( ESNUG 338 Item 8 ) --------------------------------------------- [12/3/99]
Subject: ( ESNUG 337 #3 ) X's, Optimism, Pessimism, Resets, & Verilog Case
> It is possible to code the RTL in a style which would intercept and
> process X-states more accurately, moderating both the pessimism and the
> optimism. For example, our case statement could be modified to intercept
> X-states and propagate their affect on the result more accurately.
>
> reg [1:0] d, e;
> ...
> begin
> case (d)
> 2'b00 : e = 2'b01;
> 2'b0X : e = 2'bX1;
> 2'b01 : e = 2'b11;
> 2'bX0 : e = 2'bXX;
> 2'bXX : e = 2'bXX;
> 2'bX1 : e = 2'bXX;
> 2'b10 : e = 2'b10;
> 2'b1X : e = 2'bX0;
> 2'b11 : e = 2'b00;
> endcase
> end
>
> This coding style quickly becomes unmanageable for all but the simplest
> examples and is prone to errors (incompleteness). In addition, we are
> very quickly loosing the designer's clear functional intent to support an
> irritating consequence of RTL X-state simulation.
>
> - Harry Foster
> Hewlett-Packard Computer Technology Lab Richardson, TX
From: Lauren Carlson <carlson@starbridgetech.com>
Hi, John,
Here are two coding examples that attempt to overcome Verilog optimism with
respect to propagation of X's through case statements in simulation. We
were burned by not finding a reset problem until late in the schedule when
our gate level simulation propagated the X correctly.
These case statements are easier to read and use than Harry's examples in
.ESNUG 337 #3.
reg [1:0] d, e;
...
begin
case (d)
2'b00: e = 2'b01;
2'b01: e = 2'b11;
2'b10: e = 2'b10;
default:
begin
// synopsys translate_off
if ((|d[1:0]) === 1'bx))
e = 2'bxx;
else
// synopsys translate_on
e = 2'b00;
end
endcase
For a fully specified case (no default statement) one could go as far as
coding it specifically to propagate the X's to find reset problems.
reg [1:0] d, e;
...
begin
case (d)
2'b00: e = 2'b01;
2'b01: e = 2'b11;
2'b10: e = 2'b10;
2'b11: e = 2'b00;
// synopsys translate_off=09
default: e = 2'bxx;
// synopsys translate_on
endcase
Our design team is currently using this methodology to find more
intialization problems earlier.
- Lauren Carlson
StarBridge Technologies, Inc. Marlboro, MA
|
|
