Loop inside case?

T

Travis

Guest
Pardon me if this is a trivial question but my VHDL experience is
limited and rusty (sorry, I'm verilog). I'm modifying a pre-existing
unit that manages access to a giant register bank. This is handled
with a huge case statement and I need to modify the case statement to
support a new subset of addresses. These addresses correspond to a big
contiguous block of data and so I was hoping I could use a loop to
simplify the coding. Essentially what I'm looking for is the ability
to do the equivalent of the following:

case addr is
...
for i in 0 to count-1 loop
when (new_base_addr + i) =>
dataout <= new_reg(i);
end loop
...
end case

I've tried a standard for loop and a for-generate loop but neither will
synthesize (using the Xilinx Synthesis Tool). If anybody can show me
how to do this without manually entering every individual case that
would be awesome. Thanks.
- Travis
 
Travis wrote:

case addr is
...
for i in 0 to count-1 loop
when (new_base_addr + i) =
dataout <= new_reg(i);
end loop
...
end case
Click to expand...
You cannot interleave a case statement and a loop statement as you try
to do. Also, the choices of a case statement must be static, so you
cannot use a variable or signal in a choice expression (so you cannot
use new_base_addr, unless it is a constant).

Why not use an IF statement, possibly IF .. ELSIF .. ELSIF ..

   for i in 0 to count-1 loop
      if addr = new_base_addr + i then
         dataout <= new_reg(i);
end if;
   end loop;

--
Paul.
 
Paul Uiterlinden wrote:
Travis wrote:

case addr is
...
for i in 0 to count-1 loop
when (new_base_addr + i) =
dataout <= new_reg(i);
end loop
...
end case

You cannot interleave a case statement and a loop statement as you try
to do. Also, the choices of a case statement must be static, so you
cannot use a variable or signal in a choice expression (so you cannot
use new_base_addr, unless it is a constant).

Why not use an IF statement, possibly IF .. ELSIF .. ELSIF ..

for i in 0 to count-1 loop
if addr = new_base_addr + i then
dataout <= new_reg(i);
end if;
end loop;

--
Paul.
Click to expand...
So there's no way to use a loop to generate the choices, even static
choices, in a case statement? That's unfortunate. As mentioned, I'm
modifying a pre-existing unit and simply updated the range of choices
in the case statement, so switching to an IF statement isn't really an
option without major refactoring. Is my only other option then to hand
code each choice (when new_base_addr + 1 ... when new_base_addr + 2 ...
etc..)? Thanks.
- Travis
 
Travis wrote:
Paul Uiterlinden wrote:
Travis wrote:

case addr is
...
for i in 0 to count-1 loop
when (new_base_addr + i) =
dataout <= new_reg(i);
end loop
...
end case
You cannot interleave a case statement and a loop statement as you try
to do. Also, the choices of a case statement must be static, so you
cannot use a variable or signal in a choice expression (so you cannot
use new_base_addr, unless it is a constant).

Why not use an IF statement, possibly IF .. ELSIF .. ELSIF ..

for i in 0 to count-1 loop
if addr = new_base_addr + i then
dataout <= new_reg(i);
end if;
end loop;

--
Paul.

So there's no way to use a loop to generate the choices, even static
choices, in a case statement? That's unfortunate. As mentioned, I'm
modifying a pre-existing unit and simply updated the range of choices
in the case statement, so switching to an IF statement isn't really an
option without major refactoring. Is my only other option then to hand
code each choice (when new_base_addr + 1 ... when new_base_addr + 2 ...
etc..)? Thanks.
- Travis
Click to expand...
Hi,

You can do

when 32 to 63 =>
dataout <= new_reg( addr );

or something similar. It will depend exactly how addr is specified as
this only works with specific types (ie ints not std_logic_vectors).

You may yet have some refactoring to do...

