Seven Segment LED Multiplexing Circuit in Verilog

The seven segment LED circuit uses seven different and individual LED's to display a hexadecimal symbol. It has 7 wires to control the individual LED's one wire to control the decimal point and one enable wire. The demo board I am using here consists of four such 7-segment LED's(As do any other demo board). To reduce the number of wires a multiplexing circuit is used to control the display. Using the multiplexing circuit the number of wires required to light up all 4 displays are reduced from 32 to 12 (8 data bits and 4 enable bits). All bits here are active low, such that to enable them a '0' is required. For example the figure below shows how to display a 3 on the seven segment.

The multiplexing circuit can take 4 inputs and have only one output. But the inputs should be displayed on the output fast enough to fool the viewer into thinking all outputs are enabled individually and simultaneously. This is achieved by having an enable signal that changes so fast that it appears that all displays are on simultaneously. The refreshing rate of the enable signal should be around 1000Hz to achieve this desired effect. Since the BASYS2 board has a 50Mhz clock and if an 18 bit counter is used and only the 2 MSB's are used to generate the enable signal then the refreshing rate of an individual enable bit will be 50MHz/(2^16) which comes to about 800 Hz.

Once the multiplexing is done the next code block is always the data bits for the seven segment. This is usually a case statement which encodes the hexadecimal digits into binary format. Since the LED's here are active low the segments that need to be enabled must be low.

The code for the LED multiplexing is shown below:

module sevenseg(
 input clock, reset,
 input in0, in1, in2, in3,  //the 4 inputs for each display
 output a, b, c, d, e, f, g, dp, //the individual LED output for the seven segment along with the digital point
 output [3:0] an   // the 4 bit enable signal
 );

localparam N = 18;

reg [N-1:0]count; //the 18 bit counter which allows us to multiplex at 1000Hz

always @ (posedge clock or posedge reset)
 begin
  if (reset)
   count <= 0;
  else
   count <= count + 1;
 end

reg [6:0]sseg; //the 7 bit register to hold the data to output
reg [3:0]an_temp; //register for the 4 bit enable

always @ (*)
 begin
  case(count[N-1:N-2]) //using only the 2 MSB's of the counter 
   
   2'b00 :  //When the 2 MSB's are 00 enable the fourth display
    begin
     sseg = in0;
     an_temp = 4'b1110;
    end
   
   2'b01:  //When the 2 MSB's are 01 enable the third display
    begin
     sseg = in1;
     an_temp = 4'b1101;
    end
   
   2'b10:  //When the 2 MSB's are 10 enable the second display
    begin
     sseg = in2;
     an_temp = 4'b1011;
    end
    
   2'b11:  //When the 2 MSB's are 11 enable the first display
    begin
     sseg = in3;
     an_temp = 4'b0111;
    end
  endcase
 end
assign an = an_temp;


reg [6:0] sseg_temp; // 7 bit register to hold the binary value of each input given

always @ (*)
 begin
  case(sseg)
   4'd0 : sseg_temp = 7'b1000000; //to display 0
   4'd1 : sseg_temp = 7'b1111001; //to display 1
   4'd2 : sseg_temp = 7'b0100100; //to display 2
   4'd3 : sseg_temp = 7'b0110000; //to display 3
   4'd4 : sseg_temp = 7'b0011001; //to display 4
   4'd5 : sseg_temp = 7'b0010010; //to display 5
   4'd6 : sseg_temp = 7'b0000010; //to display 6
   4'd7 : sseg_temp = 7'b1111000; //to display 7
   4'd8 : sseg_temp = 7'b0000000; //to display 8
   4'd9 : sseg_temp = 7'b0010000; //to display 9
   default : sseg_temp = 7'b0111111; //dash
  endcase
 end
assign {g, f, e, d, c, b, a} = sseg_temp; //concatenate the outputs to the register, this is just a more neat way of doing this.
// I could have done in the case statement: 4'd0 : {g, f, e, d, c, b, a} = 7'b1000000; 
// its the same thing.. write however you like it

assign dp = 1'b1; //since the decimal point is not needed, all 4 of them are turned off


endmodule