What is Adder: Definition and 51 Discussions

An adder is a digital circuit that performs addition of numbers.
In many computers and other kinds of processors adders are used in the arithmetic logic units or ALU. They are also used in other parts of the processor, where they are used to calculate addresses, table indices, increment and decrement operators and similar operations.
Although adders can be constructed for many number representations, such as binary-coded decimal or excess-3, the most common adders operate on binary numbers.
In cases where two's complement or ones' complement is being used to represent negative numbers, it is trivial to modify an adder into an adder–subtractor.
Other signed number representations require more logic around the basic adder.

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  1. V

    Comp Sci Digital Logic 8 Bit Full Adder Circuit

    I am unsure of how to approach this problem, we begin with two 8-bit full adders, is the goal to just combine them in some way so that it becomes one entire circuit while also adding NAND and NOR gates where necessary? By the end will we then have 16 1-bit full adders?
  2. altruan23

    Engineering Opamp adder circuit: Where does the "-" come from?

    So i used KCL and both currents are flowing into the node, and then leaving together to go to the resistor R3. So my eq can be seen in the picture. I was looking in a book and they had a minus infront of the parantheses. Is the current flowing from R3 into the node??
  3. M

    MHB Carry select adder - Time calculation

    Hey! :giggle: a) Suppose a binary pick-up adder (carry-select) of $32$-bits, comprising $4$ sub-sections adders spreading carry of range $8$ bits. Show the values obtained internally in the circuit of this adder to perform the addition of the numbers $01011001101100111110110011001101$ and...
  4. Lord Doppler

    Comp Sci Why Does My 19-bit Adder Using 74LS283 ICs Show Incorrect Results in Proteus?

    Hello, I'm doing a 19 bit adder in Proteus with the 74LS283 IC and I have a problem, I'm adding 1110111000110011010 (487834) with 0001110110001100111 (60519) which the result is 10000101111000000001 (548353) and Proteus doesn't the correct result. It is not taking into account the carry, someone...
  5. U

    Make half and full adder without chips

    Could some one recommend a good kit to build a simple half adder and full adder with breadboard, wires, transistors and diodes? We understand how diodes and transistors work. We may not get beyond the full adder. We really do not want to use a chip at this point because we can not see inside...
  6. jisbon

    Engineering 2-bit Bin Full Adder Truth Table Derivation

    Hi all, I am having some troubles deriving the truth table of the following: I don't understand how does one gets to the highlight parts. For example, 10 10, when the carry is low, shouldn't the output be 0 0 1? I fully understand how to add if it is a single bit, but now with 2 bits, it is a...
  7. Glenn Emmers

    Combining Signals of Hall Sensor & Rogowski Coil

    Dear all, I'm trying to combine the singals of both a hall sensor and a rogowski coil. The proposed circuit is given in the following figure: This should be the physical form of the implementation given in the following figure: I can somewhat see a summing amplifier formed by A2, where the...
  8. C

    Designing a 2-bit full adder using nothing but NAND gates?

    Greetings, I just wanted to post a 2-bit full adder I have been working on and which I think is my final design. I wanted to ask if there was any way that I could make it with less logic gates? Any advice is appreciated!
  9. T

    Implementing a function using an adder and a multiplier

    Homework Statement http://imgur.com/a/3Cn7c [/B]Z is an unsigned 9 bit number and X is an unsigned 3 bit number. The only available components are an 8 bit adder and a 4-bit x 4-bit multiplier. c.) We have a single 8 bit adder and a single 4-bit x 4-bit multiplier. We would like to implement...
  10. Narkon

    Understanding the Sklansky Adder Algorithm

    Hello. I am unsure if this subforum is the right place for this question, but without any alternatives, I will dare to post it here. A few days ago I received an assignment which is about the creation in VHDL of a Sklansky adder. However, before I move to the coding part, I have trouble...
  11. D

    Full Adder Truth Table: Solving the Mystery

    I don't understand the second row of the truth table. If A is 0 and B is also 0 how is there a carry of 1 and a sum of 1. 0+0=0
  12. nmaganzini

    Is 70ps a Normal Propagation Delay for a 90nm Full Adder?

