Using case statement and if-else at the same time? - verilog

Question

I am trying to write Verilog HDL behavioral description of the machine specified in the state diagram below.
I am using if-else statements inside a case statement, and this gives me syntax errors regarding those lines. Do you see what the problem is?
My code is attached below:
module foo(y_out, state, x_in, clk, reset);
input x_in, clk, reset;
output reg y_out;
parameter s0 = 3'b000, s1 = 3'b001, s2 = 3'b010, s3 = 3'b011, s4 = 3'b100;
output reg[2:0] state;
reg[2:0] next_state;
always #(posedge clk) begin
if(reset == 1'b0) state <= s0;
else state <= next_state;
end
always #(state, x_in) begin
y_out = 0;
next_state = s0;
case(state, x_in)
s0:
if (!x_in) begin
next_state = s3;
y_out = 1'b0;
end
else begin
next_state = s4;
y_out =1'b1;
end
s1:
if (!x_in) begin
next_state = s1;
y_out = 1'b0;
end
else begin
next_state = s4;
y_out =1'b1;
end
s2: if (!x_in) begin
next_state = s2;
y_out = 1'b0;
end
else begin
next_state = s0;
y_out =1'b1;
end
s3: if (!x_in) begin
next_state = s1;
y_out = 1'b0;
end
else begin
next_state = s2;
y_out =1'b1;
end
s4: if (!x_in) begin
next_state = s2;
y_out = 1'b0;
end
else begin
next_state = s3;
y_out =1'b0;
end
default begin
next_state = s0;
y_out = 1'b0;
end
endcase
end
endmodule
module t_foo;
wire t_y_out, t_state;
reg t_x_in, t_clock, t_reset;
foo M1(t_y_out, t_state, t_x_in, t_clock, t_reset);
initial #200 $finish;
initial begin
t_reset = 0;
t_clock = 0;
#5 t_reset = 1;
repeat (16)
#5 t_clock = ~t_clock;
end
initial begin
t_x_in = 0;
#15 t_x_in = 1;
repeat (8)
#10 t_x_in = ~t_x_in;
end
initial begin
$monitor("ABC: %d, x_in: %d, Clock: %d, Reset: %d", state, t_x_in, t_clock, t_reset);
$dumpfile("5_41_wv.vcd");
$dumpvars;
end
endmodule

Answer 1

case statements expect a single item if this is to be based on multiple wire/regs then they need to be concatenated using {}.
I would avoid using things like always #(state, x_in) begin and just write always #* begin. The #* will take care of the sensitivity list.
Using the concatenation operator would allow you to remove the if statements:
always #* begin
y_out = 0;
next_state = s0;
case({state, x_in}) //Added {}
{s0, 1'b0}:
begin
next_state = s3;
y_out = 1'b0;
end
{s0, 1'b1}:
begin
next_state = s4;
y_out = 1'b1;
end
{s1, 1'b0}:
begin
next_state = s1;
y_out = 1'b0;
end
{s1, 1'b1}:
begin
next_state = s4;
y_out = 1'b1;
end
Using a casez would allow you to add do not cares to the next_state logic:
always #* begin
y_out = 0;
next_state = s0;
casez({state, x_in}) //Added {}
{s0, 1'bx}: //Do not care about the state of x_in
begin
next_state = s3;
y_out = 1'b0;
end
{s1, 1'b0}:
begin
next_state = s1;
y_out = 1'b0;
end
{s1, 1'b1}:
begin
next_state = s4;
y_out = 1'b1;
end

Answer 2

Change:
case(state, x_in)
to:
case(state)
That fixes a compile error for me. The case items in your code only depend on your state parameters, not x_in.
I also get a compile error in your testbench module. To fix it, change:
$monitor("ABC: %d, x_in: %d, Clock: %d, Reset: %d", state, t_x_in, t_clock, t_reset);
to:
$monitor("ABC: %d, x_in: %d, Clock: %d, Reset: %d", t_state, t_x_in, t_clock, t_reset);
And fix a warning by changing:
wire t_y_out, t_state;
to:
wire t_y_out;
wire [2:0] t_state;

Answer 3

Using state in case expression and xin in if condition is working fine. Please find below working code.
module fsm_state(
input clk,
input rst_n,
input xin,
output reg yout
);
reg [2:0] state;
reg [2:0] next_state;
parameter s0 = 3'b000,
s1 = 3'b001,
s2 = 3'b010,
s3 = 3'b011,
s4 = 3'b100;
always # (posedge clk) begin
if (!rst_n)
state = s0;
else
state = next_state;
end
always #*
begin
yout = 1'b0;
case (state)
s0: begin
if (xin) begin
yout = 1'b1;
next_state = s4;
end
else
next_state = s3;
end
s1: begin
if (xin) begin
yout = 1'b1;
next_state = s4;
end
else
next_state = s1;
end
s2: begin
if (xin) begin
yout = 1'b1;
next_state = s0;
end
else
next_state = s2;
end
s3: begin
if (xin) begin
yout = 1'b1;
next_state = s2;
end
else
next_state = s1;
end
s4: begin
if (xin)
next_state = s3;
else
next_state = s2;
end
endcase
end
endmodule
Thanks