if you are using the traditional 0V and 5V, they will work together fine. Just remember when using CMOS ICs to never leave any floating inputs, tie them to either one logic state(ground or 5v).
     
  3. lightingman

    Senior Member

    Apr 19, 2007
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    Also... remember that CMOS driving TTL will have a limited fan-out.... I.E. you may only be able to drive 1 TTL input with a CMOS output.... The fan-out being the number of outputs that can drive a number of inputs for a given logic family...Daniel.
     
  4. RyanS

    Thread Starter New Member

    Aug 15, 2007
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    Thanks. That's just what I needed to know.:)
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    It's coming from TTL to CMOS that may present the most trouble. CMOS likes the input to swing all the way to Vcc, wher some TTL is only able to go to around 4 volts. There are CMOS buffers (4049 comes to mind) that are able to handle lower input swings.
     
  6. RyanS

    Thread Starter New Member

    Aug 15, 2007
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    I have a 4049 that I can use to buffer 6 lines, but I will have 8 lines at the TTL level. Will transistors work to buffer out the other lines?

    [edit]
    Something like the image I attached.
     
  7. Salgat

    Active Member

    Dec 23, 2006
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    Assuming he's using the same 5V for both TTL and CMOS, there should be no issues correct? I've done this myself with no issues on my digital clock.
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    Depends on the TTL type. 74HC would be just fine. 74LS might not be satisfactory without buffering, because the output swing won't go past 4.4 volts, which is a bit too little for a CMOS input. Original TTL would absolutely have to be buffered coming and going.
     
  9. cumesoftware

    Senior Member

    Apr 27, 2007
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    When driving a CMOS gate with a TTL gate, remember to add a pull-up resistor to the output of the TTL gate. You can refer to here:
    http://www.kpsec.freeuk.com/components/ic.htm

    The 74HC family is a CMOS family, not a TTL one. HC stands for high speed CMOS. There is also the 74HCT family, which is compatible with TTL. Never confuse the 74HC or 74HCT CMOS families with other families such as 74, 74S, 74L, 74LS and 74F (TTL families). A common mistake.
     
  10. RyanS

    Thread Starter New Member

    Aug 15, 2007
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    I'm using the 74S series. I tested it and found that the outputs from the TTL ICs could drive the inputs of a CMOS IC just as they were. I'll try it without any buffering or pullup resistors and see what happens.
     
  11. Salgat

    Active Member

    Dec 23, 2006
    215
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    Why would it have to be buffered?
     
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