[FPGA Tutorial] Seven-Segment LED Display on Basys 3 FPGA

This FPGA tutorial will guide you how to control the 4-digit seven-segment display on Basys 3 FPGA Board. A display controller will be designed in Verilog for displaying numbers on the 4-digit 7-segment LED display of the Basys 3 FPGA. Full Verilog code for the seven-segment LED display controller will also be provided.

The Basys 3 FPGA has a common-anode 4-digit 7-segment LED display as shown in the following figure.

Seven anodes of the seven segments in a single LED are connected together to one common anode node while its cathodes are separate as shown in the following figure. DP-segment is to illuminate the dot so we omit the DP-segment for now since it is not contributing to the number value of the seven-segment display.

To illuminate LED segments such as A-G, anodes of these segments need to be at the 'high' logic level and cathodes should be at the 'low' logic level. Thus, the common anode node should be high to activate a single seven-segment LED display. The logic levels of cathodes can be varying from low (illuminated) to high(un-illuminated) to show different decimal values on a single 7-segment LED display as follows. 

As shown in the figure above, we can display the numbers from zero to nine on the seven-segment LED display by turning on and off the seven segments of the single seven-segment LED display. For example, to show number "1" on the LED, the segments A, F, G, E, and D are un-illuminated or their cathodes should be high, and B-C segments are illuminated or their cathodes should be low. Similarly, we can obtain the following decoder table for displaying ten numbers on seven-segment LED display:

The table shows the needed cathode patterns for showing corresponding numbers on a single 7-segment LED display of the Basys 3 FPGA board.

For now, we know how to display numbers on a single seven-segment LED display on Basys 3 FPGA board. However, the cathodes of four seven-segment LEDs on Basys 3 are not separate but connected together as shown in the following figure.

Thus, to display 4 different numbers on the 4-digit seven-segment LED display, we have to control the cathodes (CA-CG) of the four seven-segment LEDs separately by activating the four seven-segment LEDs at different times. For example, when we activate LED 1 by driving A1 high and the other three LEDs (LED 2, LED 3, LED 4) are deactivated (A2, A3, and A4 not driven), the cathode pattern (CA-CG) will be used for displaying numbers on LED 1. Similarly, LED 2 - LED 4 can be displayed by using the same way at different times. As shown in the figure above, we can control activating four seven-segment LEDs by using 4 PNP transistors AN0-AN3. When a transistor is on or the Base terminal (W4, U4, V4, or U2) is low, the corresponding anode is driven high to activate or enable the corresponding seven-segment LED. 

When a LED is deactivated after illuminating, it will darken. To avoid the displaying discontinuity perceived by the human eye, the four seven-segment LEDs should be continuously refreshed at about 1KHz to 60Hz or it should be refreshed at every 1ms to 16ms. 

In this FPGA tutorial, a seven-segment LED display controller is designed for displaying numbers on the four-digit 7-segment LED display of the Basys 3 FPGA board. The following is the timing diagram for the seven-segment LED display controller on Basys 3 FPGA:

Below is an example timing diagram for displaying "1234" on the 4-digit 7-segment LED display of Basys 3 FPGA board with the refresh rate of 60Hz or period of 16ms:

Now, we know how to control and display numbers on the four-digit seven-segment LED display on Basys 3 FPGA. Next, let's design the 7-segment LED display controller.

The Basys 3 FPGA has a clock source of 100MHz and we need a 1ms-16ms refresh period or a 1KHz-60Hz refresh rate. I will choose a refresh period of 10.5ms (digit period = 2.6ms) so that we can use a 20-bit counter for creating the refresh period with the first 2 MSB bits of the counter for creating LED-activating signals (digit period of 2.6ms) as shown in the timing diagram above.

Below is an example Verilog code for creating the refresh signal and LED-activating signals:

reg [19:0] refresh_counter; 

// the first 18-bit for creating 2.6ms digit period
// the other 2-bit for creating 4 LED-activating signals

wire [1:0] LED_activating_counter; 

// count        0    ->  1  ->  2  ->  3

// activates    LED1    LED2   LED3   LED4
// and repeat
always @(posedge clock_100Mhz or posedge reset)
begin 
 if(reset==1)
  refresh_counter <= 0;
 else
  refresh_counter <= refresh_counter + 1;
end 
assign LED_activating_counter = refresh_counter[19:18];

After that, we need to generate the anode signals (W4, V4, U4, U2) for the four-digit 7-segment LED display based on the LED-activating counter. The LED-activating counter will repeatedly count from zero to three for continuously activating and updating the four seven-segment LEDs.

