Replacing discrete 7400 logic devices with CPLDs

Whether you are working on a retro computing device, or experimenting with digital logic, you’ll find that in most instances CPLDs can perform the same operations as a cluster of 7400 series discrete logic chips. Discrete logic devices, long considered the workhorses of the semiconductor industry, held a unit cost advantage over programmable logic devices for several decades. However, times have changed significantly. Advances in semiconductor process technology for CPLDs have driven the costs of these devices down to a point where they now offer a highly compelling discrete logic replacement alternative.

Xilinx engineers have prepared several white papers outlining the benefits and processes involved in replacing discrete logic 7400 series logic chips with CPLDs, such as those found in our CPLD Development Boards.

TTL “Burn Rate” for Xilinx CPLDs familiarizes logic designers that have previously not used programmable logic with the benefits of Xilinx CPLDs, and shows how to go about translating existing TTL logic into similar CPLD solutions.

Another relevant document is The Advantages of Migrating from Discrete 7400 Logic Devices to CPLDs comparing the costs of a discrete logic component-based circuit with those of a Xilinx CPLD.

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  1. Keep in mind that the cost savings is not always realized. I recently completed a design where 4 discrete logic chips – an octal D-type latch, decoder, 2-in OR and 3-in NOR – all cost less than the cheapest CPLD or SPLD. My cost for all 4 chips is $0.60, and I couldn’t find anything in the CPLD world that cheap.

    Granted, this was a case of reasonably simple glue logic, but I think it pays to know how to manually design for discrete logic instead of relying upon high level language descriptions and compilers.

    The whole topic begs the question: What is the cheapest programmable device?

    Mouser shows SPLD parts as low as $1.40 each and CPLD parts as low as $1.39. At the other end of the spectrum, SPLD parts go up to $2.82, while CPLD seems to go all the way to $457.88 for a single part! Sorting by price, I had to go through 75 pages of CPLD parts before the price dropped below $10 in single quantities. You can build a lot of custom, discrete circuits for those prices.

    1. I usually buy CPLDs at Digikey (Xilinx), and the cheapest are around $1.30 (32 or 36 macrocell xc2c or xc95). 72 macrocells is more usable, and double the cost.

      At the top range there are huge CPLDs, the kind you see doing video card duties in student projects. Those can be $50+ (a few non-stock go to the 100s range).

      Other disadvantages:
      1. Single supplier
      2. Proprietary tools
      3. Prices don’t come down much in volume unless you negotiate with your distributor

      There are lots of advantages that might make it worth the extra $0.80, depending on stage of project and volume:
      1.No searching for parts, reading sheets, sourcing 3 extra parts (in your example)
      2. Easier routing (straight connections) and fewer chips/placements saves time and board space (cost)
      3.In prototyping and even production you can upgrade/fix problems with the logic without a new board design/build/test/bring online cycle.

      1. You forgot one disadvantage:
        4. Part does not perform your intended function until after it has been programmed.

        As for advantages, my particular glue logic also consists mostly of straight connections that make routing easy – only a few of the signals must weave between chips. Also, I didn’t make any mistakes :-) so the design/build/test cycle did not suffer in this case.

        The corollary of the above (4) is that discrete logic works as soon as the parts are installed. There is no need for a programming step, and it’s impossible to ship a board that somehow skipped the programming phase.

        So, I’m just saying that it’s fairly easy to find reasons to choose either one – discrete or programmable.

  2. Interestingly you did not mention the biggest disadvantage, Cplds need much more power than logic devices.

    On the other hand, getting rid of all those logic ic families, is like a dream coming true,…

      1. Yes and no.
        I have a hugh stack of those chips, just in case I would ever need one,…
        A custom programmable dil 14 or 18 device instead would indeed be a dream, at least for me.

  3. (thanks for the information)

    5) CLPD is great for rapid development and testing.

    Another advance is that you/me only needs one CLPD for development, while one need a almost whole family of 74xx-chips to do the job. If you/me need a new 74xx-device, the CLPD needs only a new program so it’ll save some time and frustration of waiting to acuire the new IC from the shop/web. I have experienced it too many times, and when I finally got the 74xx I needed, I have forgotten what I actually needed it for! :)

    I haven’ gotted around to program a CPLD but I am looking forward to the day I’ve got some spare time.

    Rubi: Don’t be so afraid of SMD, it’ll need a careful and patient person.
    I love to solder SMD’s, it’s easier and faster than DIL. I use pasta and a toothstick, applying small portions on each pads.

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