    Hi guys, I was just wondering, I'm designing a full adder for a bitslice of a 16 bit ALU. I have SPICED my design and I am getting a propagation delay for the AND mode between the two bits of about 70ps. I'm working in a low voltage 90nm process. Am I in the right ballpark in terms of...
  13. ichabodgrant

    Comparator design using a 4-bit adder

    Homework Statement Hi, it's me again. Now I am going to design a 4-bit magnitude comparator using just ONE 4-bit adder and infinitely large number of gates (AND, OR, NOT, NAND, NOR, XOR, XNOR) for signed numbers (negative binary). Homework Equations A > B => A3barB3 + A2barB2x3 +...
  14. 010101

    Optical Adder Using 2 Lasers & Selectable Intensities

    If we use 2 lasers with selectable light intensities as input, with each level of intensity corresponding to decimal number. Like level 0 intensity - decimal 0 (MIN) level 1 intensity - decimal 1 level 2 intensity - decimal 2 ... level 9 intensity - decimal 9 (MAX) and directly shine both of...
  15. K

    Can You Simplify a Parallel Adder SOP Expression Using Karnaugh Maps?

    My working: S3 = A S2 = B S1 = C' S0 = D' F = Sum-Minterm(0, 1, 2, 4, 5, 8) (Basically i find when is X smaller then Y) Used Kmap to arrive at A'C' + B'C'D' + A'B'D' Am i correct?
  16. beyondlight

    Designing a Carry Lookahead adder in Excel sheet

    I'm designing a carry lookahead 8bit adder. My problem is that i don't know how to calculate the group generate signal. I have 4 G signals: First level: G1:0 G3:2 G5:4 G7:6 And these have to be added together: Second level: G3:0 G7:4 Third level: G7:0 Two different formulas for computing...
  17. B

    Efficient Design of a Full Adder with Karnaugh Maps

    Just a quick apology for the long post. Recently I have been looking into digital circuit design and how karnaugh maps can help you simplify a logic table to simple gates. I extended this idea to trying to derive the most efficient full adder design and came up with the following logic table for...
  18. B

    Engineering Full adder circuit using NAND v NOT, AND, OR v PLA logic

    I have designed a full adder circuit, first of all implementing NOT, AND, OR logic, then redesigned NAND logic and finally Programmable Logic Array. I would like to talk evaluate my designs a little and need a bit of help. When using NOT, AND, OR gates I used the following; NOT x 3 = 1...
  19. D

    Ripple-Carry Adder for 2s complement

    Hi, I have designed a simple 4 bit adder for adding two 4-bit unsigned integers A and B. The adder consists of a chain of 4 Full-Adders (Ripple-Carry adder). I am wondering if I need to change the circuit when A and B are in 2s complement. I don't think so. Isn't the advantage of 2s...
  20. L

    Vout of non-inverting adder circuit

    Homework Statement I need to derive this expression v_{out}=\frac{1}{2}(v_{1}-v_{2})(\frac{R_{3}+R_{2}}{R_{3}}) Using the circuit I'm given. I uploaded it. The Attempt at a Solution So far I have v_{in}=v_{+}+v_{-} which is from "rule #2 of op amps", which is "The feedback in...
  21. L

    Design 8 bit adder: Steps & Tips

    Hi I already created a 4 bit adder but for my assignment I need an 8 bit one. Is it correct if I design to separate 4bit adders and connect their outputs (Cout) with an AND gate? 4 bit adder: https://www.dropbox.com/s/ao60s6dgtrnhal3/4bit%20adder.PNG
  22. D

    Designing a 2-Bit Half Adder: How to Add Two 2-Bit Numbers Without Carry?

    I had a cpre exam last night and this was the last question. Can some one tell me the answer, I'm curious if I got it right or not. The part that tripped me up was a 2 bit "half adder". I assumed I wasn't supposed to use a full adder. So I'm not sure what to do with the carry from the first...
  23. Psinter

    Can this 4 bit adder be further simplified?

    Hello, I'm new to logic circuits and I was making a 4 bit binary adder and I thought I could maybe simplify it. However, I can't find anything simpler than what I already got. What I wanted to know is: is my circuit already in its most simple form or am I making something wrong in my attempts to...
  24. C

    16-bit addition/subtraction with an 8-bit adder

    Homework Statement Suppose we have only one 8-bit ripple carry adder but need to do 16-bit addition and subtraction. Design a sequential circuit (FSM) with only one 8-bit ripple carry adder to implement a 16-bit adder-subtractor. You are allowed to use MUXs and need to generate the overflow...
  25. M

    Full Adder modified to make Full Subtractor

    If I was to make a modification to an 8 bit full adder to make it a an eight bit full subtractor would I put an inverter at the carry in input for the first full adder and then an inverter on one of each of the two inputs going into each adder. eg an inverter on A3, A2, A1, A0 and leave B3, B2...
  26. T

    What Is the Role of R5 in a Full Adder Circuit?