Below is an example Verilog code for creating the anode signals and updating values of the four 7-segment LEDs on Basys 3 FPGA:

    // anode activating signals for 4 LEDs
    // decoder to generate anode signals 
    always @(*)
    begin
        case(LED_activating_counter)
        2'b00: begin
            Anode_Activate = 4'b0111; 
            // activate LED1 and Deactivate LED2, LED3, LED4
            LED_BCD = displayed_number[15:11];
            // the first hex-digit of the 16-bit number
             end
        2'b01: begin
            Anode_Activate = 4'b1011; 
            // activate LED2 and Deactivate LED1, LED3, LED4
            LED_BCD = displayed_number[10:8];
            // the second hex-digit of the 16-bit number
                end
        2'b10: begin
            Anode_Activate = 4'b1101; 
            // activate LED3 and Deactivate LED2, LED1, LED4
            LED_BCD = displayed_number[7:4];
             // the third hex-digit of the 16-bit number
              end
        2'b11: begin
            Anode_Activate = 4'b1110; 
            // activate LED4 and Deactivate LED2, LED3, LED1
             LED_BCD = displayed_number[3:0];
             // the fourth hex-digit of the 16-bit number 
               end   
        default:begin
             Anode_Activate = 4'b0111; 
            // activate LED1 and Deactivate LED2, LED3, LED4
            LED_BCD = displayed_number[15:11];
            // the first hex-digit of the 16-bit number
            end
        endcase
    end

Last but not least, the example Verilog code for BCD to 7-segment decoder based on the decoder table above:

reg[6:0] LED_out;
// Cathode patterns of the 7-segment LED display 
always @(*)
begin
 case(LED_BCD)
 4'b0000: LED_out = 7'b0000001; // "0"  
 4'b0001: LED_out = 7'b1001111; // "1" 
 4'b0010: LED_out = 7'b0010010; // "2" 
 4'b0011: LED_out = 7'b0000110; // "3" 
 4'b0100: LED_out = 7'b1001100; // "4" 
 4'b0101: LED_out = 7'b0100100; // "5" 
 4'b0110: LED_out = 7'b0100000; // "6" 
 4'b0111: LED_out = 7'b0001111; // "7" 
 4'b1000: LED_out = 7'b0000000; // "8"  
 4'b1001: LED_out = 7'b0000100; // "9" 
 default: LED_out = 7'b0000001; // "0"
 endcase
end

The design for 7-segment LED display controller is basically completed. Let's use the controller to display a 16-bit counting number on the four-digit seven-segment LED display of the Basys 3 FPGA board with the counting period of 1 second.

Full Verilog code of the seven-segment LED display controller for displaying counting numbers on Basys 3 FPGA: 

// fpga4student.com: FPGA projects, Verilog projects, VHDL projects
// FPGA tutorial: seven-segment LED display controller on Basys  3 FPGA
module Seven_segment_LED_Display_Controller(
    input clock_100Mhz, // 100 Mhz clock source on Basys 3 FPGA
    input reset, // reset
    output reg [3:0] Anode_Activate, // anode signals of the 7-segment LED display
    output reg [6:0] LED_out// cathode patterns of the 7-segment LED display
    );
    reg [26:0] one_second_counter; // counter for generating 1 second clock enable
    wire one_second_enable;// one second enable for counting numbers
    reg [15:0] displayed_number; // counting number to be displayed
    reg [3:0] LED_BCD;
    reg [19:0] refresh_counter; // 20-bit for creating 10.5ms refresh period or 380Hz refresh rate
             // the first 2 MSB bits for creating 4 LED-activating signals with 2.6ms digit period
    wire [1:0] LED_activating_counter; 