Cheers,

Andy
 
Travis wrote:


So there's no way to use a loop to generate the choices, even static
choices, in a case statement? That's unfortunate. As mentioned,
I'm modifying a pre-existing unit and simply updated the range of
choices in the case statement, so switching to an IF statement isn't
really an
option without major refactoring. Is my only other option then to
hand code each choice (when new_base_addr + 1 ... when new_base_addr
+ 2 ...
etc..)? Thanks.
Click to expand...
I'm not sure if this would work (no compiler at hand at the moment):

for i in 0 to count-1 loop
case addr is
when new_base_addr + i =>
dataout <= new_reg(i);
when ...
end case;
end loop;

After all, i is a constant. But VHDL is a bit peculiar with regards to
choices in the case statement. See e.g.
http://www.vhdl.org/comp.lang.vhdl/FAQ1.html#static.

--
Paul.
 
"Travis" <travis@spencertechnologies.com> wrote in message
news:1158178421.183984.61050@m73g2000cwd.googlegroups.com...
Pardon me if this is a trivial question but my VHDL experience is
limited and rusty (sorry, I'm verilog). I'm modifying a pre-existing
unit that manages access to a giant register bank. This is handled
with a huge case statement and I need to modify the case statement to
support a new subset of addresses. These addresses correspond to a big
contiguous block of data and so I was hoping I could use a loop to
simplify the coding. Essentially what I'm looking for is the ability
to do the equivalent of the following:

case addr is
...
for i in 0 to count-1 loop
when (new_base_addr + i) =
dataout <= new_reg(i);
end loop
...
end case

I've tried a standard for loop and a for-generate loop but neither will
synthesize (using the Xilinx Synthesis Tool). If anybody can show me
how to do this without manually entering every individual case that
would be awesome. Thanks.
- Travis
How about something like this
Click to expand...

if (Address is in the new range) then
Do your new stuff here
else
case -- The existing case statement
end case;
end if;

KJ
 
KJ wrote:
"Travis" <travis@spencertechnologies.com> wrote in message
news:1158178421.183984.61050@m73g2000cwd.googlegroups.com...
Pardon me if this is a trivial question but my VHDL experience is
limited and rusty (sorry, I'm verilog). I'm modifying a pre-existing
unit that manages access to a giant register bank. This is handled
with a huge case statement and I need to modify the case statement to
support a new subset of addresses. These addresses correspond to a big
contiguous block of data and so I was hoping I could use a loop to
simplify the coding. Essentially what I'm looking for is the ability
to do the equivalent of the following:

case addr is
...
for i in 0 to count-1 loop
when (new_base_addr + i) =
dataout <= new_reg(i);
end loop
...
end case

I've tried a standard for loop and a for-generate loop but neither will
synthesize (using the Xilinx Synthesis Tool). If anybody can show me
how to do this without manually entering every individual case that
would be awesome. Thanks.
- Travis
How about something like this

if (Address is in the new range) then
Do your new stuff here
else
case -- The existing case statement
end case;
end if;

KJ
Click to expand...
Thanks to all for your suggestions. I think KJ's suggestion is
probably the only method that wouldn't require major rework but I had
already gone the route of putting together a perl script to generate
the code for me. I'm pretty happy with the result so I thought I'd
share it incase somebody else might find it useful. Thanks again.

------------------------------------------------

#!/usr/local/bin/perl -w
# Script:
# textgen.pl
# Description:
# Generates repeating strings with auto-increment integer or hex
values.
# Since double-quotes cannot be inside the input string, use '\q' to
insert a
# placeholder for a double-quote (").
# Arguments:
# 1 - The number of repetitions
# 2 - String with %#d (integer), %#x (hex) or %#b (binary)
placeholders.
# This uses the C-style printf formatting native to Perl.
# 3+ - The third and all remaining arguments are indicators of how
to
# update the values in the string. They take the format of
XąY, where
# X is the initial value, ą is increment or decrement and Y
is the
# amount to increment or decrement.
# Sample:
# textgen.pl 3 "#%d, 0x%02x\n, \q%05b\q" 10-3 15+16 1+4
# > Output:
# #10, 0x0F, "00001"
# #7, 0x1F, "00101"
# #4, 0x2F, "01001"
###############################################################################

use strict;

if($#ARGV < 1) {
die "Too few arguments";
}

my $i;
my $reps = $ARGV[0];
my $text = $ARGV[1];

