conditional on a don't care

[janeruth]

Hi,

I have a when-else conditional assignment which contains some don't care values Code is simplified as bellow;

en2 <= '1' when SIG = "010-" else
'1' when SIG ="---1" else
'0';

However, in simulation, en2 only goes high if SIG actually equals "010-" or "---1" as opposed to considering the "-" values as don't cares, the same is true for using "X".

I'm using isim from Xilinx, could anyone tell me if this is a isim thing specifically or if there's another way of coding such that the simulator treats these as real don't care states?

I can imagine that the code would synthesis fine to behave as i wish but am obviously concerned that the simulator doesn't translate it as such?

Much thanks

 

[jeppe]

Hi Janeruth

if you create a truthtable for SIG (with 16 entries) will you find that "010-" or "---1" covers all cases.

Jeppe

 

[joris]

I think you should use the std_match function, like (untested code)

en2 <= '1' when std_match(SIG, "010-") else
'1' when std_match(SIG, "---1") else
0;

 

[sridar]

Re:

Instead, you can the following boolean equation

En2 <= ( Sig(3) and sig(2) and sig(1) ) or sig(0);

 

[janeruth]

Thanks all,

Am still surprised at the simulator but turns out the std_match function works perfectly and most neatly for this.

Unfortunately, I have a similar issue elsewhere, where I am using with-select instead of when-else. I can't use the std_match in that case and will have to go with a more complicated condition