How to understand this code in a package definition

[fl]

Hi,
How to understand the part:
----------------
constant simulating : boolean := false
-- synopsys translate_off
or true
-- synopsys translate_on
;

---------------
This is part of the code generated by System Generator 10.1i. It can
compile through by Modelsim. I am curious about "or true" line. It is
like broken code, but it works. Could you explain its meaning to me?
Thanks in advance.








library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;
package conv_pkg is
constant simulating : boolean := false
-- synopsys translate_off
or true
-- synopsys translate_on
;
constant xlUnsigned : integer := 1;
constant xlSigned : integer := 2;
....
....
end conv_pkg;

 

[Mike Treseler]

fl wrote:

This is part of the code generated by System Generator 10.1i. It can
compile through by Modelsim. I am curious about "or true" line. It is
like broken code, but it works. Could you explain its meaning to me?
Thanks in advance.

-- modelsim sees:
constant simulating : boolean := false or true; -- = true

-- quartus/ise sees:
constant simulating : boolean := false; -- = false

 

[Andy Peters]

Mike Treseler wrote:

fl wrote:


This is part of the code generated by System Generator 10.1i. It can
compile through by Modelsim. I am curious about "or true" line. It is
like broken code, but it works. Could you explain its meaning to me?
Thanks in advance.

-- modelsim sees:
constant simulating : boolean := false or true; -- = true

-- quartus/ise sees:
constant simulating : boolean := false; -- = false

Seems to me that ModelSim does the right thing. The expression on the
RHS is evaluated (and we should know that 1 or 0 is 1) at analysis
time and assigned to the constant.

ISE and Quartus appear to be wrong.

-a

 

[Brian Drummond]

On Thu, 31 Jul 2008 14:45:27 -0700 (PDT), Andy Peters <google@latke.net>
wrote:

Mike Treseler wrote:
fl wrote:


This is part of the code generated by System Generator 10.1i. It can
compile through by Modelsim. I am curious about "or true" line. It is
like broken code, but it works. Could you explain its meaning to me?
Thanks in advance.

-- modelsim sees:
constant simulating : boolean := false or true; -- = true

-- quartus/ise sees:
constant simulating : boolean := false; -- = false

Seems to me that ModelSim does the right thing. The expression on the
RHS is evaluated (and we should know that 1 or 0 is 1) at analysis
time and assigned to the constant.

ISE and Quartus appear to be wrong.

They are all three right, because the synthesis tools must obey the
"synthesis translate_on/off" pragmas.

This constant can be used to generate different behaviour in synthesis
and simulation.

Which is not usually a good idea; though there are occasionally good and
safe uses for it. (For example, to greatly speed up a serial interface
to reduce wasted simulation time, or to bypass a very long
initialisation delay for some external component such as a PLL, or the
DLL in DDR memory)

- Brian

 

[Amal]

One can use the following pragma to remove code that is only used for
simulation purposes and not translated to logic during synthesis:

-- pragma translate_off
...
-- pragma translate_on

I think you need to make sure the the pragma is placed correctly
around the "or TRUE" as follows:

constant SIMULATING : boolean := FALSE
-- pragma translate_off
or TRUE
-- pragma translate_on
;

During simulation the pargma is not seen and the constant becomes
TRUE. But synthesis would ignore "or TRUE" and the constant becomes
FALSE.

Basically, I think the purpose of this constant is to EXCLUDE code for
simulation, like the way you exclude code for synthesis using the
translate_off/translate_on pragmas. Example could be, debugging logic
that you do not want to be there in your simulation, but you want them
for synthesis. FPGA examples could be the XILINX ChipScope and ALTERA
SignalTap. I do not want or care about these to be in my simulation,
but I need them for debugging in the lab.

g_SYNTHESIS_ONLY: if ( not SIMULATING ) generate
-- Put debugging code that does not show up in simulation
i_chipscope_icon icon port map ( ... );
i_chipscope_ila ila port map ( ... );
end generate g_SYNTHESIS_ONLY;

Cheers,
-- Amal