'x' state on one bit of the input bus of an adder cause the

[Haiwen]

I have an adder:
module adder(
input [1:0] add1,
input [1:0] add2,
output [2:0] addout
);
assign addout = add1 + add2;
endmodule

The input is:
add1 = 2'bx0;
add2 = 2'b00;

The RTL simulation result is:
addout = 3'bxxx;

The Timing simulation result is:
addout = 3'b0x0;
and this is what I expected.

How can I solve the mismatch in RTL simulation?

 

[Chris Maryan]

The "timing" simulation is probably on a post synthesis or post fit netlist, where the "+" has been converted to a proper adder in whatever logic is appropriate for your application. Describe a complete adder out of plain gates and you will get what you want. (i.e. describe a ripple-carry adder or some other architecture)

Moreover, why do you care about the details of the output of an adder with an X input?

Chris

 

[Snowy]

It takes some bit manipulation to handle unknowns exactly. That takes
programmer time and CPU time (I've done it). Most likely they traded off
exactness for speed. Usually, you don’t care about the result unless it is
totally defined. As someone else suggested, you might need to write your
own model if you want exactness.

Gary

"Haiwen" wrote in message
news:3d0b033f-3994-43d5-a1e6-0fb463b0a86f@n8g2000pbc.googlegroups.com...

I have an adder:
module adder(
input [1:0] add1,
input [1:0] add2,
output [2:0] addout
);
assign addout = add1 + add2;
endmodule

The input is:
add1 = 2'bx0;
add2 = 2'b00;

The RTL simulation result is:
addout = 3'bxxx;

The Timing simulation result is:
addout = 3'b0x0;
and this is what I expected.

How can I solve the mismatch in RTL simulation?

 

[Andy]

On Feb 6, 8:42 am, Chris Maryan <kmar...@gmail.com> wrote:

Moreover, why do you care about the details of the output of an adder with an X input?

Chris

Because the actual behavior of the HW is implementation dependent, the
RTL operator does not attempt to define its behavior for inputs
containing meta-values, beyond the whole result being 'unknown'.

To compare the results, I would take the RTL outputs, run them through
a 'X' to '-' ('unknown' to 'dont care') conversion (probably need to
write your own function), and then use std_match() to compare the RTL
vs gate level outputs.

This in effect says that for verification, if the output of the RTL is
not known, then the output of the gate level sim is of no consequence.

Andy