Seven Segment Multiplexing on Basys2

Question

this is my first post so I hope I'm doing this correctly. I'm trying to output a "4 3 2 1" on a four digit seven segment display on a BASYS2 board. I have checked to make sure that 0 enables the signal and that I have the ports mapped correctly. I believe the error is within my multiplexing logic since I am only able to display a single digit. I'm new to Verilog (am used to C) and would appreciate any suggestions. Thanks

`timescale 1ns / 1ps

module main (clock, AN0, AN1, AN2, AN3, CA, CB, CC, CD, CE, CF, CG, CDP);


//USED FOR SEVEN SEG
input clock;

output AN0, AN1, AN2, AN3, CA, CB, CC, CD, CE, CF, CG, CDP;

reg [7:0] cathodedata; //cathode data
reg [3:0] anodedata; //anode data
reg [2:0] digit = 1;
reg [6:0] data;
reg setdp;
reg [19:0] counter = 0;

assign CA = cathodedata [7];
assign CB = cathodedata [6];
assign CC = cathodedata [5];
assign CD = cathodedata [4];
assign CE = cathodedata [3];
assign CF = cathodedata [2];
assign CG = cathodedata [1];
assign CDP = cathodedata [0];
assign AN3 = anodedata [3];
assign AN2 = anodedata [2];
assign AN1 = anodedata [1];
assign AN0 = anodedata [0];
//USED FOR SEVEN SEG

    //Multiplexing
    //Board Clock: 50MHz
    //p = t*f
    //t = 16ms
    //p = 16ms * 50*10^6 = 800,000 cycles
    //200,000 cycles for each digit
    //Refreshed every 16ms (~60Hz)

always@(negedge clock)
begin
    if (digit == 1)
        begin
            if (counter == 200_000)
                begin
                    digit = 2;
                end
            else
                begin
                counter = counter + 1;
                data = 4;
                end
        end
    else if (digit == 2)
        begin
            if (counter == 400_000)
                begin
                    digit = 3;
                end
            else
                begin
                    counter = counter + 1;
                    data = 3;
                end
        end
    else if (digit == 3)
        begin
            if (counter == 600_000)
                begin
                    digit = 4;
                end
            else
                begin
                    counter = counter + 1;
                    data = 2;
                end
        end
    else if (digit == 4)
        begin
            if (counter == 800_000)
                begin
                    digit = 1;
                    counter = 0;
                end 
            else
                begin
                    counter = counter + 1;
                    data = 1;
                end
        end 
end


always @ (*)
begin

    case (data)
        6'd0: cathodedata = 8'b00000011; //0
        6'd1: cathodedata = 8'b10011111; //1
        6'd2: cathodedata = 8'b00100101; //2
        6'd3: cathodedata = 8'b00001101; //3
        6'd4: cathodedata = 8'b10011001; //4
        6'd5: cathodedata = 8'b01001001; //5
        6'd6: cathodedata = 8'b01000001; //6
        6'd7: cathodedata = 8'b00011111; //7
        6'd8: cathodedata = 8'b00000001; //8
        6'd9: cathodedata = 8'b00001001; //9
        6'd10: cathodedata = 8'b00010001; //A
        6'd11: cathodedata = 8'b11000001; //B
        6'd12: cathodedata = 8'b01100011; //C
        6'd13: cathodedata = 8'b10000101; //D
        6'd14: cathodedata = 8'b00100001; //E
        6'd15: cathodedata = 8'b01110001; //F
        default: cathodedata = 8'b11111111; //default all off
    endcase

    if (setdp == 1) //decimal point
        cathodedata = cathodedata & 8'hFE;

    case(digit)
        0: anodedata = 4'b1111; //all OFF
        4: anodedata = 4'b1110; //AN0
        3: anodedata = 4'b1101; //AN1
        2: anodedata = 4'b1011; //AN2
        1: anodedata = 4'b0111; //AN3
        default:
        anodedata = 4'b1111; //all OFF
    endcase

end 
endmodule

Answer 1

Here's a modified version of one of my pet projects. It should do exactly as you wanted: display 4 3 2 1 on the 4dig7seg display. I tested it on an Amani GTX CPLD, and it should transfer cleanly to your board.

