Discrete logic design

Question

I have been tasked with building a simple alarm device. It just needs to measure a few inputs and the outputs will respond accordingly (to put it very simply!). To me, it seemed that using a few discrete logic gates would get the job done, but a colleague (who was working on it with me) decided that we should use programmable logic instead. His case won, because firstly, he is more senior than me, and secondly, his main argument was that programmable devices are the future and we want to make future proof products.

My question is if you have a design which could be easily implemented by a few discrete logic gates, is it worth designing with discrete logic any more? IS there any advantage to using them over programmable? Or is it slowly going to be phased out completely by programmable logic? To be clear, I don't want answers of 'I believe this is the case' or 'personally I think this but...' I'd like to know if there are any actual advantages to designing with discrete over programmable and is it worth designing with them these days in consumer electronics products?

Answer 1

Discrete logic design won't be fully phased out. There will always be applications where using a discrete logic IC is preferable. As has been pointed out, speed is a big advantage, although in a lot of applications, the speed difference is just not that important.

When it comes to the design phase, if you design a circuit that only needs to perform 2 or 3 simple logic functions at different points in the circuit, it will be better to use discrete gates, just to save design time on having to also write the program too.

For systems that need to do more complex logic functions, it would be silly to spend all the time working out a truth table, then figuring out which logic gates go where etc when you could just write a small program. Usually, the more inputs mean the more gates required and the longer it takes to design in discrete.

The place where discrete logic really has the advantage though is in learning. When you are first learning about logic design and how gates work etc, that is where getting hands on with actual logic gates and designing different functions with discrete parts is great. Always a good idea to get an understanding of fundamentals. So, because of that, discrete logic is always going to have a place in this world. As for consumer electronics? The future is definitely programmable.

Answer 2

I'm not a professional electronics engineer at all (actually just a beginner), but my few cents are that dedicated discrete logic ICs should only be used if timing is important or if it is a requirement not to use programmable logic.

With a microcontroller you can implement much more complicated logic and it is more flexible. Also it can be reprogrammed without having to change the hardware.

Also when very fast timings are needed that software cannot keep up with, dedicated logic ICs could be used. But for an alarm system this seems unneeded (it does not need a response time of nanoseconds).

Below is my interpretation of the advantages/disadvantages:

                          Discrete logic (ICs)      Programmable logic/
                                                    (Microcontrollers)
 Nanosecond speed                  x                         -
 Just a few 'operators' needed     x                         -
 No tool chain needed              x                         -
 Cost Efficiency *                 -                         x
 PCB / proto size                  -                         x
 Flexibility for changing          -                         x
 Production cost                   -                         x
 Possibility to extend features    -                         x

* Assuming more than a few (different) logic functions needed

Answer 3

If you're going to create a commercial product, you really shouldn't choose a design because "it's programmable" or because a more senior colleague proposed it. Instead you should estimate risks and costs associated with each design, and pick the one with the lowest cost and acceptable risk level. For a start:

• price of individual components adds to the cost
• required PCB size and complexity adds to the cost
• required design effort and tools adds to the cost
• required testing and certification adds to the cost

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• lack of flexibility (e.g. changes in discrete logic requiring a new PCB) is a risk
• lack of experience with a particular technology in your team is a risk
• inability to meet project requirements (e.g. target power consumption) is a risk

If sourcing discrete logic ICs for your particular project is cheaper, your team has experience with such design and you consider the lack of flexibility is not critical, there is no reason not to use discrete logic.

Answer 4

One aspect the other answers are forgetting is safety. Discrete logic circuits are way more reliable than much more complex microcontroller designs. I helped building a hydrogen prototype car, all the safety-circuits were designed using discrete logic. Safety and reliability is an aspect you'd might want to consider designing an alarmsystem.

Answer 5

I have to admit, whenever I have to do some complex combinatorial logic plus some timers, I don't bother with discrete logic at all but always code a minimal assembler program for an ATtiny (use a PIC if you like those better).

The combinatorial logic is at maximum 20 lines of assembler (several lookup tables). Each software timer adds another 10 lines. Hardware timers even less. You even gain advantage of having an A/D converter, Comparator, PWM generation on board, should you need those.

The only disadvantage is you had to flash this part. Sometimes that's a show-stopper, but it's seldom the case. The big advantage is you need much less space on the board, routing is much simpler, and you can easily change the logic functions if required.

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The remaining use-case for separate logic ICs is when things have to work quicker than 1µs.

Answer 6

In the end product - probably not many advantages to discrete logic if we are talking a consumer device. Exceptions would be something that needs to be very rugged against adverse conditions, or very easy to completely characterize (the actual complexity and potential for hidden bugs is much higher with something firmware based), or that you want to be able to build again from very similar parts for decades to come (74xx footprints change very very slowly even if the technology letters change :) ).

Something that you can actually do better with discretes is self-timing, asynchronous logic (completion of one thing immediately triggers another thing). The legitimacy of such designs is a holy war topic. Do not worry, the synchronous people will want to beat you but you just have to take their clock away, they will be nonsensically waiting forever for it.

When it comes to design methodology, I'd say it depends on the preferred style of the designer - a CPLD device that you could literally rewire live ("let's touch that wire to pins until something clicks in"), in a ratsnest view, and under power from a host PC (with simulated sparks when you touch a wire to a pin, preferrably, just for ambience) would certainly be well liked by people preferring discrete logic :)