what's the difference between VHDL 93 CONCATENATION and VHDL 87 CONCATENATION?

[walala]

Dear all,

In my code I have the following line:

index1:=CONV_INTEGER('0' & count(4 downto 2));

I want to take the 4 downto 2 bits of "count", then put a "0" in front of it
and then change to INTEGER type, the reason why I prefix a "0" is to get a
unsigned integer result.

After synthesis by Synposys DC, I got the following warning:

Warning: VHDL-93 generates different concatenation results from VHDL-87
in routine myidct line 138 in file
'/home/min/a/xding/EE495d/Lab4/source/myidct.vhd'. (VHDL-2285)
Warning: VHDL-93 generates different concatenation results from VHDL-87
in routine myidct line 104 in file
'/home/min/a/xding/EE495d/Lab4/source/myidct.vhd'. (VHDL-2285)

I don't have either 93 or 87 on my hand... can anybody tell me what is the
difference? And will I get my expected results?

Thanks a lot,

-Walala

 

[David Jones]

In my code I have the following line:

index1:=CONV_INTEGER('0' & count(4 downto 2));

I want to take the 4 downto 2 bits of "count", then put a "0" in front of it
and then change to INTEGER type, the reason why I prefix a "0" is to get a
unsigned integer result.

After synthesis by Synposys DC, I got the following warning:

Warning: VHDL-93 generates different concatenation results from VHDL-87
in routine myidct line 138 in file
'/home/min/a/xding/EE495d/Lab4/source/myidct.vhd'. (VHDL-2285)
Warning: VHDL-93 generates different concatenation results from VHDL-87
in routine myidct line 104 in file
'/home/min/a/xding/EE495d/Lab4/source/myidct.vhd'. (VHDL-2285)

I don't have either 93 or 87 on my hand... can anybody tell me what is the
difference? And will I get my expected results?

The only difference is the bounds of the result, which changed from VHDL-87
to VHDL-93. I'm not sure what those bounds are in this case, and it
doesn't matter. In 99% of situations, the slice is used in a context
where it doesn't matter, and the Synopsys warning is pure noise.

You can ignore this warning; you will get expected results.

BTW, what math package are you using? If you use ieee.numeric_std like
you should, then you can say:

index1 := to_integer(unsigned(count(4 downto 2)));

Much cleaner and no warnings. Even better would be to declare "count"
as unsigned.

 

[Egbert Molenkamp]

For your concatenation qustion see
http://www.vhdl.org/vi/comp.lang.vhdl/FAQ1.html#concat

Do you need the concatenation with a '0' ?
From 'conv_...' is assume you are using the synopsys package(s):
- If count is a of type std_logic_vector and you are using the package
std_logic_UNSIGNED the conv_integer will interpreted it as an unsigned (no
zero extension is needed)

- if count is a of type unsigned and you are using the package
std_logic_arith the conv_integer will interpreted it as an unsigned (no zero
extension is needed)

- If count is a of type std_logic_vector and you are using the package
std_logic_SIGNED the conv_integer will interpreted it as a signed (the zero
extension is needed)

- if count is a of type signed and you are using the package std_logic_arith
the conv_integer will interpreted it as an signed; now you have two
solutions: a) extend with a '0', or b) type conversion
CONV_INTEGER(unsigned( count(4 downto 2)));

I prefer to use the type SIGNED or UNSIGNED for a vector has such an
interpretation in a design. In that case you can use the synopsys package
std_logic_arith (the std_logic_unsigned and std_logic_signed are not
needed). Or consider moving to the ieee package numeric_std. This package is
very similar to std_logic_arith. One change is that functions that start
with 'conv_..' are to be replaced with 'to_..'.

Egbert Molenkamp

 

[walala]

Dear Egbert,

Thanks for your answer.

Yeah, exactly, I am using the SIGNED package and "count" is of type
std_logic_vector,...

But I meant to make it unsigned, because it's not very good to
interpreate a counter to be a signed value... however, I used SINGED
package because I need to use signed multiplication somewhere else in
my circuit...

So I am kind of mixing signed and unsigned here... It is great that I
found them work through experiment... Thanks for pointing out that for
me!

-Walala