I like separating projects into separate modules to keep organized. The top level module takes the on board clock as an input and outputs 14 signals to a four-digit-seven-segment serial display. Make sure the pins are correctly assigned.

module SevenSeg(clk, odig, onum, col, ap); 

input clk;         // 50 MHz oscillator 
output [3:0] odig; // selected digit output
output [7:0] onum; // selected display output
output col = 1;    // turns the colon off 
output ap  = 1;    // turns the apostrophe off 

wire clock;        // divided oscillator to slow the display output 

parameter a   = 8'b11111110; // low means on, high means off  
parameter b   = 8'b11111101; // these are just parameters  
parameter c   = 8'b11111011; // defining each of the 
parameter d   = 8'b11110111; // seven segments 
parameter e   = 8'b11101111; // making life easier
parameter f   = 8'b11011111; 
parameter g   = 8'b10111111; 
parameter dp  = 8'b01111111; 

parameter off = 8'b11111111; 

parameter one    =  b & c;             // parameters for outputs
parameter two    =  a & b & d & e & g; 
parameter three  =  a & b & c & d & g; 
parameter four   =  b & c & f & g; 


    wire [7:0] port1 = one;   // This is set up so these can change dynamically...
    wire [7:0] port2 = two;   // ... if so desired.     
    wire [7:0] port3 = three;
    wire [7:0] port4 = four;


slowclk m1(clk, clock);   // divides clk by 500, for 50 MHz in, 100 kHz out 

digitize m2(clock, port1, port2, port3, port4, odig, onum); // rotates the digit outputs 

endmodule 

---

Next I have a simple clock divider. The case count = 500 can be modified to divide your clock to whatever frequency you like. The output should look fine in the range of about 120 Hz to about 2 MHz.

module slowclk(clk, clock); 

input clk; 
output reg clock; 
integer count; 

always @(posedge clk) 
case(count)      
500: begin clock <= clock + 1; count <= 0; end   // Change 500 to any divisor you like
default:  count <= count + 1;   
endcase 
endmodule   

---

Then we have to rotate the digits. Note if you change the anodes and cathodes at the "same" time, there will be a small leakage current into the neighboring segments causing a "ghost" effect.

module digitize(clock, num1, num2, num3, num4, odig, onum); 

input wire clock; 
input [7:0] num1; // rightmost digit 
input [7:0] num2; 
input [7:0] num3; 
input [7:0] num4; // leftmost digit 

output reg [3:0] odig; 
output reg [7:0] onum; 

parameter [7:0] off = 8'b11111111; 

reg [3:0] dstate; 

always @(posedge clock) 
case(dstate) 
0:begin   odig   <= 4'b0001; dstate <=  1; end 
1:begin   onum   <= num1;    dstate <=  2; end 
2:begin   onum   <= off;     dstate <=  3; end  // off states prevent 'ghosting' 
3:begin   odig   <= 4'b0010; dstate <=  4; end  // when changing output digit 
4:begin   onum   <= num2;    dstate <=  5; end 
5:begin   onum   <= off;     dstate <=  6; end 
6:begin   odig   <= 4'b0100; dstate <=  7; end 
7:begin   onum   <= num3;    dstate <=  8; end 
8:begin   onum   <= off;     dstate <=  9; end 
9:begin   odig   <= 4'b1000; dstate <= 10; end 
10:begin  onum   <= num4;    dstate <= 11; end 
11:begin  onum   <= off;     dstate <=  0; end 
default:  dstate <= 0; 
endcase 
endmodule 

Answer 2

Towards the end you have :

always @(*) begin
...
  if (setdp == 1) //decimal point
    cathodedata = cathodedata & 8'hFE;

always @* implies a combinatorial block, that is no flip-flop to break up loops etc. Your simulator may not have caught this but I would expect it to have real problems in synthesis.

Basically you have cathodedata driving itself.

I would replace this with something like:

always @(*) begin
...
  if (setdp == 1) //decimal point
    cathodedata_mask = cathodedata & 8'hFE;
  else 
    cathodedata_mask = cathodedata;

Answer 3

I too am using a basys2 and looking for a 7-seg driver. Nathan G's code did not work for me (using system verilog perhaps?), but I found this article: http://simplefpga.blogspot.co.uk/2012/12/scrolling-or-moving-text-using-7.html?showComment=1362783256904#c5783969326158067433 and I cannibalized it to my own ends.The modified code is below. It should take (although I haven't checked the decoding fully yet) four hex values and display them on the 7-seg. In my example my board now says 'FAAF' (because getting this working was faf)

ucf file taken from digilent site:

# Pin assignment for DispCtl
# Connected to Basys2 onBoard 7seg display
NET "seg<0>" LOC = "L14"; # Bank = 1, Signal name = CA
NET "seg<1>" LOC = "H12"; # Bank = 1, Signal name = CB
NET "seg<2>" LOC = "N14"; # Bank = 1, Signal name = CC
NET "seg<3>" LOC = "N11"; # Bank = 2, Signal name = CD
NET "seg<4>" LOC = "P12"; # Bank = 2, Signal name = CE
NET "seg<5>" LOC = "L13"; # Bank = 1, Signal name = CF
NET "seg<6>" LOC = "M12"; # Bank = 1, Signal name = CG

