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Microprocessor class question

  1. Aug 31, 2004 #1
    OK, my new professor in a microprocessor class asked this question, which I believe to be slightly open to interpretation:

    If we needed and AND gates function, but had no AND gate, what single chip could we use to take it's place?

    Now, this question was verbal, and he sent us home to 'think' about it.

    Well, I am figuring that one way to reproduce an AND gate, without using one, is to use inverters with an OR gate. An inverter on each input and an inverter on the output provides the same logical function as a single AND gate.

    However, I have a feeling that the professor is looking for us to come back to class with an IC number of the single CHIP that can complete this task.

    SO, is there a single chip with OR gates and inverters on it? If so, how do I go about researching it and coming up with the IC number?

    I have visited the TI site where they list all their semiconductors and specs, but I have not turned up this combo. Am I just searching wrong? Am I interpreting the prof's question incorrectly?

    Two notes: this question was asked verbally, not written on the board. Therefore I cannot scour over his exact verbiage.
    Secondly, this class is the next step beyond a digital logic class. We are expected to already have logic and gates in our heads. This is a microprocessor class where we will be learning to program and microprocessors and build circuits. Maybe this info will help someone to interpret the question better.

    Thanks in advance.

    edited for dumm spellng misstakess
  2. jcsd
  3. Aug 31, 2004 #2
    i rather doubt he was talking about finding a chip with no AND gates. i think most EE professors just want you to be aware that the AND gate is a bit superfluous. it's just that it is more efficient to physically use actual AND gates on hardware. which is why you were having a tough time looking for a chip without one.
  4. Sep 1, 2004 #3
    Well, he did say in this scenario, that we were supposed to assume for whatever reason, there is no chip with AND gates available.

    If there was no AND gate available for use, but we needed that function, what single chip could we use to provide it? That is what I took away from his delivery of the question.

    That is why I think we are supposed to come up with a chip.
  5. Sep 1, 2004 #4
    you just designed one didn't you? it has two inputs, one output and its internal circuitry uses no AND gates. now you could use this chip as a template to replace every instance of an AND gate in the design any other more complex chip.
  6. Sep 1, 2004 #5
    A simpler way?

    Is there a simpler way to replicate the AND gate function that using the 4 gates I have specified (3 inverters and an OR gate)?

    By simpler I mean less gates.
  7. Sep 1, 2004 #6
    You can make AND gates using NOR or NAND gates (you should be aware of this already). I know Motorola makes ICs having only NAND gates or only NOR gates. Search their website to find out. If you have a profesional EE design software package (e.g. Xilinx's ISE), you can search its IC package repertoire to find the make and model number of a specific IC package.
  8. Sep 2, 2004 #7
    Silly me. I can use two NAND gates! The rust is slowly breaking loose, it was a long summer. Thanks for the push.
  9. Sep 3, 2004 #8
    a nand gate is just an and gate and a not gate combined. so internally u'll still have the 'and'. Besides u needn't use 2 nands for this. u can use the nand and a not gate. now if u meant using the 2nd nand as a not then u were right.

    still, i feel one cannot substitute an AND gate with any other single gate capable of doing the same function. if so, that would be called an and gate too!
  10. Sep 3, 2004 #9
    Yes, I put the output of the first NAND gate to both inputs of the second NAND gate. That allowed it to operate as an inverter.

    This class met again last night, and I found out the professors preferred answer. It turns out that he was using this question as a way to review Demorgan's Law, AB = (A'+B')'. So the answer he was actually looking for was a negative logic OR gate. Essentially that is what my first idea was with the OR gate and inverters on both inputs and output. He wanted us to draw a single gate in such a way that it would reproduce the AND function. He threw off everyone with the word chip.

    Thanks all for you contributions. It certainly helped me to review almost forgotten material. The gears are turning again.
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