# Convert input text linebreaks to real linebreaks
$text =~ s/\\n/\n/g;
# Convert \q to double-quote
$text =~ s/\\q/\\\"/g;

# Extract the value/increment pairs into arrays
my @values = splice(@ARGV, 2, $#ARGV);
my @inc = @values;

# Convert value/increment pairs to just values
for($i = 0; $i <= $#values; $i++) {
$values[$i] =~ /(\d+)[\+|-]\d+/;
$values[$i] = $1;
}
# Convert value/increment pairs to just increments
for($i = 0; $i <= $#inc; $i++) {
$inc[$i] =~ /\d+([\+|-]\d+)/;
$inc[$i] = $1;
# Remove '+' signs
$inc[$i] =~ s/\+//g;
}
# Extract the placeholders
my @places = ($text =~ /(%\d*[d|x|b])/g);

# Verify placeholders and arguments compatibility
if($#places != $#values) {
die "Invalid argument list";
}

# Print output
for(; $reps > 0; $reps--) {
# Create the value list
my $val;
my $valuelist = '';
foreach $val (@values) {
$valuelist .= ", $val";
}

# Create printf function from input string and current values
my $evalstr = "printf(\"$text\"$valuelist)";
# Evaluate (print out) printf string
eval $evalstr;

# Update the values
for($i = 0; $i <= $#values; $i++) {
$values[$i] += $inc[$i];
}
}
 
Since loops are unrolled in synthesis (and therefore must be staticly
bound), the index is "treated as static" in synthesis. This treatment
only exists in the backend, and has no effect on legal vhdl staticness.

Case statement targets must be locally static, meaning that at analysis
time, before elaboration, the compiler must be able to know the value
of each target. Among other things, even generic values and deferred
constants (i.e. whose values are only defined in a package body) cannot
be used as case statement targets, since the compiler must be able to
verify, at analysis time, that all possible choices are covered exactly
once.

Andy


Paul Uiterlinden wrote:
Travis wrote:


So there's no way to use a loop to generate the choices, even static
choices, in a case statement? That's unfortunate. As mentioned,
I'm modifying a pre-existing unit and simply updated the range of
choices in the case statement, so switching to an IF statement isn't
really an
option without major refactoring. Is my only other option then to
hand code each choice (when new_base_addr + 1 ... when new_base_addr
+ 2 ...
etc..)? Thanks.

I'm not sure if this would work (no compiler at hand at the moment):

for i in 0 to count-1 loop
case addr is
when new_base_addr + i =
dataout <= new_reg(i);
when ...
end case;
end loop;

After all, i is a constant. But VHDL is a bit peculiar with regards to
choices in the case statement. See e.g.
http://www.vhdl.org/comp.lang.vhdl/FAQ1.html#static.

--
Paul.
Click to expand...
 
Andy wrote:

Since loops are unrolled in synthesis (and therefore must be
staticly bound), the index is "treated as static" in synthesis. This
treatment only exists in the backend, and has no effect on legal
vhdl staticness.

Case statement targets must be locally static, meaning that at
analysis time, before elaboration, the compiler must be able to know
the value of each target. Among other things, even generic values
and deferred constants (i.e. whose values are only defined in a
package body) cannot be used as case statement targets, since the
compiler must be able to verify, at analysis time, that all possible
choices are covered exactly once.
Click to expand...
Thanks for the clarification. Of course I knew all this, but it had
sunken too deep for direct access. ;-)

--
Paul.
 
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