Verilog multiple drivers

Question

I'm trying to make BCD Counter using Verilog that will be connected to 7-segment decoder.
After I synthesize it, the error occured like this:

Multi-source in Unit <BCDcountmod> on signal <BCD0<3>>; this signal is connected to multiple drivers.>**And more.....

***Any solution?* (Here's my code below)

module BCDcountmod(
  input Clock, Clear, up, down,
  output [3:0] BCD1_1, BCD0_0 );
reg [3:0] BCD1, BCD0;
//reg [3:0] BCD1_1, BCD0_0;
always @(posedge Clock) begin
  if (Clear) begin
    BCD1 <= 0;
    BCD0 <= 0;
    end
end


 always @(posedge up) begin
      if (BCD0 == 4'b1001) begin
        BCD0 <= 0;
        if (BCD1 == 4'b1001)
          BCD1 <= 0;
        else
          BCD1 <= BCD1 + 1;
      end
      else
        BCD0 <= BCD0 + 1;
    end


always @(posedge down) begin
      if (BCD0 == 4'b0000) begin
        BCD0 <= 4'b1001;
        if (BCD1 == 4'b1001)
          BCD1 <= 4'b1001;
        else
          BCD1 <= BCD1 - 1;
      end
      else
        BCD0 <= BCD0 - 1;
    end

 assign BCD1_1 = BCD1;
 assign BCD0_0 = BCD0;

endmodule

Answer 1

You cannot modify BCD from different always blocks. Any modification should be perfomed in only one always. Something like:

module BCDcountmod(
  input Clock, Clear, up, down,
  output [3:0] BCD1_1, BCD0_0 );
  reg [3:0] BCD1, BCD0;
//reg [3:0] BCD1_1, BCD0_0;

  assign BCD1_1 = BCD1;
  assign BCD0_0 = BCD0;  

  always @(posedge Clock) begin
    //---- IS IT CLEAR? --------------
    if (Clear) begin
      BCD1 <= 0;
      BCD0 <= 0;
    end
    //---- IS IT UP? --------------
    else if (up) then begin
      if (BCD0 == 4'b1001) begin
        BCD0 <= 0;
        if (BCD1 == 4'b1001)
          BCD1 <= 0;
        else
          BCD1 <= BCD1 + 1;
      end
    end
    //---- IS IT DOWN? --------------
    else if (down) begin
      if (BCD0 == 4'b0000) begin
        BCD0 <= 4'b1001;
        if (BCD1 == 4'b1001)
          BCD1 <= 4'b1001;
        else
          BCD1 <= BCD1 - 1;
      end
      else
        BCD0 <= BCD0 - 1;
    end
  end
endmodule