    Ok, so I am pretty sure I understand this Full Adder diagram fairly well. But I have one question... I included the picture of course. Take a look at R5 1K. What is the purpose of this resistor? Why doesn't current always flow through the 1K R5 resistor rather than going to the 10K R6...
  27. T

    Engineering Optimizing Full Adder Circuit: Truth Table & Gate Count Reduction Tips

    Heres my truth table A|B|Cin|Sum|Cout 0 0 0 0 0 0 0 1 1 0 0 1 0 1 0 0 1 1 0 1 1 0 0 1 0 1 0 1 0 1 1 1 0 0 1 1 1 1 1 1 Here's my two equations from the K-maps Sum = A+B+CIn Cout = A.B+CIn(A+B) Now what I want to ask is, is there a way the gate count be reduced further by...
  28. L

    Full adder with look ahead carry

    Homework Statement Draw the logic block for the third cell (s2) if a full adder with carry look ahead show all inputs and output write equation for Go Po So using inputs of AoBoCoHomework Equations The Attempt at a Solution here is what i have, is this what they are asking? here is a image...
  29. H

    8 bit adder/sub with 4 bit ripple adder and muxes only

    Hi I'm having trouble with coming up with a minimal solution to this problem: create a 8 bit adder/sub using only one 4 bit ripple carry adder and muxes. This is what I'm thinking but I don't know how to make it any bit simpler: We can calculate the sum of first 4 bits using the ripple carry...
  30. Andy Resnick

    'comptometer'- a mechanical adder

    Here's a small selection of images I've been working on for the past couple of weeks. The object is a 'comptometer'- a mechanical adder. Mine was given to me by a friend who rescued it from his fabric factory- it's a 1940's model. It mostly works, but there's no way I am skilled enough to take...
  31. S

    2 Bit Adder: Inputs 0,1,0,1 Outputs

    Hello! I've been working out the truth table to a logic gate which I've been told is a 2 Bit Adder. The inputs, in the order of the truth table below are 0, 1, 0, 1. Inputs: Outputs: 0 0 0 0 0 0 0 1 0 0 0 0 1 0 0 1 0 0 0 0 1 0 0 1 0 0 1 0 0 0 0 1 0 0 1 1 1 0 0 0 1 1 1...
  32. D

    Creating a full adder using a 3-to-8 decoder

    I'm trying to create a full adder using one 3-to-8 decoder and some nand gates. As of now I know I will have X, Y, and C_in as my inputs. I am having trouble with figuring out what the 8 outputs of the decoder should be, so I am unsure about where and how to use the nand gates. Anyone able to...
  33. P

    Verilog Serial Adder in Behavioral Description

    Homework Statement My homework is to design a Serial Adder in Verilog using a shift register module, a full adder module, and a D Flip-Flop module. I know my full adder and flip flop modules are correct, but I am not so sure about my shift register. The shift register is 8 bits: Inputs for...
  34. F

    Sequential vs combinational adder

    Homework Statement Write the VHDL code for an n-bit sequential adder and an n-bit combinational adder using a full adder and observe the FPGA for each implementation. Homework Equations None The Attempt at a Solution My issue isn't the coding itself or anything of that sort (which...
  35. C

    Carry lookahead adder - How is this possible?

    Homework Statement I have a book which says that the gate delay for generating Ci is 2 logr(n) + 1, where r is the fan-in for each gate and n is the number of bits. This implies that with a fan-in of 2 and 4 bits, the delay for a generating C5 as shown below should be 5 gate delays. How...
  36. L

    Designing a Two-Bit Adder Using Verilog Primitive Gates

    designing a two-bit adder using verilog's primitive gates_I get "z" for my outputs Hi, I am trying to write verilog code for a two bit adder using verilog's primitive gates. I had implemented a two bit adder before...using gates I myself had implemented with nmos and pmos transistors, with...
  37. F

    A four bit adder logic circuit block diagram

    pretty new to this whole computer architure thing, so i appreciate the answers given to my previous question, went through a lecture today in class, came out pretty clueless...what i would like to know is how exactly would the block diagram for a four bit adder circuit look? thank you for...
  38. I