                 // count     0    ->  1  ->  2  ->  3
              // activates    LED1    LED2   LED3   LED4
             // and repeat
    always @(posedge clock_100Mhz or posedge reset)
    begin
        if(reset==1)
            one_second_counter <= 0;
        else begin
            if(one_second_counter>=99999999) 
                 one_second_counter <= 0;
            else
                one_second_counter <= one_second_counter + 1;
        end
    end 
    assign one_second_enable = (one_second_counter==99999999)?1:0;
    always @(posedge clock_100Mhz or posedge reset)
    begin
        if(reset==1)
            displayed_number <= 0;
        else if(one_second_enable==1)
            displayed_number <= displayed_number + 1;
    end
    always @(posedge clock_100Mhz or posedge reset)
    begin 
        if(reset==1)
            refresh_counter <= 0;
        else
            refresh_counter <= refresh_counter + 1;
    end 
    assign LED_activating_counter = refresh_counter[19:18];
    // anode activating signals for 4 LEDs, digit period of 2.6ms
    // decoder to generate anode signals 
    always @(*)
    begin
        case(LED_activating_counter)
        2'b00: begin
            Anode_Activate = 4'b0111; 
            // activate LED1 and Deactivate LED2, LED3, LED4
            LED_BCD = displayed_number/1000;
            // the first digit of the 16-bit number
              end
        2'b01: begin
            Anode_Activate = 4'b1011; 
            // activate LED2 and Deactivate LED1, LED3, LED4
            LED_BCD = (displayed_number % 1000)/100;
            // the second digit of the 16-bit number
              end
        2'b10: begin
            Anode_Activate = 4'b1101; 
            // activate LED3 and Deactivate LED2, LED1, LED4
            LED_BCD = ((displayed_number % 1000)%100)/10;
            // the third digit of the 16-bit number
                end
        2'b11: begin
            Anode_Activate = 4'b1110; 
            // activate LED4 and Deactivate LED2, LED3, LED1
            LED_BCD = ((displayed_number % 1000)%100)%10;
            // the fourth digit of the 16-bit number    
               end
        endcase
    end
    // Cathode patterns of the 7-segment LED display 
    always @(*)
    begin
        case(LED_BCD)
        4'b0000: LED_out = 7'b0000001; // "0"     
        4'b0001: LED_out = 7'b1001111; // "1" 
        4'b0010: LED_out = 7'b0010010; // "2" 
        4'b0011: LED_out = 7'b0000110; // "3" 
        4'b0100: LED_out = 7'b1001100; // "4" 
        4'b0101: LED_out = 7'b0100100; // "5" 
        4'b0110: LED_out = 7'b0100000; // "6" 
        4'b0111: LED_out = 7'b0001111; // "7" 
        4'b1000: LED_out = 7'b0000000; // "8"     
        4'b1001: LED_out = 7'b0000100; // "9" 
        default: LED_out = 7'b0000001; // "0"
        endcase
    end
 endmodule

Pin constraint file for the four-digit seven-segment LED display on Basys 3 FPGA:

# Clock signal
set_property PACKAGE_PIN W5 [get_ports clock_100Mhz]       
 set_property IOSTANDARD LVCMOS33 [get_ports clock_100Mhz]
set_property PACKAGE_PIN R2 [get_ports reset]     
 set_property IOSTANDARD LVCMOS33 [get_ports reset]
#seven-segment LED display
set_property PACKAGE_PIN W7 [get_ports {LED_out[6]}]                    
   set_property IOSTANDARD LVCMOS33 [get_ports {LED_out[6]}]
set_property PACKAGE_PIN W6 [get_ports {LED_out[5]}]                    
   set_property IOSTANDARD LVCMOS33 [get_ports {LED_out[5]}]
set_property PACKAGE_PIN U8 [get_ports {LED_out[4]}]                    
   set_property IOSTANDARD LVCMOS33 [get_ports {LED_out[4]}]
set_property PACKAGE_PIN V8 [get_ports {LED_out[3]}]                    
   set_property IOSTANDARD LVCMOS33 [get_ports {LED_out[3]}]
set_property PACKAGE_PIN U5 [get_ports {LED_out[2]}]                    
   set_property IOSTANDARD LVCMOS33 [get_ports {LED_out[2]}]
set_property PACKAGE_PIN V5 [get_ports {LED_out[1]}]                    
   set_property IOSTANDARD LVCMOS33 [get_ports {LED_out[1]}]
set_property PACKAGE_PIN U7 [get_ports {LED_out[0]}]                    
   set_property IOSTANDARD LVCMOS33 [get_ports {LED_out[0]}]

set_property PACKAGE_PIN U2 [get_ports {Anode_Activate[0]}]                    
   set_property IOSTANDARD LVCMOS33 [get_ports {Anode_Activate[0]}]
set_property PACKAGE_PIN U4 [get_ports {Anode_Activate[1]}]                    
   set_property IOSTANDARD LVCMOS33 [get_ports {Anode_Activate[1]}]
set_property PACKAGE_PIN V4 [get_ports {Anode_Activate[2]}]               
   set_property IOSTANDARD LVCMOS33 [get_ports {Anode_Activate[2]}]
set_property PACKAGE_PIN W4 [get_ports {Anode_Activate[3]}]          
   set_property IOSTANDARD LVCMOS33 [get_ports {Anode_Activate[3]}]

Now, create a new project in Vivado, choose the device part number of XC7A35T-1CPG236C for Artix-7 FPGA on Basys 3 FPGA board. Then, add the source and constraint files, and generate the bitstream. Program the FPGA using the bit stream and see how it works on the Basys 3 FPGA board.

Demo video for seven-segment LED display controller on Basys 3 FPGA board:

Here is the full VHDL code for seven-segment display on Basys 3 FPGA.
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