Chapter 3 Code
To comply the Chapter 3 Verilog codes within the book titled Fundamental Digital Circuits and FPGA
Section 3.1: Three-input XOR Gate
Module Definition
Verilog Code
Code 3.1: Construction of a 3-input XOR Gate circuit using the Gate-level description method
/*---------------------------------------------------------------------------------------------------
*- File name: XOR3_1.v
*- Top Module name: XOR3_1
- Submodules: xor (Verilog built-in primitive gates)
*- Description: Implementation of a 3 input XOR gate
*-
*- Example of Usage:
You can implement this code on all variants of STEPFPGA family boards.
If you want to implement this code on board and observe the logic behaviors:
- assign A, B, C to 3 on-board switches
- assign Y to any on-board LEDs
*- Copyright of this code: MIT License
--------------------------------------------------------------------------------------------------- */
module XOR3_1 (
input wire A,
input wire B,
input wire C,
output wire Y
) ;
xor (Y, A, B, C) ;
endmodule
Implementation
Section 3.2: One-bit Full Adder
Module Definition
Verilog Code
Written in Data-flow Description style
Code 3.2: Constructing a 1-bit Full Adder
/*---------------------------------------------------------------------------------------------------
*- File name: full_adder.v
*- Top Module name: full_adder
- Submodules: N/A
*- Description: Implementation of a 1-bit Full Binary Adder
*-
*- Example of Usage:
You can implement this code on all variants of STEPFPGA family boards.
If you want to implement this code on board and observe the logic behaviors:
- assign a, b, cin to 3 on-board switches
- assign sum, co to any 2 on-board LEDs
*- Copyright of this code: MIT License
--------------------------------------------------------------------------------------------------- */
module full_adder (
input wire a,
input wire b,
input wire cin,
output wire sum,
output wire co
);
assign sum = a^b^cin;
assign co = ((a^b)&cin)|(a&b);
endmoduleWritten in Structrual (Gate) Level Description style
module full_adder (
input wire a,
input wire b,
input wire cin,
output wire sum,
output wire co
);
// XOR gates for sum
wire xor_ab, xor_abc;
xor (xor_ab, a, b);
xor (xor_abc, xor_ab, cin);
// AND & OR gates for carry out
wire and_ab, and_xor_ab_cin, or_final;
and (and_ab, a, b);
and (and_xor_ab_cin, xor_ab, cin);
or (or_final, and_ab, and_xor_ab_cin);
// Assignments
assign sum = xor_abc;
assign co = or_final;
endmodule
Implementation
Section 3.3: Two by Four Decoder
Module Definition
Verilog Code
Code 3.3: Construction of a complete 2-4 Decoder
/*---------------------------------------------------------------------------------------------------
*- File name: decoder24.v
*- Top Module name: decoder24_en
- Submodules: N/A
*- Description: Implementation of a 2-4 decoder module (without Enable)
*-
*- Example of Usage:
You can implement this code on all variants of STEPFPGA family boards.
If you want to implement this code on board and observe the logic behaviors:
- assign A[1] , A[0] to 2 on-board switches
- assign Y[3]...Y[0] to 4 on-board LEDs
*- Copyright of this code: MIT License
--------------------------------------------------------------------------------------------------- */
module decoder24 (
input [1: 0] A,
output reg [3: 0] Y
);
always @ (A) begin
case (A)
2'b00: Y = 4'b0001;
2'b01: Y = 4'b0010;
2'b10: Y = 4'b0100;
2'b11: Y = 4'b1000;
endcase
end
endmodule
Implementation
Section 3.4: Three by Eight Decoder
Module Definition
Verilog Code
Code 3.4: Behavioral description of a 2-4 Decoder with enable input signal
/*---------------------------------------------------------------------------------------------------
*- File name: decoder38.v
*- Top Module name: decoder38
- Submodules: decoder24_en upper
*- Description: Implementation of a 3-8 decoder module
*-
*- Example of Usage:
You can implement this code on all variants of STEPFPGA family boards.
If you want to implement this code on board and observe the logic behaviors:
- assign X[2] ... X[0] to 3 on-board switches
- assign D[7] ... D[0] to 8 on-board LEDs
*- Copyright of this code: MIT License
--------------------------------------------------------------------------------------------------- */
module decoder38
(
input wire [2: 0] X,
output wire [7: 0] D
) ;
decoder24_en upper (
.A (X[1: 0]) ,
.EN (X[2]) ,
.Y (D[7: 4])
) ;
decoder24_en lower (
.A (X[1: 0]) ,
.EN (!X[2]) ,
.Y (D[3: 0])
) ;
endmodule
module decoder24_en (
input wire [1:0] A,
input wire EN,
output reg [3:0] Y
);
always @ (EN, A) begin
if (EN == 1'b1)
case (A)
2'b00: Y = 4'b0001;
2'b01: Y = 4'b0010;
2'b10: Y = 4'b0100;
2'b11: Y = 4'b1000;
endcase
else
Y = 4'b0000;
end
endmoduleImplementation
Section 3.5: Seven segment display
Module Definition
Verilog Code
Code 3.6: Common Cathode 7-segment driver
/*---------------------------------------------------------------------------------------------------
*- File name: segment7.v
*- Top Module name: segment7
- Submodules: N/A
*- Description: Implementation of a 7-segment display driver (common cathod)
*-
*- Example of Usage:
You can implement this code on all variants of STEPFPGA family boards.
