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Digital Logic Fundamentals help

  1. Oct 14, 2008 #1
    I have recently started my second year at Uni and one of the areas I am studying is Digital Logic Fundamentals. However due to a mixup with my timetable I missed the first few weeks of the course. I am trying to catch up but have become stuck on part of the course that wasn't explained well in the online notes.

    Does anyone have any good resources or books that can help me understand this area especially understanding TTL's, Totem Pole outputs and just generally anything in that area?

  2. jcsd
  3. Oct 14, 2008 #2


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    Not to sound snarky here, but friends who are also taking this course? The professor / grad student teaching it (you can often ask for a longer appointment if you go prepared)? If you only have spot troubles (i.e. generally understand the material, but have (lots of?) little problems here and there), they'd be your best bet.

    EDIT: Was there a course textbook assigned? Your prof may be able to recommend one available from your bookstore or library. I doubt that Mano and Kime, "Logic and Computer Design Fundamentals", the textbook I used, is still in print. Or someone else on this board may be able to recommend one.
  4. Oct 14, 2008 #3
    is TTL even used anymore in the real world?
  5. Oct 14, 2008 #4
    I have got the course text suggested but it is one of those books which is not easy to comprehend with no knowledge of the subject. I have tried talking to the lecturer but he just keeps suggesting the course texts. I was just hoping someone here had some experience of books or sites they have used that explains it fully (as in how they work) and not just what they do.
  6. Oct 14, 2008 #5
  7. Oct 15, 2008 #6


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    I'd suggest that instead of trying to find a full explanation, that you instead look for a simplified model, and then build up your understanding as you pick up transistor theory (second semester course? third year?) A high level explanation may be more suited, especially if you don't have any / much experience (in other courses) with even BJTs. If it may seem that they've gone over a disproportionate amount of material, that's probably because they glossed over a lot of the (low-level) details, which is what you'd expect for a (introductory digital) course. Once you've got this high-level understanding down, you can start delving downwards. Unless you guys have already done standard circuit analysis and covered transistor operation in other courses.

    It may seem daunting when presented with circuits involving 2, 3, 4+ transistors, but the key is to simplify the functionality to on or off states (the whole digital paradigm). Thus, when you have an NPN (BJT arrow Not Pointed iN--my memory technique) and the base receives a '1' (possibly 5V through a resistor) it's on, and you pretend that the collector-emitter are connected together (with no connection to the base--this isn't true but is functionally correct for this first-level analysis). The same thing happens when a PNP (BJT arrow Pointed iN Polarity) has a '0' applied to its base.

    When the opposite of these apply (a '0' to an NPN base, '1' to a PNP) make the simplification that the Collector and Emitter are disconnected (and that, again, neither of these are connected to the base).

    When the output stage of a logic circuit contains a PNP and an NPN with the collectors tied together, or one NPN pulling low (output gets connected to ground when on) and one NPN pulling high (output gets connected to 5V, Vcc, or '1' when on), this is known as a totem pole output, because the amount of current that can be supplied or sunk from the output is (roughly) the same (when properly designed--neglecting device specifics). This is as opposed to an open collector configuration, or an open collector with a pull-up resistor attached to the output.

    I hope this helps. But here's a (very) high-level intro that doesn't require transistor analysis, and also goes over other varieties of logic:
  8. Oct 16, 2008 #7
  9. Oct 16, 2008 #8
    Thanks for the help, the asic-world site looks like the sort of thing I need. Also I have been told that by the end of the course I need to:
    Anyone know any good sites for teaching me this side of it?
  10. Oct 16, 2008 #9
    OK, where is it used other than stuff like legacy military equipment?

    the reason it concerns me is i'm not convinced it is a good use the students' tuition money, since most of them will not be using it.
  11. Oct 16, 2008 #10


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    An ALU (Arithmetic Logic Unit) is at the heart of any CPU (Central Processing Unit), and is the module that performs logical operations (e.g. 0xf AND 0x4--0b0111 AND 0b0100) and mathematical operations e.g. ROTATE (this is analogous to division/multiplication by powers of 2), ADD, etc.:

    This is probably more of a final project / lab thing, rather than something you'd do on an exam (so no need to panic!) I would suggest showing up to class, and (pre)reading through the textbook. And coming here whenever you've got really intractable problems.
  12. Oct 16, 2008 #11


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    I think it's important in the sense that it's an introduction to non-amplifier use of the BJT. By the same token, how many EECS students are are actually likely to do low-level logic design? I think this just adds to cumulative knowledge, and the overall knowledge base that an EE should have (and in and of itself provides a nice historical perspective).

    It's handy when you need to use a single gate instead of a full quad pack (or whatever multiple it is that you're using). So using a BJT or few, you can save 40 or 50 cents and a little bit of space. Or when you've goofed and need to add a logic function when you've already got an otherwise good board manufactured. Again, just more cumulative EE knowledge.
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