'driving_value attribute

[Peter Hermansson]

Hi,

I have no luck in using the 'driving_value attribute. The following
simple code creates a "No driver in this region for 'driving or
'driving_value attribute of signal v" error message in Modelsim.

library ieee;
Use ieee.std_logic_1164.all;

Entity test Is
Port(
a : In Std_logic;
b : In Std_logic;
y : Out Std_logic;
v : Out Std_logic
);
End test;

Architecture RTL Of test Is

Begin

v <= a or b;
y <= a and v'driving_value;

End Architecture RTL;

I have activated the VHDL93 switch....
Have I misunderstood how to use this attribute?

Thanks in advance,

Peter

 

[Jonathan Bromley]

I have no luck in using the 'driving_value attribute. The following
simple code creates a "No driver in this region for 'driving or
'driving_value attribute of signal v" error message in Modelsim.
[...]
Architecture RTL Of test Is
Begin
v <= a or b;
y <= a and v'driving_value;
End Architecture RTL;

Have I misunderstood how to use this attribute?

Yes, I think so. You have created two distinct
processes: one for the assignment to v, one for
the assignment to y. 'driving_value applies only
inside a process, where it provides visibility of
the value that THE CURRENT PROCESS is driving
on to the chosen signal.

What are you trying to do? I don't really see
what meaning you ascribe to v'driving_value in
the above code.
--

Jonathan Bromley, Consultant

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

Doulos Ltd. Church Hatch, 22 Market Place, Ringwood, Hampshire, BH24 1AW, UK
Tel: +44 (0)1425 471223 mail: (e-mail address removed)
Fax: +44 (0)1425 471573 Web: http://www.doulos.com

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

 

[Mike Treseler]

Peter Hermansson wrote:
luck in using the 'driving_value attribute. The following

simple code creates a "No driver in this region for 'driving or
'driving_value attribute of signal v" error message in Modelsim.

library ieee;
Use ieee.std_logic_1164.all;
Entity test Is
Port(
a : In Std_logic;
b : In Std_logic;
y : Out Std_logic;
v : Out Std_logic
);
End test;
Architecture RTL Of test Is
Begin
v <= a or b;
y <= a and v'driving_value;
End Architecture RTL;

I have activated the VHDL93 switch....
Have I misunderstood how to use this attribute?

'driving_value is only visible inside the driving process.
So let's combine your two processes into one.

comb : process (a, b) is
begin -- process comb
v <= a or b;
y <= a and v'driving_value;
end process comb;


But consider using an explicit variable instead, like this:

library ieee;
use ieee.std_logic_1164.all;
entity driving_var is
port(
a : in std_logic;
b : in std_logic;
y : out std_logic;
v : out std_logic
);
end driving_var;

architecture demo of driving_var is
begin
comb : process (a, b) is
variable v_v : std_logic;
begin -- process comb
v_v := a or b;
v <= v_v;
y <= a and v_v;
end process comb;
end architecture demo;

This adds clarity to the logic
and always works for synthesis.

Last I tested leo, 'driving_value
compiles ok, but the synthesis is
incorrect.


-- Mike Treseler

 

[Peter Hermansson]

I have no luck in using the 'driving_value attribute. The following
simple code creates a "No driver in this region for 'driving or
'driving_value attribute of signal v" error message in Modelsim.
[...]
Architecture RTL Of test Is
Begin
v <= a or b;
y <= a and v'driving_value;
End Architecture RTL;

Have I misunderstood how to use this attribute?

Yes, I think so. You have created two distinct
processes: one for the assignment to v, one for
the assignment to y. 'driving_value applies only
inside a process, where it provides visibility of
the value that THE CURRENT PROCESS is driving
on to the chosen signal.

What are you trying to do? I don't really see
what meaning you ascribe to v'driving_value in
the above code.
--

Jonathan Bromley, Consultant

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

Doulos Ltd. Church Hatch, 22 Market Place, Ringwood, Hampshire, BH24 1AW, UK
Tel: +44 (0)1425 471223 mail: (e-mail address removed)
Fax: +44 (0)1425 471573 Web: http://www.doulos.com

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

My intention was to read the value of the signal "v" sent out from the
entity, without using a "dummy" signal.

Regards, Peter