The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Modeling >> Behavioral Models >> Strange XNOR behavior in veriloga using spectre
https://designers-guide.org/forum/YaBB.pl?num=1176481506

Message started by naone on Apr 13^th, 2007, 9:25am

Title: Strange XNOR behavior in veriloga using spectre
Post by naone on Apr 13^th, 2007, 9:25am

Hello,

I am using veriloga in spectre. When I use the xnor built in function of ^~ or ~^ I get funny results.

integer a,b
a=1;
b=1;

c=a~^b;

c evaluates to -1

integer a,b;
a=0;
b=1;
c=a~^b;

c evaluates to -2

I think what is going on is all the integers are represented as a multi-bit integer and the ~^ is going the operation bit-wise. The resulting number is then a negative number in 2s complement. Is there a way to implement a and b as a binary value?

BTW c=!(a^b) works for me now but I would like to understand this a little better.

Thanks,

Naone

Title: Re: Strange XNOR behavior in veriloga using spectr
Post by Andrew Beckett on Apr 16^th, 2007, 3:53am

This seems reasonable behaviour to me.

If you only want to use an integer for single bit operation, you can always and the results with 1:

(a~^b)&1

which will give what you're expecting.

In Verilog-AMS you can use logic signals, and so define the bit length of the word you want. But in Verilog-A, you just have integers.

Regards,

Andrew.