Delayed assign in Verilog

[shaiko]

Hello,

1. Is the following legal in Verilog?

assign #5 a = ~a;

2. Will it have the same effect as the following VHDL code?

a <= not a after 5ns ;

3. Is it possible in Verilog to explicitly set the time delay units in code? (such as with the 5ns example in VHDL).

 

[sharath666]

1. It is legal. (You wait for 5 time units, invert value of a and assign it to a)
2. This is different. you are modelling transport delay here.
3.Define the timescale accordingly..

 

[shaiko]

With VHDL, this will generate a periodic waveform:

a <= not a after 5ns ;

Why should this yield a different result?

assign #5 a = ~a;

We assign the inverted value of a to itself with a delay of 5 - and we do it all the time. It sounds like this block of code should also yield a periodic waveform with a duty cycle of 50%.

 

[sharath666]

I agree with you. The result should be the same if your intention is to generate clocks.

 

[andre_luis]

As far as I know, assign just models combinational logic, but the code above has a time-dependent command, which also could not synthesize.

 

[shaiko]

As far as I know, assign just models combinational logic, but the code above has a time-dependent command, which also could not synthesize.

Of course. My question is about simulation.

 

[ads-ee]

Uh, assign #5 a = ~a; isn't going to work in simulation. All you'll get is x as a cannot be initialized as it is defined as a wire and you can't initialize a continuous assignment.

So you won't be able to generate a clock in this fashion.

Use one of these to generate a clock in the testbench in Verilog:

---

Code Verilog - [expand]
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// define a as a reg
reg a;
 
// using forever
initial begin
  a = 0;
  forever #5 a = ~a;
end
 
// using always
initial begin
  a = 0;
end
always begin
  #5 a = ~a;
end
 
// alternative using always (starts off as x unless you add an initial block)
always begin
  #5 a = 0;
  #5 a = 1;
end

---

 

[dave_59]

Uh, assign #5 a = ~a; isn't going to work in simulation. All you'll get is x as a cannot be initialized as it is defined as a wire and you can't initialize a continuous assignment.

This will work in SystemVerilog if a is defined as a bit.

bit a;
assign #5ns a = ~a;

 

[shaiko]

This will work in SystemVerilog if a is defined as a bit.

Is bit a standard type?
Is it synthesizable with SV ?

 

[ads-ee]

This will work in SystemVerilog if a is defined as a bit.

bit a;
assign #5ns a = ~a;

Just tested this in both Modelsim and Vivado's simulator. Works fine in Modelsim, but doesn't work correctly in Vivado's simulator (a changes one time from 0 to 1 and never updates after that.

Unfortunately we only have a couple of Modelsim licenses and they get hogged by others so I have to use Vivado's simulator and unfortunately it's SV support is spotty at best and isn't even officially documented.

I used the following code to test this.

---

Code Verilog - [expand]
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`timescale 1ns/1ns
 
module assign_test;
 
  bit a;
 
  assign #5 a = ~a;
 
  initial begin
    $monitor ("%t - a = %b", $time, a);
  end
 
endmodule

---

Vivado output:

Time resolution is 1 ns
xsim% run 100  ns
                   0 - a = 0
                   5 - a = 1

modelsim output:

Time resolution is 1 ns
xsim% run 50
                   0 - a = 0
                  10 - a = 1
                  15 - a = 0
                  20 - a = 1
                  25 - a = 0
                  30 - a = 1
                  35 - a = 0
                  40 - a = 1
                  45 - a = 0
                  50 - a = 1

- - - Updated - - -

Bit is a standard SV type, and will synthesize.