I am trying to design a state machine that counts through and replaces values between 47 and 58. In my waveform though, I keep getting XXX for my R_data in the register file, and thus the rest of my top level design is thrown off. I can't seem to find a reason as to why R_data is outputting XXX for any address with R_en = 1. I'm on Vivado 2020.2; thank you for any help, and please let me know if I need to clarify anything.
Register:
`timescale 1ns / 1ps
module RegFile16x8(R_Addr, W_Addr, R_en, W_en, R_Data, W_Data, Clk, Rst);
input [3:0] R_Addr, W_Addr;
input Clk, Rst, R_en, W_en;
output reg [7:0] R_Data;
input [7:0] W_Data;
reg [7:0] RegFile [0:15];
always @(posedge Clk) begin
if (Rst == 1) begin
RegFile[0] <= 8'd48;
RegFile[1] <= 8'd53;
RegFile[2] <= 8'd68;
RegFile[3] <= 8'd57;
RegFile[4] <= 8'd55;
RegFile[5] <= 8'd59;
RegFile[6] <= 8'd40;
RegFile[7] <= 8'd49;
RegFile[8] <= 8'd31;
RegFile[9] <= 8'd38;
RegFile[10] <= 8'd54;
RegFile[11] <= 8'd50;
RegFile[12] <= 8'd63;
RegFile[13] <= 8'd58;
RegFile[14] <= 8'd70;
RegFile[15] <= 8'd51;
end
else if (W_en==1) begin
RegFile[W_Addr] <= W_Data;
end
end
always @(*) begin
if (R_en==1)
R_Data <= RegFile[R_Addr];
else
R_Data <= 8'bZZZZZZZZ;
end
endmodule
Top Level:
`timescale 1ns / 1ps
module PartA(Clk, Rst, go, done, count);
input Clk, Rst, go;
output reg [6:0] count;
output reg done;
reg [2:0] State, StateNext;
parameter s1 = 0, s2 = 1, s3 = 2, s4 = 3, s5 = 4, s6 = 5, s7 = 6, s8 = 7;
reg [4:0] i;
reg [7:0] temp;
reg R_en, W_en;
reg [7 :0] a_i;
wire [7:0] a_o;
//RegFile16x8(R_Addr, W_Addr, R_en, W_en, R_Data, W_Data, Clk, Rst);
RegFile16x8 r1(i[3:0], i[3:0], R_en, W_en, a_o, a_i, Clk, Rst);
always @(State, Rst, count) begin
R_en = 0;
W_en = 0;
case(State)
s1: begin
if(go == 1)
StateNext = s2;
else
StateNext = s1;
end
s2: begin
done = 0;
count = 0;
i = 0;
StateNext = s3;
end
s3: begin
if(i < 16)
StateNext = s4;
else
StateNext = s5;
end
s4: begin
R_en = 1;
temp = a_o;
if ((temp > 47) && (temp < 58))
StateNext = s7;
else
StateNext = s6;
end
s5: begin
done = 1;
StateNext = s1;
end
s6: begin
StateNext = s8;
end
s7: begin
W_en = 1;
count = count + 1;
a_i = temp - 48;
StateNext = s8;
end
s8: begin
i = i + 1;
StateNext = s3;
end
default: begin
StateNext = s1;
end
endcase
end
always @(posedge Clk) begin
if (Rst == 1)
State = s1;
else
State = StateNext;
end
endmodule
Test Bench:
`timescale 1ns / 1ps
module partA_tb();
reg Clk, Rst, go;
wire [6:0] count;
wire done;
PartA a1(Clk, Rst, go, done, count);
always begin
Clk = 0;
#200
Clk = 1;
#200;
end
initial begin
Rst = 1;
#200
Rst = 0;
go = 1;
end
endmodule
You need to keep Rst in the testbench high for a longer amount of time. The 1st posedge of Clk happens at time 200ns, and that is when you release the reset. You need to keep the reset asserted until after the 1st posedge of the clock in order to properly reset your RegFile, since it is a synchronous reset.
This testbench change allows RegFile to be reset to known values:
initial begin
Rst = 1;
#400
Rst = 0;
go = 1;
end
The change above removes the XXX from R_data.
I chose an arbitrary delay of 400, but it can be anything greater than 200. The key is that you need at least one posedge of Clk to sample Rst when it is high.