If you want to implement this code on board and observe the logic behaviors:
- assign seg_data[3]...seg_data[0] to the 4 on-board swiches
- assign segment_led[8] to SEG, segment_led[7] to DP
- assign segment_led[6]...segment_led[0] to 'g, f, ... a' accordingly
* - Additional comments:
If you want to display decimal numbers only, the 4-bit input data must be in BCD converted
form; will explain in Chapter 5 when implement the Elevator project.
*- Copyright of this code: MIT License
--------------------------------------------------------------------------------------------------- */
module segment7 (
input wire [3:0] seg_data,
output reg [8:0] segment_led
// MSB~LSB = SEG, DP, g, f e, d, c, b, a
) ;
always @ (seg_data) begin
case (seg_data)
4'b0000: segment_led = 9'h3f; // 0
4'b0001: segment_led = 9'h06; // 1
4'b0010: segment_led = 9'h5b; // 2
4'b0011: segment_led = 9'h4f; // 3
4'b0100: segment_led = 9'h66; // 4
4'b0101: segment_led = 9'h6d; // 5
4'b0110: segment_led = 9'h7d; // 6
4'b0111: segment_led = 9'h07; // 7
4'b1000: segment_led = 9'h7f; // 8
4'b1001: segment_led = 9'h6f; // 9
4'b1010: segment_led = 9'h77; // A
4'b1011: segment_led = 9'h7C; // b
4'b1100: segment_led = 9'h39; // C
4'b1101: segment_led = 9'h5e; // d
4'b1110: segment_led = 9'h79; // E
4'b1111: segment_led = 9'h71; // F
endcase
end
endmoduleImplementation
Section 3.6: Decimal counter
Illustration
Verilog Code
Code 3.6: Using four counters to control the on-off time of four LED lights respectively
/*---------------------------------------------------------------------------------------------------
*- File name: LED_sequence.v
*- Top Module name: LED_sequence
- Submodules: N/A
*- Description: Experiment with counters
*-
*- Example of Usage:
You can implement this code on all variants of STEPFPGA family boards.
If you want to implement this code on board and observe the logic behaviors:
- assign clk to PCLK (12MHz) on-board clock
- assign rst_n to any push-buttons on-board
- assign led[3] ... led[0] to 4 on-board LEDs
*- Copyright of this code: MIT License
--------------------------------------------------------------------------------------------------- */
module LED_sequence (
input clk,
input rst_n,
output [3:0] led
) ;
reg [25: 0] cnt;
parameter t_1s = 12_000_000,
t_2s = 24_000_000,
t_3s = 36_000_000,
t_4s = 48_000_000;
always @ (posedge clk or negedge rst_n) begin
if (!rst_n)
cnt <= 0;
else
cnt <= cnt + 1'b1;
end
assign led[0] = (cnt < t_1s) ? 1 : 0; // Conditional assignment
assign led[1] = (cnt < t_2s) ? 1 : 0; // If (true), assign to 1; otherwise assign to 0
assign led[2] = (cnt < t_3s) ? 1 : 0;
assign led[3] = (cnt < t_4s) ? 1 : 0;
endmodule
A simple D-Flipflop
/*---------------------------------------------------------------------------------------------------
*- File name: dff.v
*- Top Module name: dff
- Submodules: N/A
*- Description: Implementation of a 1-bit D Flip-Flop
*-
*- Example of Usage:
You can implement this code on all variants of STEPFPGA family boards.
If you want to implement this code on board and observe the logic behaviors:
- assign clk to PCLK signal (12MHz)
- assign D to any on-board switches
- assign Q to any on-board LEDs
*- Copyright of this code: MIT License
--------------------------------------------------------------------------------------------------- */
module dff(input clk, D, output reg Q);
always @ (posedge clk) begin
Q <= D;
end
endmoduleImplementation
Section 3.7: Frequency divider
Illustration
Verilog Code
Code 3.7: Frequency division by any integer
/*
File name: divider_integer
Module Function: Implementation of a clock frequency divider
This code implements a clock divider which supports for all integer divisors.
Note: if the divisor is 'odd' number, the output duty cycle is not exactly 50%; but if the divider is higher than
99 you can practically neglect this.
Copyright License: MIT
*/
// To set for different divisor, you only need to change these two parameters
module divider_integer # (
parameter N = 12000000, // the divisor
parameter WIDTH = 24 // the minimum bit-width to hold this divisor
)
(
input clk,
output reg clkout
);
reg [WIDTH-1:0] cnt;
always @ (posedge clk) begin
if(cnt>=(N-1))
cnt <= 1'b0;
else
cnt <= cnt + 1'b1;
clkout <= (cnt<N/2)?1'b1:1'b0;
end
endmodule
Implementation
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