Hello tech enthusiasts! Today, we’re diving into the basics of digital electronics with a crucial component: the Half Adder, implemented in Verilog. Whether you’re just starting out or looking to brush up on your knowledge, this post will guide you through understanding how a Half Adder operates and how to code it in Verilog.
What is a Half Adder?
A half adder is a simple digital circuit that adds two single-bit binary numbers, A and B. It produces two outputs: the Sum (S) and the Carry (C). Here’s how it works:
The Components: XOR and AND Gates
The half adder uses two types of logic gates:
- XOR Gate: This gate outputs 1 when the inputs are different. It calculates the Sum (S) of A and B.
- AND Gate: This gate outputs 1 only when both inputs are 1. It calculates the Carry (C) from the addition of A and B.
The Truth Table
To understand the half adder’s operation, look at its truth table:
| A | B | Sum (S) | Carry (C) |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 1 | 0 |
| 1 | 1 | 0 | 1 |
How It Works
- Inputs A and B: These are the two binary numbers you want to add.
- Sum (S): Calculated by the XOR gate. If A and B are different, S is 1; if they are the same, S is 0.
- Carry (C): Calculated by the AND gate. It’s 1 only if both A and B are 1.
Verilog Code: Half Adder
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`timescale 1ns / 1ps module HalfAdder( input clkin, input a, input b, output s, output cout ); assign s = a ^ b; assign cout = a & b; endmodule |
Verilog testbench code : HalfAdder
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`timescale 1ns / 1ps module HalfAdder_tb; // Inputs reg a; reg b; reg clkin; // Outputs wire s; wire cout; // Instantiate the Unit Under Test (UUT) HalfAdder uut ( .clkin(clkin), .a(a), .b(b), .s(s), .cout(cout) ); initial begin // Initialize Inputs clkin = 0; a = 0; b = 0; // Wait 100 ns for global reset to finish #100; // Add stimulus here $display("Testing HalfAdder..."); $monitor("At time %t: a = %b, b = %b, s = %b, cout = %b", $time, a, b, s, cout); // Test all combinations of a and b a = 0; b = 0; #1; a = 0; b = 1; #1; a = 1; b = 0; #1; a = 1; b = 1; #1; // Finish simulation $finish; end always #5 clkin = ~clkin; endmodule |
Constraints file : Basys3 board
Modify this file when you are preparing for synthesis, implementation, and bitstream generation. Be sure to activate the necessary properties when using it. You can ignore this file for simulation purposes.
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## This file is a general .xdc for the Basys3 rev B board ## To use it in a project: ## - uncomment the lines corresponding to used pins ## - rename the used ports (in each line, after get_ports) according to the top level signal names in the project ## Clock signal set_property PACKAGE_PIN W5 [get_ports clkin] set_property IOSTANDARD LVCMOS33 [get_ports clkin] create_clock -add -name sys_clk_pin -period 10.00 -waveform {0 5} [get_ports clkin] ## Switches set_property PACKAGE_PIN V17 [get_ports {sw[0]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[0]}] set_property PACKAGE_PIN V16 [get_ports {sw[1]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[1]}] set_property PACKAGE_PIN W16 [get_ports {sw[2]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[2]}] set_property PACKAGE_PIN W17 [get_ports {sw[3]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[3]}] set_property PACKAGE_PIN W15 [get_ports {sw[4]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[4]}] set_property PACKAGE_PIN V15 [get_ports {sw[5]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[5]}] set_property PACKAGE_PIN W14 [get_ports {sw[6]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[6]}] set_property PACKAGE_PIN W13 [get_ports {sw[7]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[7]}] set_property PACKAGE_PIN V2 [get_ports {sw[8]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[8]}] set_property PACKAGE_PIN T3 [get_ports {sw[9]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[9]}] set_property PACKAGE_PIN T2 [get_ports {sw[10]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[10]}] set_property PACKAGE_PIN R3 [get_ports {sw[11]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[11]}] set_property PACKAGE_PIN W2 [get_ports {sw[12]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[12]}] set_property PACKAGE_PIN U1 [get_ports {sw[13]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[13]}] set_property PACKAGE_PIN T1 [get_ports {sw[14]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[14]}] set_property PACKAGE_PIN R2 [get_ports {sw[15]}] set_property IOSTANDARD LVCMOS33 [get_ports {sw[15]}] ## LEDs set_property PACKAGE_PIN U16 [get_ports {led[0]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[0]}] set_property PACKAGE_PIN E19 [get_ports {led[1]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[1]}] set_property PACKAGE_PIN U19 [get_ports {led[2]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[2]}] set_property PACKAGE_PIN V19 [get_ports {led[3]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[3]}] set_property PACKAGE_PIN W18 [get_ports {led[4]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[4]}] set_property PACKAGE_PIN U15 [get_ports {led[5]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[5]}] set_property PACKAGE_PIN U14 [get_ports {led[6]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[6]}] set_property PACKAGE_PIN V14 [get_ports {led[7]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[7]}] set_property PACKAGE_PIN V13 [get_ports {led[8]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[8]}] set_property PACKAGE_PIN V3 [get_ports {led[9]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[9]}] set_property PACKAGE_PIN W3 [get_ports {led[10]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[10]}] set_property PACKAGE_PIN U3 [get_ports {led[11]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[11]}] set_property PACKAGE_PIN P3 [get_ports {led[12]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[12]}] set_property PACKAGE_PIN N3 [get_ports {led[13]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[13]}] set_property PACKAGE_PIN P1 [get_ports {led[14]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[14]}] set_property PACKAGE_PIN L1 [get_ports {led[15]}] set_property IOSTANDARD LVCMOS33 [get_ports {led[15]}] ##7 segment display set_property PACKAGE_PIN W7 [get_ports {seg[0]}] set_property IOSTANDARD LVCMOS33 [get_ports {seg[0]}] set_property PACKAGE_PIN W6 [get_ports {seg[1]}] set_property IOSTANDARD LVCMOS33 [get_ports {seg[1]}] set_property PACKAGE_PIN U8 [get_ports {seg[2]}] set_property IOSTANDARD LVCMOS33 [get_ports {seg[2]}] set_property PACKAGE_PIN V8 [get_ports {seg[3]}] set_property IOSTANDARD LVCMOS33 [get_ports {seg[3]}] set_property PACKAGE_PIN U5 [get_ports {seg[4]}] set_property IOSTANDARD LVCMOS33 [get_ports {seg[4]}] set_property PACKAGE_PIN V5 [get_ports {seg[5]}] set_property IOSTANDARD LVCMOS33 [get_ports {seg[5]}] set_property PACKAGE_PIN U7 [get_ports {seg[6]}] set_property IOSTANDARD LVCMOS33 [get_ports {seg[6]}] set_property PACKAGE_PIN V7 [get_ports dp] set_property IOSTANDARD LVCMOS33 [get_ports dp] set_property PACKAGE_PIN U2 [get_ports {an[0]}] set_property IOSTANDARD LVCMOS33 [get_ports {an[0]}] set_property PACKAGE_PIN U4 [get_ports {an[1]}] set_property IOSTANDARD LVCMOS33 [get_ports {an[1]}] set_property PACKAGE_PIN V4 [get_ports {an[2]}] set_property IOSTANDARD LVCMOS33 [get_ports {an[2]}] set_property PACKAGE_PIN W4 [get_ports {an[3]}] set_property IOSTANDARD LVCMOS33 [get_ports {an[3]}] ##Buttons set_property PACKAGE_PIN U18 [get_ports btnC] set_property IOSTANDARD LVCMOS33 [get_ports btnC] #set_property PACKAGE_PIN T18 [get_ports btnU] #set_property