I am running this code
module neuron_xor(x0, x1, y4);
input signed [4:0] x0, x1;
reg signed [4:0] w02, w03, w12, w13, w24, w34;
reg signed [4:0] th2, th3, th4;
wire signed [4:0] y2, y3;
output signed [4:0] y4;
neuron n2(x0, x1, w02, w12, th2, y2);
neuron n3(x0, x1, w03, w13, th3, y3);
neuron n4(y2, y3, w24, w34, th4, y4);
initial begin
w02 = 2; w03 = -2;
w12 = 2; w13 = -2;
w24 = 2; w34 = 2;
th2 = 1; th3 = -3; th4 = 3;
end
endmodule
With this testbench:
`timescale 1ns/100ps
module tb_neuron_xor();
reg signed [4:0] x0, x1;
reg signed [4:0] w02, w03, w12, w13, w24, w34;
reg signed [4:0] th2, th3, th4;
wire signed [4:0] y2, y3;
wire signed [4:0] y4;
reg signed [4:0] ctrl;
integer i, j, flag;
neuron n2(x0, x1, w02, w12, th2, y2);
neuron n3(x0, x1, w03, w13, th3, y3);
neuron n4(y2, y3, w24, w34, th4, y4);
initial begin
$dumpfile("tb_neu_xor.vcd");
$dumpvars;
flag = 0;
#10;
for (i = -16; i < 15; i = i + 1) begin
for (j = -16; j < 15; j = j + 1) begin
x0 = i; x1 = j;
ctrl = x0 ^ x1;
if (ctrl != y4) begin
flag = 1;
$display("Error: x0=%b, x1=%b, y4=%b, ctrl=%b", x0,x1,y4,ctrl);
end
$display("y4=%d, ctrl=%d", y4, ctrl);
end
end
if (flag == 0) begin
$display("No Error!");
end
#10;
$finish;
end
endmodule
I get y4=X for each and every iteration. As far as I understand, this happens, because no initial value has been given to y4, and the default is X. However, I cannot change the type of y4 to reg in order to assign a value to it. That would result in an error message saying in the test bench, line 15, neuron n4:
tb_neuron_xor.v:15: error: reg y4; cannot be driven by primitives or continuous assignment. tb_neuron_xor.v:15: error: Output port expression must support continuous assignment. tb_neuron_xor.v:15: : Port y0 of neuron is connected to y4
How could I work my way around this?
In case a better context is needed, here is how neuron module is defined:
module neuron(x0, x1, w0, w1, th0, y0);
input signed [4:0] x0, x1;
input signed [4:0] w0, w1;
input signed [4:0] th0;
output reg signed [4:0] y0;
reg signed [11:0] a0, a1, a2;
always @(x0 or x1 or w0 or w1 or th0)
begin
a0 = x0*w0;
a1 = x1*w1;
a2 = a0 + a1;
if (a2 >= th0)
y0 = 1;
else
y0 = 0;
end
endmodule
