Synthesis Problem

[Joe Lancaster]

Hi everyone!

I've run into a problem with a design I'm working on and I hope someone
may have had a similar problem before. I have a narrow but deep
SelectRam with some data in it, that I am trying to read into a
std_logic_vector. The problem is when I try to synthesize the design
using Synplify Pro, it gives me the following error:

"Expecting constant expression"

which refers to this line of code:

q_out_buff(q_up_idx downto q_idx) <= q_dout(1 downto 0);

The above code is in a clocked process, and executes a finite number of
times to load the vector. On each clock, q_up_idx and q_idx get
incremented and q_dout is the output from the SelectRam.

Now, the synthesizer doesn't like the dynamic index into my vector, even
though functionally it is correct. Does anyone know of a more explicit
way to write this so Synplify can understand what I am trying to do?

Thanks in advance,
Joe Lancaster

 

[Ralf Hildebrandt]

q_out_buff(q_up_idx downto q_idx) <= q_dout(1 downto 0);

Maybe your synthesis tool does not like this, because there are two
_independent_ values for upper and lower bound: q_up_idx and q_idx. What
happens if you just write:

q_out_buff(q_idx+1 downto q_idx) <= q_dout(1 downto 0); --?

Furthermore: Should q_out_buff be a latch (this is the case te the
moment)? What value do you want for the bits, that are not assigned with
the new value q_dout?


Otherwise I would start with modelling every bit of q_out_buff manually:

-- all latches!
q_out_buff(0) = q_dout(0) when q_idx=0;
q_out_buff(1) = q_dout(1) when q_idx=0 else
q_dout(0) when q_idx=1; -- take care! muxed latch!
-- and so on...

I guess, you will find a way to describe it with a for-generate loop.


Ralf

 

[Tim Hubberstey]

Hi everyone!

I've run into a problem with a design I'm working on and I hope someone
may have had a similar problem before. I have a narrow but deep
SelectRam with some data in it, that I am trying to read into a
std_logic_vector. The problem is when I try to synthesize the design
using Synplify Pro, it gives me the following error:

"Expecting constant expression"

which refers to this line of code:

q_out_buff(q_up_idx downto q_idx) <= q_dout(1 downto 0);

The above code is in a clocked process, and executes a finite number of
times to load the vector. On each clock, q_up_idx and q_idx get
incremented and q_dout is the output from the SelectRam.

Now, the synthesizer doesn't like the dynamic index into my vector, even
though functionally it is correct. Does anyone know of a more explicit
way to write this so Synplify can understand what I am trying to do?

Try splitting the assignment up like this:

q_out_buff(q_up_idx) <= q_dout(1);
q_out_buff(q_idx) <= q_dout(0);

Also, you may have to make sure that your code defines a fixed (not just
finite) number of iterations.

If that doesn't work, you may need to define the implied multiplexer
explicitly and generate an explicit select signal.

 

[jtw]

I had the same issue recently with Synplify; everything fine in Modelsim,
but then ... it wouldn't synthesize. I just coded mine into a loop (I had
more bits than the two used here), and each pass of the loop assigned each
bit individually. (My implementation was more a barrel shifter.)

Synthesizer is now happy.

Jason

 

[Joe Lancaster]

Hello again!

Thanks to everyone for your help.

Good call Tim, your method worked great and I didn't have to do loops or
enumeration! I just modified

q_out_buff(q_up_idx downto q_idx) <= q_dout(1 downto 0);

to be

q_out_buff(q_idx + 1) <= q_dout(1);
q_out_buff(q_idx) <= q_dout(1);

and Synplicity is happy (and so am I).

Thanks again,
Joe