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How to think digital electronics?

  1. Dec 10, 2014 #1
    I'm having difficulty grasping digital circuits. I'm not even sure how to think of it..
    For eg. In analog electronics when it comes to transistors I can clearly and perfectly visualize how current amplification or power amplification occurs.

    To further elaborate my problem,, I recently learnt about flip-flops and I had no idea what was happening in class. There were logic gates and there were input and output values in terms of 0s and 1s... 0 meant no voltage and 1 meant +5.1V. This is as far as I can go regarding understanding logic gates. And I also realize if I don't understand this now I'm gonna have a hard time going anywhere beyond where I am now.
    So is there a way to visualize what is happening inside these circuits?
    To make this problem bigger,, I don't have a very good teacher. I realize I should be as independent in my studies as possible but sometimes it cant be helped. :-(

    Any help much appreciated :-))
     
  2. jcsd
  3. Dec 10, 2014 #2

    Doug Huffman

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    I'm not certain where lies your difficulty. Isolated Logic gates are a classroom teaching tool. Next comes networks of them and that is a much more practical and common sight.
     
  4. Dec 10, 2014 #3
    My problem is that I don't know how to think about it.
    With physics and analog electronics visualization is the way to go.
    Along with knowing the results, we are expected to know more about how those results are obtained when a particular circuit is being taught. And in understanding how its working lies my problem

    I'm still at the very beginning at the digital course so use of ICs is strictly discouraged.
     
  5. Dec 10, 2014 #4

    Bystander

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    Op-amp with no feedback? Tickle it and it saturates and latches.
     
  6. Dec 10, 2014 #5
    I haven't learnt op-amp yet. So I don't know what your'e talking about.
     
  7. Dec 10, 2014 #6

    Bystander

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    Just take your analog amplifier circuit and remove the feedback, same thing.
     
  8. Dec 10, 2014 #7
    I still don't get it
    what are you trying to explain here?
     
  9. Dec 10, 2014 #8

    Bystander

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    You do have feedback in your amplifiers --- an amplifier without feedback has two output states, off and wide open.
     
  10. Dec 10, 2014 #9
    Heres how I understand it. Analog signals both low power or high power are typically linear aka sine wave , they have pretty streched out rise and fall times
    Now take the mosfet of a digital circuit or from a computers cpu ,it switches on fast and switches off fast again creating a square like signal , it goes fast from non conducting to conducting and back again to non conducting which then in the code language translates to 0 then 1 then 0, basically that's it , real current and voltage is present at both cases analog or digital just that.

    by switching various voltages at a given order very fast you ca create a wave that represents a sine wave , this is the way a computer reproduces sound as sound is a linear sine wave but a computer cant deal with such so it chops the sine wave into many different peaks and lows.
     
  11. Dec 10, 2014 #10

    jim hardy

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    The subject of logic circuits used to be introduced using simple mechanical switches.

    Simply put, a logic IC has a switch on its output and a high gain amplifier that controls the switch.

    The way to think of them hardware-wise is not as an electronic circuit but a switch. The switch changes state when its input pin crosses about midpoint between the supply rails.

    Your introductory course should start out teaching simple logic functions and Boolean algebra..
    All logic circuits can be boiled down to these five functions:
    AND, OR, INVERT, TIME DELAY and MEMORY.
    You should think of them in terms of True-False. That's because logic circuits can use high to represent True or they can use low to represent True.
    When you have grasped how those five building blocks operate True-False you can move on to the complications of "is high true or false?" . It's painful troubleshooting systems where they mixed those two methods.

    So it's a different world than your analog audio.......

    have fun
     
  12. Dec 10, 2014 #11
    and4.gif
    (from hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/ietron/and4.gif)
    Consider this analog circuit (assume the supply is +5V instead of 6V)
    What happens if you supply +5 Volts to A and 0 Volts to B. What will be the voltage at the terminal marked Out?
    What happens why you supply 0 Volts to A and 0 Volts to B?
    Consider all situations of supplying either 0V or 5V to A and B, and try figuring out what the output voltage will be.
    Now, this is a digital AND gate. Actually, digital circuits are a lot simpler to understand than analog circuit because, in digital circuits, the input is either 0V or 5V. You don't need to think about inputs lying between 0V and 5V * (as a beginner).
     