    Engineering Creating a 4 bit full adder logic circuit

    Hello, i had a quick question, basically i am trying to build a 4 bit full adder logic circuit on a motherboard. i been trying to research but i came up empty...i would like to know what are the exact parts that i need to successfully build this logic circuit? thanks..
  39. K

    Full Adder Sum Formula (Logic)

    Hi, I've been trying to work out the formula for the sum for the full adder logic, however have come across a gap which I don't know how to fill. S = (¬A.¬B.C) + (¬A.B.¬C) + (A.¬B.¬C) + (A.B.C) S = ¬A.(¬B.C + B.¬C) + A.(¬B.¬C) S = ¬A.(B \oplus C) + A.( do not know what to do at this point to...
  40. D

    Deriving full adder sum and carry outputs using boolean algebra

    Homework Statement Hi, I am trying to write the sum and output of a full adder in terms of XOR logical functions using boolean logic and Karnaugh maps. I've got the expressions from the Karnaugh maps fine but I can't seem to rearrange them into the expected form shown at the end of my...
  41. K

    Add DC & AC Voltages: Learn How to Do It!

    Hi I've learned about voltage adder circuits using op-amp that add two or more DC voltages. But is it possible to add AC voltage with a DC voltage? What i mean is can we add (for eg 2V DC with a AC of 5v peak value,50hz) so that the output is again AC with peak of 7V and frequency is 50hz...
  42. K

    Can Adder and Divider Circuits Be Used to Create Closed Loop Inverter Equations?

    Hello Every1 I'm doing a project on Closed Loop inverter and There are 2 eqns which i would like to realize using electrical circuits. They are as follows: B=2/M(V1/V2) V0=0.1666(B-1/B) where B,M,V1,V2,V0 are voltages. Is it possible for to realize the above equations using a...
  43. M

    Full Adder using a 3-to-8 Decoder in VHDL

    I need to design a full adder using a 3-to-8 decoder. I have the code for the 3-to-8 decoder but don't know how to use it as a full adder. Please help. Thanks //3-to-8 Decoder library ieee; use ieee.std_logic_1164.all; use ieee.numeric_std.all; entity Decoder is port (...
  44. M

    Engineering Need help badlyabout full adder circuit

    hey guys.. can u pls help me solve this problem. i have some answers but I am just not really sure if they're right. pls help me. my exam starts in a few hours:frown: here's the problem: The truth table of a full adder circuit is given below, where A and B are two inputs and Cin is the...
  45. R

    Can Two 8-bit Adders Be Used to Construct a 16-bit 2's Complement Adder?

    16 bit 2's complement adder. Due tomorrow! You are given two adders. Each adder has two 8-bit operand inputs and a 1-bit carry input. Each adder calculates the sum of the two 8-bit 2's complement operands and the 1-bit carry and outputs an 8-bit sum and a 1-bit 2's complement overflow bit...
  46. A

    Propagation delay for carry lookahead adder

    Homework Statement 1) I'm using a 4 bit carry lookahead adder to build a 8 bit parallel adder. I'm to calculate the maximum propagation delay time, assuming each gate introduces a unit time of propagation delay. I'm assuming not carry lookahead generators. Would it just be 8 units of...
  47. E

    How Does an Adder/Subtractor Unit Handle Negative Inputs?

    If two 4 bit numbers are being added in the Adder/Subtractor unit e.g. X and Y. Then according to my textbook it is written that the Add/Sub Unit is built in a way that the add/sub signal (which chooses whether to add/subtract) is XOR with Y (the position where the number that is to be...
  48. E

    How Can I Create a Testbench for a 1-Bit Adder Circuit?

    So, I am trying to design a testbench for 1-bit adder first, I have 3 inputs A, B and Cin with 2 outputs Sum and Cout; would I just have 3 loops, one for each input? thanks.
  49. H

    How can a byte-wide adder be created using full adders?

    Can someone please explain this to me.. Once we have a full adder, then we can string eight of them together to create a byte-wide adder and cascade the carry bit from one adder to the next.
  50. D

    Boolean help. adder and subtractor

    A boolean circuit has 4 inputs, a, b, cin and K. when K=0, the circuit is an adder. When K=1, the ciruit is a subtractor. WHere do I start? [?]