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Constant Values of 0

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laserbeak43

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Oct 27, 2009, 7:58:12 AM10/27/09
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Hello,
ISE is giving me a warning:
":Xst:1710 - FF/Latch <RDB_2> (without init value) has a constant
value of 0 in block <init>. This FF/Latch will be trimmed during the
optimization process"

When clicking the warning link the only thing that pops up in the
knowledge base is titled:
"How do I filter messages out of the XST log file?"

So, looking in my init module, I search for instances of RDB which is
a register, 4-bits wide.
it's in an always block's case statement that assigns(non-blocking)
either 4'h2 or 4'h3 to it.
and one blocking assignment outside of the always block that says:
assign dbo = RDB;
dbo is a 4-bit output.

I'm lost and don't understand this warning. I have similar ones in my
code and would like to understand why xst is addressing this, and if
it's even a problem.
Could someone please give me some info?

Thanks,
Malik

glen herrmannsfeldt

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Oct 27, 2009, 1:30:03 PM10/27/09
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laserbeak43 <laser...@gmail.com> wrote:

> ISE is giving me a warning:
> ":Xst:1710 - FF/Latch <RDB_2> (without init value) has a constant
> value of 0 in block <init>. This FF/Latch will be trimmed during the
> optimization process"

(snip)

> So, looking in my init module, I search for instances of RDB which is
> a register, 4-bits wide.
> it's in an always block's case statement that assigns(non-blocking)
> either 4'h2 or 4'h3 to it.

(snip)

If the only values are 4'h2 and 4'h3 then only one bit ever changes.
By the time of P&R everything is separate bits, so I would
expect the other three bits to be optimized out. Are there
also messages for the other two constant bits?

-- glen

laserbeak43

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Oct 27, 2009, 2:16:22 PM10/27/09
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Nope!!
So i guess that's correct then! I just wanted an answer from someone
more knowledgeable.
Thanks!!
Malik

On Oct 27, 1:30 pm, glen herrmannsfeldt <g...@ugcs.caltech.edu> wrote:

gabor

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Oct 29, 2009, 3:21:19 PM10/29/09
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Are you sure the 4-bit register isn't initialized to 0 first
and then only allowed to be 2 or 3 in the case statement? In
that case only bits that remain 0 (not 1) would never change and
therefore get optimized out. If you have a clocked always
block with no reset function, generally a bad practice, the
default initialization value for the register would be zero
for XST, although a simulator would start it up at 4'bXXXX.

Regards,
Gabor

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