  13. Dec 10, 2014 #12

    analogdesign

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    Following on what I_am_learning says, if you understand analog amplifiers, you understand digital circuits. A digital gate is simply one or more analog amplifiers in parallel or series. Imagine driving an analog amplifier really hard, its output would go a power supply, right? That is a digital circuit.

    Since you understand transistors, imagine a simple common-emitter amplifier. If the transistor is biased in forward-active, and the input signal is "small", then the output is an inverted, amplified version of the input. If the input signal is huge (like from one power supply to the other) then the output is an inverted version of the input expect it goes to the power supplies because that is the limit of the output voltage. This is just a digital inverter, and in the old days sometimes inverters (and more complex gates) were made just in this way.
     
  14. Dec 11, 2014 #13

    meBigGuy

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    Logic gates are made up of electronically controlled switches. Transistors are used, and are always either fully on, or fully off, based on the inputs. This results in the output being either at the supply, or at ground. The inputs are also either at supply, or ground.

    The simplest logic circuit is an inverter, which can be made by 1 n-channel mos-fet and a pullup resistor. The output is the pullup-drain node. When the gate is at supply voltage, then the transistor is on and shorts the pullup to ground.

    It is all about voltage controlled switches arranged in a way to provide logic functions.

    A flip-flop is a bit tougher, but typically it is a collection of switches such that when the clock input rises, the flip-flip's output goes to, and holds, the signal level (logic level) that was at its input.

    This is an AND gate made from mosfet transistors.

    th?id=HN.608005333268563388&pid=15.1&P=0.jpg

    You need to track through something like the above AND gate, which is built with a 4 transistor NAND followed by a 2 transistor inverter. When a circled gate is high (pmos), its drain-source is an open circuit (OFF). When an uncircled gate is high, its drain-source is shorted (ON). And, vice-versa. The inputs and outputs are either at supply or ground --- never in between.

    Once you track through the above gate it will be clear what is going on. At that point you can just think in terms of the logic functions, and not about what is going on internally.
     
  15. Dec 11, 2014 #14

    sophiecentaur

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    Digital electronics can be learned perfectly well without the need for a teacher. There is so much information to be found on the Web on that subject and you can always find something that's on your particular level. So do not use your non-perfect teacher as a reason for not getting on with Digital Electronics. Any eejit (myself included) can (and did) learn most of it for themselves and without a teacher. Just don't expect to run before you can walk.
    From what you write, I can't be sure whether your problem is how the components within individual circuits function or whether it's the logic operations they perform. The actual numbers of volts that a particular logic technology uses is not relevant to the logic behind the operation performed. It's often enough to talk of a transistor being turned 'On" or 'Off" and the "1" or the "0" will often just refer to whether the output level is zero or +some voltage. When you turn the lights on or off in your house, you don't particularly care whether the supply volts are 220V or 245V - you just describe them as 'ON' or 'OFF'.
    Any binary digital circuit will look at its input signals and 'decide' whether they are high or low; intermediate values are not acknowledged and will only occur when there is a fault (illogical Captain!).
    You really should just trawl through Google searches, using terms like Logic Gate Circuits. There seem to be many more links about combining the basic logic elements and truth tables than actually how the gates etc. are constructed internally.
    No one uses Resistor Transistor Logic (RTL) in real work these days (afaik) but it is a very easy logic system to look at. See this link and this link.
     
  16. Dec 11, 2014 #15

    jim hardy

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    Perfect link Sophie, that hyperphysics one, for the "where do i start" question.

    In my beginning days (1960's) i found it useful to talk out loud my way through circuits.

    Looking at that top picture in Sophie's first link:

    Diode-Resistor AND Gate
    ck.gif AND Gate
    nck.gif OR Gate
    diand.gif Some logic gates can be produced with just diodes and resistors (called diode resistor logic or DRL).
    Basic Gates
    Index

    Here's what i would have said, aloud, to myself:
    "IF
    A is low then output is low, irrespective of B.
    B is low then output is low, irrespective of A.
    Output is high.only if A and B are both high.
    So - this is an AND gate, its logic equation is Out = A AND B ."

