Unable to Implement Simple ALU

Question

I have a basic 8-bit ALU described in Verilog. I am trying to implement the design, but I am getting error messages:

ERROR:NgdBuild:809 - output pad net 'quotient<1>' has an illegal load: pin I3 on block Mmux_opcode[2]_GND_1_o_wide_mux_8_OUT81 with type LUT6

The design does the following operation, addition, subtraction, multiplication, division, AND, OR, XOR, and XNOR. The interesting thing about it is the fact that Xilinx XST cannot synthesis a divider unless the dividend is being divided by a factor of 2 (basically shifting right). So to take care of this I used a CORE IP component generated by Xilinx Core Generator. It takes in a single clock (no clock enable or synchronous clear, and outputs the correct quotient and remainder after about 20 or so clock cycles. The core itself can be found under math functions in the Core Generator program. Anyway, here's my code:

`timescale 1ns / 1ps
module ALU8(A,B,opcode,clk,OUT);

// I/O
// We have two 16-bit inputs
input [7:0] A,B;
// The opcode determines our next operation
input [2:0] opcode;
// The processor clock
input clk;
// A 32-bit output
output [15:0] OUT;

// The inputs are wires
wire [7:0] A,B;
wire [2:0] opcode;

// The output is a register
reg [15:0] OUT;

// The quotient and remainder for tyhe divider
wire [7:0] quotient,remainder;
// Declare an internal dividing unit
Divider8 divider(.rfd(), .clk(clk), .dividend(A), .quotient(quotient), .divisor(B), .fractional(remainder));

// Define operation codes, there's only 9 so far
parameter   ADD = 3'b000;
parameter   SUB = 3'b001;
parameter   MUL = 3'b010;
parameter   DIV = 3'b011;
parameter   AND = 3'b100;
parameter   OR  = 3'b101;
parameter   XOR = 3'b110;
parameter   XNOR    = 3'b111;

// On the rising-edge of the clock
always @(posedge clk)
begin
    // The output is determined by the operation
    // Think of it as a MUX
    // A MUX8 will be added in later
    case(opcode)
        ADD: OUT <= A + B;
        SUB: OUT <= A - B;
        MUL: OUT <= A * B;
        DIV: OUT <= {quotient,remainder};
        AND: OUT <= A & B;
        OR:  OUT <= A | B;
        XOR: OUT <= A ^ B;
        XNOR: OUT <= A ~^ B;
        default: OUT <= 16'b0000000000000000;
    endcase
end
endmodule

Obviously my code terrible and my comments are probably erroneous, but I am just a beginner with Verilog. However, I do plan on improving this code greatly and adding more operations for me to practice. The module itself does successfully synthesis and simulate correctly, but I am unable to implement it on any FPGA. Anyone know if there is a problem with the code, or Xilinx ISE (which is full of bugs as usual), or maybe the project settings?

EDIT: I've made a few changes to the code, to reflect the advice provided by the answers.

Answer 1

Do you have the "Create netlist wrapper with IO pads" option selected in your coregen project? From the error it sounds like the Divider8 core has an OBUF or similar output buffer driving the quotient output. OBUF can only drive a signal that is leaving the FPGA.

Another note, although this is not related to the ngdbuild error: normally non-blocking assignments are used in sequential blocks ("always @(posedge clk)"), e.g. "OUT <= A + B". The short explanation of this is that it defers updating the OUT signal until all other events for the current time have been processed, which avoids race conditions in the simulator.

Answer 2

This is not an answer, but I think these tips might improve your code.

Since I do not have your Divider8 module, I can't compile your code, unless I comment out the divider instance. It seems like the error message is related to that instance.

For many synthesis tools initial blocks are not synthesizable. Since I do not use xilinx, I can not comment on its support. Perhaps you could just delete the initial block.

It is better to use non-blocking assignments (<=) for synthesizing sequential logic. For example:

case(opcode)
    ADD: OUT <= A + B;
    SUB: OUT <= A - B;

You could re-code your constant as:

    default: OUT = {16{1'b0}};