NET "dp" LOC = "N13"; # Bank = 1, Signal name = DP

NET "an<3>" LOC = "K14"; # Bank = 1, Signal name = AN3
NET "an<2>" LOC = "M13"; # Bank = 1, Signal name = AN2
NET "an<1>" LOC = "J12"; # Bank = 1, Signal name = AN1
NET "an<0>" LOC = "F12"; # Bank = 1, Signal name = AN0

Top level module has this:

module top_level(     

//clock input
input CLK,

output [7:0] seg,
output dp,
output [3:0] an

);    

scrolling_name scrolling_name(
.clock(CLK),
.reset(RESET),
.a(seg[0]),
.b(seg[1]),
.c(seg[2]),
.d(seg[3]),
.e(seg[4]),
.f(seg[5]),
.g(seg[6]),
.dp(dp),
.an(an),
//.XPosition(XPosition),
//.YPosition(YPosition)
.XPosition(8'hFA),
.YPosition(8'hAF)
);

endmodule

and the file I took from the guy in the link and have edited is:

`timescale 1ns / 1ps

module scrolling_name(
input clock,
input reset,
output a,
output b,
output c,
output d,
output e,
output f,
output g,
output dp,
output [3:0] an,
input [7:0] XPosition,
input [7:0] YPosition
);

reg [28:0] ticker; //to hold a count of 50M
wire click;
reg [3:0] fourth, third, second, first; // registers to hold the LED values

always @ (posedge clock or posedge reset) //always block for the ticker
begin
if(reset)
ticker <= 0;
else if(ticker == 50000000) //reset after 1 second
ticker <= 0;
else
ticker <= ticker + 1;
end

reg [3:0] clickcount; //register to hold the count upto 9. That is why a 4 bit     register is used. 3 bit would not have been enough.

assign click = ((ticker == 50000000)?1'b1:1'b0); //click every second

always @ (posedge click or posedge reset)
begin
 if(reset)
  clickcount <= 0;
 else if(clickcount == 8)
   clickcount <= 0;
  else
  clickcount <= clickcount + 1;

end

always@(posedge clock)
begin
fourth = XPosition[7:4]; 
third = XPosition[3:0]; 

second = YPosition[7:4]; 
first = YPosition[3:0]; 
end

//see my other post on explanation of LED multiplexing.

localparam N = 18;

reg [N-1:0]count;

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

reg [6:0]sseg;
reg [3:0]an_temp;

always @ (*)
 begin
  case(count[N-1:N-2])

   2'b00 :
    begin
     sseg = first;
     an_temp = 4'b1110;
    end

   2'b01:
    begin
     sseg = second;
     an_temp = 4'b1101;
    end

   2'b10:
    begin
     sseg = third;
     an_temp = 4'b1011;
    end

   2'b11:
    begin
     sseg = fourth;
     an_temp = 4'b0111;
    end
  endcase
 end
assign an = an_temp;

reg [6:0] sseg_temp;

always @ (*)
 begin
  case(sseg)
   0 : sseg_temp = 7'b1000000; //to display 0
   1 : sseg_temp = 7'b1001111; //to display 1
   2 : sseg_temp = 7'b0100100; //to display 2
   3 : sseg_temp = 7'b0110000; //to display 3
   4 : sseg_temp = 7'b0011001; //to display 4
   5 : sseg_temp = 7'b0010010; //to display 5
   6 : sseg_temp = 7'b0000011; //to display 6
   7 : sseg_temp = 7'b1111000; //to display 7
   8 : sseg_temp = 7'b0000000; //to display 8
   9 : sseg_temp = 7'b0011000; //to display 9
   10 : sseg_temp = 7'b0001000; //to display A
   11 : sseg_temp = 7'b0000011; //to display B
   12 : sseg_temp = 7'b1000110; //to display C
   13 : sseg_temp = 7'b0100001; //to display D
   14 : sseg_temp = 7'b0000110; //to display E
   15 : sseg_temp = 7'b0001110; //to display F

default : sseg_temp = 7'b1111111; //blank
endcase
end

assign {g, f, e, d, c, b, a} = sseg_temp;
assign dp = 1'b1;

endmodule

Please excuse the horrible formatting/indenting - there is a combination of his, mine, and adding four spaces to get stack overflow to recognize it as code. I don't have time to make things pretty unfortunately. I can bundle all this together and put it on dropbox if you would like.

Nathan