IOSTANDARD LVCMOS33 [get_ports btnU] #set_property PACKAGE_PIN W19 [get_ports btnL] #set_property IOSTANDARD LVCMOS33 [get_ports btnL] set_property PACKAGE_PIN T17 [get_ports btnR] set_property IOSTANDARD LVCMOS33 [get_ports btnR] #set_property PACKAGE_PIN U17 [get_ports btnD] #set_property IOSTANDARD LVCMOS33 [get_ports btnD] ##Pmod Header JA ##Sch name = JA1 #set_property PACKAGE_PIN J1 [get_ports {JA[0]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JA[0]}] ##Sch name = JA2 #set_property PACKAGE_PIN L2 [get_ports {JA[1]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JA[1]}] ##Sch name = JA3 #set_property PACKAGE_PIN J2 [get_ports {JA[2]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JA[2]}] ##Sch name = JA4 #set_property PACKAGE_PIN G2 [get_ports {JA[3]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JA[3]}] ##Sch name = JA7 #set_property PACKAGE_PIN H1 [get_ports {JA[4]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JA[4]}] ##Sch name = JA8 #set_property PACKAGE_PIN K2 [get_ports {JA[5]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JA[5]}] ##Sch name = JA9 #set_property PACKAGE_PIN H2 [get_ports {JA[6]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JA[6]}] ##Sch name = JA10 #set_property PACKAGE_PIN G3 [get_ports {JA[7]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JA[7]}] ##Pmod Header JB ##Sch name = JB1 #set_property PACKAGE_PIN A14 [get_ports {JB[0]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JB[0]}] ##Sch name = JB2 #set_property PACKAGE_PIN A16 [get_ports {JB[1]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JB[1]}] ##Sch name = JB3 #set_property PACKAGE_PIN B15 [get_ports {JB[2]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JB[2]}] ##Sch name = JB4 #set_property PACKAGE_PIN B16 [get_ports {JB[3]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JB[3]}] ##Sch name = JB7 #set_property PACKAGE_PIN A15 [get_ports {JB[4]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JB[4]}] ##Sch name = JB8 #set_property PACKAGE_PIN A17 [get_ports {JB[5]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JB[5]}] ##Sch name = JB9 #set_property PACKAGE_PIN C15 [get_ports {JB[6]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JB[6]}] ##Sch name = JB10 #set_property PACKAGE_PIN C16 [get_ports {JB[7]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JB[7]}] ##Pmod Header JC ##Sch name = JC1 #set_property PACKAGE_PIN K17 [get_ports {JC[0]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JC[0]}] ##Sch name = JC2 #set_property PACKAGE_PIN M18 [get_ports {JC[1]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JC[1]}] ##Sch name = JC3 #set_property PACKAGE_PIN N17 [get_ports {JC[2]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JC[2]}] ##Sch name = JC4 #set_property PACKAGE_PIN P18 [get_ports {JC[3]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JC[3]}] ##Sch name = JC7 #set_property PACKAGE_PIN L17 [get_ports {JC[4]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JC[4]}] ##Sch name = JC8 #set_property PACKAGE_PIN M19 [get_ports {JC[5]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JC[5]}] ##Sch name = JC9 #set_property PACKAGE_PIN P17 [get_ports {JC[6]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JC[6]}] ##Sch name = JC10 #set_property PACKAGE_PIN R18 [get_ports {JC[7]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JC[7]}] ##Pmod Header JXADC ##Sch name = XA1_P #set_property PACKAGE_PIN J3 [get_ports {JXADC[0]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JXADC[0]}] ##Sch name = XA2_P #set_property PACKAGE_PIN L3 [get_ports {JXADC[1]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JXADC[1]}] ##Sch name = XA3_P #set_property PACKAGE_PIN M2 [get_ports {JXADC[2]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JXADC[2]}] ##Sch name = XA4_P #set_property PACKAGE_PIN N2 [get_ports {JXADC[3]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JXADC[3]}] ##Sch name = XA1_N #set_property PACKAGE_PIN K3 [get_ports {JXADC[4]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JXADC[4]}] ##Sch name = XA2_N #set_property PACKAGE_PIN M3 [get_ports {JXADC[5]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JXADC[5]}] ##Sch name = XA3_N #set_property PACKAGE_PIN M1 [get_ports {JXADC[6]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JXADC[6]}] ##Sch name = XA4_N #set_property PACKAGE_PIN N1 [get_ports {JXADC[7]}] #set_property IOSTANDARD LVCMOS33 [get_ports {JXADC[7]}] ##VGA Connector #set_property PACKAGE_PIN G19 [get_ports {vgaRed[0]}] #set_property IOSTANDARD LVCMOS33 [get_ports {vgaRed[0]}] #set_property PACKAGE_PIN H19 [get_ports {vgaRed[1]}] #set_property IOSTANDARD LVCMOS33 [get_ports {vgaRed[1]}] #set_property PACKAGE_PIN J19 [get_ports {vgaRed[2]}] #set_property IOSTANDARD LVCMOS33 [get_ports {vgaRed[2]}] #set_property PACKAGE_PIN N19 [get_ports {vgaRed[3]}] #set_property IOSTANDARD LVCMOS33 [get_ports {vgaRed[3]}] #set_property PACKAGE_PIN N18 [get_ports {vgaBlue[0]}] #set_property IOSTANDARD LVCMOS33 [get_ports {vgaBlue[0]}] #set_property PACKAGE_PIN L18 [get_ports {vgaBlue[1]}] #set_property IOSTANDARD LVCMOS33 [get_ports {vgaBlue[1]}] #set_property PACKAGE_PIN K18 [get_ports {vgaBlue[2]}] #set_property IOSTANDARD LVCMOS33 [get_ports {vgaBlue[2]}] #set_property PACKAGE_PIN J18 [get_ports {vgaBlue[3]}] #set_property IOSTANDARD LVCMOS33 [get_ports {vgaBlue[3]}] #set_property PACKAGE_PIN J17 [get_ports {vgaGreen[0]}] #set_property IOSTANDARD LVCMOS33 [get_ports {vgaGreen[0]}] #set_property PACKAGE_PIN H17 [get_ports {vgaGreen[1]}] #set_property IOSTANDARD LVCMOS33 [get_ports {vgaGreen[1]}] #set_property PACKAGE_PIN G17 [get_ports {vgaGreen[2]}] #set_property IOSTANDARD LVCMOS33 [get_ports {vgaGreen[2]}] #set_property PACKAGE_PIN D17 [get_ports {vgaGreen[3]}] #set_property IOSTANDARD LVCMOS33 [get_ports {vgaGreen[3]}] #set_property PACKAGE_PIN P19 [get_ports Hsync] #set_property IOSTANDARD LVCMOS33 [get_ports Hsync] #set_property PACKAGE_PIN R19 [get_ports Vsync] #set_property IOSTANDARD LVCMOS33 [get_ports Vsync] ##USB-RS232 Interface #set_property PACKAGE_PIN B18 [get_ports RsRx] #set_property IOSTANDARD LVCMOS33 [get_ports RsRx] #set_property PACKAGE_PIN A18 [get_ports RsTx] #set_property IOSTANDARD LVCMOS33 [get_ports RsTx] ##USB HID (PS/2) #set_property PACKAGE_PIN C17 [get_ports PS2Clk] #set_property IOSTANDARD LVCMOS33 [get_ports PS2Clk] #set_property PULLUP true [get_ports PS2Clk] #set_property PACKAGE_PIN B17 [get_ports PS2Data] #set_property IOSTANDARD LVCMOS33 [get_ports PS2Data] #set_property PULLUP true [get_ports PS2Data] ##Quad SPI Flash ##Note that CCLK_0 cannot be placed in 7 series devices. You can access it using the ##STARTUPE2 primitive. #set_property PACKAGE_PIN D18 [get_ports {QspiDB[0]}] #set_property IOSTANDARD LVCMOS33 [get_ports {QspiDB[0]}] #set_property PACKAGE_PIN D19 [get_ports {QspiDB[1]}] #set_property IOSTANDARD LVCMOS33 [get_ports {QspiDB[1]}] #set_property PACKAGE_PIN G18 [get_ports {QspiDB[2]}] #set_property IOSTANDARD LVCMOS33 [get_ports {QspiDB[2]}] #set_property PACKAGE_PIN F18 [get_ports {QspiDB[3]}] #set_property IOSTANDARD LVCMOS33 [get_ports {QspiDB[3]}] #set_property PACKAGE_PIN K19 [get_ports QspiCSn] #set_property IOSTANDARD LVCMOS33 [get_ports QspiCSn] set_property BITSTREAM.GENERAL.COMPRESS TRUE [current_design] |
Schematic : Half Adder
Simulation: Half Adder
Conclusion
The half adder is a crucial component in digital electronics, forming the basis for more complex circuits like the full adder. Understanding it is essential for anyone interested in digital design.
It can also be use to developed using full Adder (FA) module.
Thanks for reading! Stay tuned for more insights into digital electronics.
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