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Designing Binary Arithmetic Circuits with VHDL Structural Modeling

Learn to implement Half Adder, Full Adder, 4-bit Ripple Carry Adder, and 4-bit Comparator using VHDL. Explore structured design techniques and apply them to complex circuits. Includes practical examples and concurrent statements.

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Designing Binary Arithmetic Circuits with VHDL Structural Modeling

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  1. Experiment 7 Design of Binary Arithmetic Circuits

  2. Experiment 6 Overview: • The Point: you used some relatively simple VHDL models to implement some relatively complicated circuits. • Include block diagrams • Concurrency = Concurrency (CSA) • VHDL code should look perfect • Properly indentation • Commented • Title banner (names, description, etc) • No line wrap! • Self commented names

  3. Instructional Objectives: • To use concurrent VHDL statements in the design of arithmetic circuits • To design a Half Adder, Full Adder, 4-bit Ripple Carry Adder, and 4-bit Comparator

  4. 7-Seg Decoder Priority Encoder B3 B2 AA-AG B1 CATH B0 I7 I6 I5 Y2 I4 Y1 I3 Y0 I2 I1 STROBE I0 Connect to ground 4 switches (sensors) Alarm Control Break-in Armed OFF/ON_L Alarm Key Comparator I3 EQ I2 I1 I0 4 switches (access code) Digital Alarm System Experiment 7 P3

  5. Binary Addition Given two 4-bit numbers, their sum A+B is calculated as follows a3 a2 a1 a0 + b3 b2 b1 b0 s3 s2 s1 s0 c2 c1 c0 cout

  6. Half Adder A device, called a Half Adder, can be design to add a0+b0 in the least significant stage of the addition problem. Inputs: ai, bi Output: ci, si Obtain equations for ci and si from the truth table for the half adder.

  7. Full Adder A device, called a Full Adder, can be design to add ai+bi, for i>0 stages of the addition problem. Inputs: ai, bi, ci-1(carry in to that stage) Output: ci, si Obtain equations for ci and si from the truth table for the full adder.

  8. 4-Bit Ripple Carry Adder Use one Half Adder (for the least significant stage) and three Full Adders. Propagate the carry through the circuit: ripple How many gate propagation delays before the carry out is valid?

  9. 4-Bit Comparator Given two 4-bit binary numbers, A and B, determine if they are equal. Multiple solutions: • Subtract B from A and test for zero result • Use XOR gates to do a bitwise comparison

  10. VHDL Concurrent StatementRefresher ARCHITECTURE mad_hatter OF myhalfadder IS BEGIN concurrent statement1; -- half adder sum equation concurrent statement2; -- half adder carry equation END myarch;

  11. VHDL Structural Modeling • Reflects modern digital circuit design • Supports readability, understandability, reuse of code • Supports preferred digital design approach: hierarchical design • Allows for the use of library-based modules • Allows for easy scalability of design • Utilizes software design techniques • Xilinx schematic capture software not reliable

  12. VHDL Structural Modeling Similar to higher level language programming Example circuit

  13. VHDL Code for “Modules”

  14. Final VHDL Code

  15. XCRPlus Development Board Carry out 4-bit sum Of A+B A B

  16. Experiment 7 Overview P1: Design, test, and implement a Half Adder P2: Design, test, and implement a Full Adder P3: Design and implement a 4-bit Ripple Carry Adder Using Structural Modeling (Modelsim printout) P4: Design and implement a 4-bit Comparator Save for use in Experiment 9

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