Do we need to match IC technologies for a circuit?

Discussion in 'General Electronics Chat' started by dumindu89, May 19, 2012.

  1. dumindu89

    Thread Starter Member

    Oct 28, 2010
    When we are selecting ICs to construct a circuit do we need to check whether that all the ICs are constructed with the same technology?

    As an example, If we construct a circuit using ICs with TTL technology, Is it OK to use ICs with CMOS technology for the same circuit? and Is it OK to use both TTL and Low Shotcky TTL ICs in the same circuit?

    Please explain with the reasons.
  2. MrChips


    Oct 2, 2009
    Yes and No.

    Try to select the same technology.

    The devil is in the details.

    You can get away with mixing TTL and LS TTL, but you have to look into the details.

    You do NOT mix TTL and CMOS.
    They have completely different input thresholds and output drive capabilities.
  3. takao21203

    Distinguished Member

    Apr 28, 2012
    Depends if you run the circuit at the speed margins, in the Mhz range.

    If not, you can mix with some consideration.
    Also TTL/CMOS with some level shifting/clipping, for low speed = resistors.

    But why not use HC chips only these days?
    They can actually work from less than 5 volts.

    CMOS simple rule: 0 to 1/2 Vcc = LOW, 1/2 Vcc to Vcc = HIGH.
    TTL simple rule: 0 to 1.8/2V = LOW, 3.8 to 5V = HIGH.

    If the TTL is not loaded much, and the right resistors are used,
    HIGH will be close to 5V, and LOW close to 0V, even if this is not guaranteed.
    Not a problem for HC, but older Standard or LS chips can cause problems.

    You simply need experience with that, and as I wrote, preferably use HC.
  4. Wendy


    Mar 24, 2008
    I lean towards post #2. There are cases you can mix incompatible logic types, but you are getting away with it, and may yet still be bit by Murphy.

    CMOS is probably the friendliest logic family out there, and has everything but the kitchen sink in terms of black box logic.

    TTL a logic high is NLT 3.4V, and a low is NGT 0.7V. CMOS tends to go to the power supply levels, but has no current drive. There are easy ways to handle this however, starting with power MOSFETs.
  5. MrChips


    Oct 2, 2009
    The question is - Why do you need to mix?

    There is almost every digital function in both LSTTL and CMOS.

    If you have to mix you can look at 74HCT series.
  6. dumindu89

    Thread Starter Member

    Oct 28, 2010
    No I asked Why do we need to MATCH. :)

    If I am constructing a circuit which work in higher frequencies (let's say around 100 MHz), what technology you prefer?
  7. MrChips


    Oct 2, 2009
    Why do we need to MIX vs why do we need to MATCH? - same difference.

    You need to MATCH because things don't work if you MIX.
    You don't MIX because all the functions are available in one technology.

    For greater than 100MHz I use 74Fxx technology.
    But I only need to at the high speed sections where it is needed. For below 60MHz I resort back to 74LSxx.

    But I have not done this in years since I can do everything with an ARM chip now.

    If you really have to, use an FPGA.