Shift right : does not compile in Modelsim VCOM

[Pasacco]

In VHDL code, following libraries and signals are defined.
-------------------
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
use IEEE.NUMERIC_STD.ALL;
.....
signal HF : std_logic_vector(15 downto 0);
signal LF : std_logic_vector(15 downto 0);
-------------------

Problem is that

Following "shift right" is not working.
--------------------
HF <= LF srl 2; -- logical shift right
--------------------

Modelsim 6.1c reports following error :
-------------------
# ** Error: HF.vhd(113): No feasible entries for infix operator "srl".
# ** Error: HF.vhd(113): Type error resolving infix expression "srl"
as type std_logic_vector.
-------------------

I tried different ways, such as "SHR, SHIFT_RIGHT" with different data
types.

If anyone had same problem, please let me know how to resolve this
problem.
thank you in advance

 

[Duane Clark]

In VHDL code, following libraries and signals are defined.
-------------------
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
use IEEE.NUMERIC_STD.ALL;

Don't mix NUMERIC_STD with STD_LOGIC_ARITH/STD_LOGIC_UNSIGNED. In fact,
you really should not be using STD_LOGIC_ARITH/STD_LOGIC_UNSIGNED at
all, ever.

....
signal HF : std_logic_vector(15 downto 0);
signal LF : std_logic_vector(15 downto 0);
-------------------

Problem is that

Following "shift right" is not working.

I don't really see a reason to bother with srl and similar operators for
that.

HF <= LF(1 downto 0) & LF(15 downto 2);

 

[Jonathan Bromley]

In VHDL code, following libraries and signals are defined.
-------------------
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
use IEEE.NUMERIC_STD.ALL;

As Duane already said, this is a total mess;
STD_LOGIC_ARITH and NUMERIC_STD both define
a bunch of things with the same names, and
the two USE clauses render *both* sets of
definitions invisible. However, that's not
the problem here.

signal HF : std_logic_vector(15 downto 0);
signal LF : std_logic_vector(15 downto 0);
...
HF <= LF srl 2; -- logical shift right

# ** Error: HF.vhd(113): No feasible entries for infix operator "srl".

Indeed so. The shift operators have no implicit (built-in)
definition, and std_logic_1164 does not provide definitions
for them.

You can sooooo easily define your own - details below - but
you can also use the shift operators defined on UNSIGNED
vectors in NUMERIC_STD (if, of course, you get rid of the
conflicting USE clause for STD_LOGIC_ARITH).

HF <= std_logic_vector( unsigned(LF) srl 2 );

Yes, I know it's ugly. So, why not make your own?

function "srl" (L: std_logic_vector; R: natural)
return std_logic_vector is
variable V: std_logic_vector(L'length-1 downto 0);
begin
if R >= L'length then
V := (others => '0');
else
V := L;
end if;
for i in V'range loop
if i < R then
V(i) := '0';
else
V(i) := V(i-R);
end if;
end loop;
return V;
end;

--
Jonathan Bromley, Consultant

DOULOS - Developing Design Know-how
VHDL * Verilog * SystemC * e * Perl * Tcl/Tk * Project Services

Doulos Ltd., 22 Market Place, Ringwood, BH24 1AW, UK
(e-mail address removed)
http://www.MYCOMPANY.com

The contents of this message may contain personal views which
are not the views of Doulos Ltd., unless specifically stated.

 

[Jonathan Bromley]

You can sooooo easily define your own [srl operator]

ahem, yes, but the code I provided does a LEFT shift
whoops, sorry. It is left as a trivial exercise for the
reader to rewrite it to do RIGHT shift as required!
(hint: make sure you iterate over the vector elements
in the correct order... and don't be frightened of
giving the internal variable a different declaration
if it makes life easier).
--
Jonathan Bromley, Consultant

DOULOS - Developing Design Know-how
VHDL * Verilog * SystemC * e * Perl * Tcl/Tk * Project Services

Doulos Ltd., 22 Market Place, Ringwood, BH24 1AW, UK
(e-mail address removed)
http://www.MYCOMPANY.com

The contents of this message may contain personal views which
are not the views of Doulos Ltd., unless specifically stated.

 

[Pasacco]

Now it compiles. Thank you for providing 3 solutions.