    Logic equations use the arithmetic symbol X to represent AND
    and the symbol + to represent OR

    so that gate's logic equation is: Out = A X B

    To your original question "How do i think of logic circuits? "
    I say "Talk your way through them."
    A major requisite for becoming a logic designer is the ability to gracefully lose arguments with yourself, so don't be embarrassed when your colleagues look askance at your mutterings.

    old jim
     
    Last edited by a moderator: Apr 18, 2017
  17. Dec 16, 2014 #16

    dlgoff

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    Without trying to be redundant, the HyperPhysics overview of Digital Electronics is a good way to see how areas are tied together.

    digcktcon.gif
     
  18. Dec 16, 2014 #17

    sophiecentaur

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    IMO, the only way to get into digital ( or almost any) electronics is to start making stuff. You will learn more from trying to make a simple kit project function properly than from reading any number of books and forums without practical experience. As you make progress you will start to see the point of that confusing theory stuff. The connection between the abstract logic and the concrete voltages soon starts to hit you.
     
  19. Dec 18, 2014 #18
    OK first of all very sorry for delayed response. I have a hard time getting internet access.

    Switching logic is interesting I never knew it existed. Thankyou :-D

    We have these circuits as construction of logic gates but my problem was when studying complicated circuits. It's not practical to convert all digital circuits into basic constructions of gates. And things like flip-flops would be overwhelmingly difficult to understand like these (I tried). This was helpful after I knew switching logic.
    I converted analog circuits into switches and from switches to gates also its easier to understand what's happening with current in switching circuits. :-D:-D

    meBigGuy
    I'm sorry I only know how bjts and ujts work. :-(
    sophiecentaur
    I understand what you're trying to say and I'm trying hard to learn stuff on my own. Although with a good teacher it's so much more easier. Guess i'm just lazy. :)
    With the internet I'm using it took more than a minute to open this page. I'll look those links when I have faster internet (in college) :-D Thankyou..
    I'm also participating in a project competition so I'll be making stuff as you said. Thankyou again : - )

    jim hardy
    Thanks. Talking my way through equations is a really good idea : - )
    dlgoff
    I'll look more info in hyperphysics.
     
  20. Dec 19, 2014 #19

    sophiecentaur

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    @Giant One thing you have to do is to separate the nuts and bolts of the electronics and the circuits from the logical functions, in your mind. When you start with very elementary logic gates you, of course, notice that there are only two signal levels on the meter or 'scope, involved with binary logic. Once past that stage, you really don't need to relate the two at all until you are really pushing the speeds or lengths of connecting wire. Stop thinking in terms of transistors (or whatever) doing all the switching and just think 'Logic'. Truth tables and zeros and ones, scribbled on a schematic are the way to work out what happens. Bottom line is to accept that it can get very complicated and that you won't understand the next level until you get there
     
  21. Dec 19, 2014 #20

    jim hardy

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    Second Sophie's suggestion.
    Learn to work logic circuits as just - i talk through them using "low-high" because some of our systems mixed low-true and high-true logic in the came circuit.... nightmare.
    That's why some folks use the word "Assert" because it doesn't assume high or low.
    When youre fluent at that is the time to look at the transistors inside the IC to see how they work.
    Every logic family has its own interface scheme. For example TTL inputs must be pulled low, they will source a milliamp that must be sunk They will drift high if disconnected but they should be actively driven else they'll pick up noise. TTL outputs will sink more current than they'll source.
    The famous 555 timer will sink or source plenty of current so it can drive most anything.

    A good designer understands what's just inside the chips so he can interface smoothly. You have an advantage there with your analog background provided you dont let it get in the way of "boolean thinking" . (i hesitate to call it 'Logical' )

    Think simple. I recommend thinking in High - Low because "Zero and One" or "True and False" can be either voltage.
     
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