Verilog vs VHDL for Loops

Started by Andre Bonin September 18, 2004
Hey all, I'me trying to convert a C algorithm to Verilog using Quartus 
II Web edition.

The following for loop doesn't compile because it says its not of 
constant loop time.

What i really need is to be able to calculate the loop time "on the fly".

Can VHDL or Verilog do this? or is this a limitation?


Thanks
Error: Verilog HDL For Statement error at XXXXXXXX.v(49): must use only 
constant expressions in terminating conditions

---- Error at while loop -
	integer X = 0;
	always
	begin
		while( X < 30 )
		begin
			X = X + 1;
		end
	end
Hi Andre,
I presume because you've posted this to CAF you're writing this code for
synthesis. In that case you should really be thinking about what you want
synthesised.
In your C algorithm, a number X is stored in a memory location somewhere.
When it's incremented, the CPU or whatever fetches the value, adds one, and
writes it back.
Now, in the synthesised version, the number X is actually implemented as a
piece of hardware. In your FPGA there's a 5 bit counter, which increments
when it gets a clock. Do you see the fundamental difference? You need to
code for this hardware. For a start, looking at your verilog code, where's
your clock?
Good luck mate, Syms.

"Andre Bonin" <Yoyoma_2@[at-]Hotmail.com> wrote in message
news:5KP2d.38134$%S.33263@pd7tw2no...
> Hey all, I'me trying to convert a C algorithm to Verilog using Quartus > II Web edition. > > The following for loop doesn't compile because it says its not of > constant loop time. > > What i really need is to be able to calculate the loop time "on the fly". > > Can VHDL or Verilog do this? or is this a limitation? > > > Thanks > Error: Verilog HDL For Statement error at XXXXXXXX.v(49): must use only > constant expressions in terminating conditions > > ---- Error at while loop - > integer X = 0; > always > begin > while( X < 30 ) > begin > X = X + 1; > end > end
Andre,

I do agree with Symon, specially if you have a software background. Be
careful when trying to synthesize (HARDWARE) your software.

Anyway, below is an example of how to implement a while_loop in VHDL.
I added some comments reagarding the conditions under which the
whil_loop is implemented;

library ieee;
use ieee.std_logic_1164.all;

entity while_ex is
port( clk: in std_logic;
	z: out boolean);
end;

architecture beh of while_ex is

begin
-- while-loops are synthesizable as long as they have a
-- valid wait statement in every possible path within the loop
-- if a while-loop does not have a single wait statement, and
-- it is bound by constantes, then the tool synthesized the 
-- design correctly
process -- no sensitivity list

variable x: integer range 0 to 35;
begin
x:= 0;
	wait until clk'event and clk='1';
		z<= false;
	wh_loop: WHILE x < 30 LOOP
		wait until clk'event and clk='1';
		          x:= x+1;
	END LOOP;
	
z <= true;   

end process; 

end beh;

regards,

cristian
cristian wrote:

> Andre, > > I do agree with Symon, specially if you have a software background. Be > careful when trying to synthesize (HARDWARE) your software.
The goal of the project is ADPCM encoding and a 10x10 integer matrix multiplication, i think that could be done within the FPGA. it would be *nice* to be able to implement any C algo within hardware but i do accept their are limitations. Though for some odd reason VHDL seems more apt and less 'picky' about these kinds of loops. Or am i being deceived?
> Anyway, below is an example of how to implement a while_loop in VHDL. > I added some comments reagarding the conditions under which the > whil_loop is implemented;
Thank you for this piece of code, it helps me get started and play around with VHDL. VHDL seems more 'higher level' then verilog. For this type of task, do you reccomend VHDL or Verilog? VHDL is an older standard but my work with verilog gives me the hint that its lower level.
> > library ieee; > use ieee.std_logic_1164.all; > > entity while_ex is > port( clk: in std_logic; > z: out boolean); > end; > > architecture beh of while_ex is > > begin > -- while-loops are synthesizable as long as they have a > -- valid wait statement in every possible path within the loop > -- if a while-loop does not have a single wait statement, and > -- it is bound by constantes, then the tool synthesized the > -- design correctly > process -- no sensitivity list > > variable x: integer range 0 to 35; > begin > x:= 0; > wait until clk'event and clk='1'; > z<= false; > wh_loop: WHILE x < 30 LOOP > wait until clk'event and clk='1'; > x:= x+1; > END LOOP; > > z <= true; > > end process; > > end beh; > > regards, > > cristian
On Sun, 2004-09-19 at 18:04 +0000, Andre Bonin wrote:
> cristian wrote: > > > Andre, > > > > I do agree with Symon, specially if you have a software background. Be > > careful when trying to synthesize (HARDWARE) your software. > > The goal of the project is ADPCM encoding and a 10x10 integer matrix > multiplication, i think that could be done within the FPGA. it would be > *nice* to be able to implement any C algo within hardware but i do > accept their are limitations. Though for some odd reason VHDL seems > more apt and less 'picky' about these kinds of loops. Or am i being > deceived? > > > Anyway, below is an example of how to implement a while_loop in VHDL. > > I added some comments reagarding the conditions under which the > > whil_loop is implemented; > > Thank you for this piece of code, it helps me get started and play > around with VHDL. VHDL seems more 'higher level' then verilog. For > this type of task, do you reccomend VHDL or Verilog? VHDL is an older > standard but my work with verilog gives me the hint that its lower level.
<snip> There is always Handel-C which supports most of the C constructs (for, while, arrays, pointers, functions, etc) in FPGA hardware.
Andre Bonin wrote:
> > cristian wrote: > > > Andre, > > > > I do agree with Symon, specially if you have a software background. Be > > careful when trying to synthesize (HARDWARE) your software. > > The goal of the project is ADPCM encoding and a 10x10 integer matrix > multiplication, i think that could be done within the FPGA. it would be > *nice* to be able to implement any C algo within hardware but i do > accept their are limitations. Though for some odd reason VHDL seems > more apt and less 'picky' about these kinds of loops. Or am i being > deceived?
I think you are being deceived. I have coded in both VHDL and Verilog and have seen little difference in the two other than syntax. Both are capable of coding the same structures in similar ways.
> > Anyway, below is an example of how to implement a while_loop in VHDL. > > I added some comments reagarding the conditions under which the > > whil_loop is implemented; > > Thank you for this piece of code, it helps me get started and play > around with VHDL. VHDL seems more 'higher level' then verilog. For > this type of task, do you reccomend VHDL or Verilog? VHDL is an older > standard but my work with verilog gives me the hint that its lower level.
A piece of code is not really going to help you understand your problem or how to code in HDL. You need to consider that an HDL is just that, a Hardware Description Language and *NOT* a software programming language. I know that not everyone thinks or works like I do, but I find I fight the language much less if I design my hardware in my head or on paper and then use the HDL to describe my hardware. If you want to do a matrix operation, you really need to design the hardware to do that, then code that hardware. Just trying to write a software description of the problem will not in general give you a reasonable solution in the generated hardware. So think about the architecture you expect and then learn how to describe that hardware in your chosen HDL. Directly translating C to HDL is not a good way to go. You might even think about getting some assistance from a consultant or a training class. I found a little bit of training to go a long way with HDLs. -- Rick "rickman" Collins rick.collins@XYarius.com Ignore the reply address. To email me use the above address with the XY removed. Arius - A Signal Processing Solutions Company Specializing in DSP and FPGA design URL http://www.arius.com 4 King Ave 301-682-7772 Voice Frederick, MD 21701-3110 301-682-7666 FAX
Andre Bonin wrote:

> cristian wrote:
>> I do agree with Symon, specially if you have a software background. Be >> careful when trying to synthesize (HARDWARE) your software.
> The goal of the project is ADPCM encoding and a 10x10 integer matrix > multiplication, i think that could be done within the FPGA. it would be > *nice* to be able to implement any C algo within hardware but i do > accept their are limitations. Though for some odd reason VHDL seems > more apt and less 'picky' about these kinds of loops. Or am i being > deceived?
(snip) Well, you can code a processor in VHDL, or use an FPGA that includes one, and then you can run any C algorithm on it. 10x10 matrix multiply in combinatorial logic is a little large even for today's FPGAs. Multiplying two 10x10 matrices is 10000 multiplies. Did you want to synthesize 10000 multipliers? I believe ADPCM encoders are well understood and not hard to implement, though I don't happen to know how to do it. Are the words coming in one at a time, one per clock cycle? How much work do you need to do on each cycle? Implement the hardware to do just that. Often the implementation of an algorithm, or even the algorithm itself is completely different done in hardware than in C. -- glen
"rickman" <spamgoeshere4@yahoo.com> wrote in message
news:414E757B.4AB4DBDB@yahoo.com...

> A piece of code is not really going to help you understand your problem > or how to code in HDL. You need to consider that an HDL is just that, a > Hardware Description Language and *NOT* a software programming > language. I know that not everyone thinks or works like I do, but I > find I fight the language much less if I design my hardware in my head > or on paper and then use the HDL to describe my hardware. > > If you want to do a matrix operation, you really need to design the > hardware to do that, then code that hardware. Just trying to write a > software description of the problem will not in general give you a > reasonable solution in the generated hardware. > > So think about the architecture you expect and then learn how to > describe that hardware in your chosen HDL. Directly translating C to > HDL is not a good way to go.
I agree entirely with this, every time I start a new VHDL module the first thing I do is sketch out the hardware on a piece of paper, then write the HDL to describe it. Nial ------------------------------------------------ Nial Stewart Developments Ltd FPGA and High Speed Digital Design Cyclone Based 'Easy PCI' proto board www.nialstewartdevelopments.co.uk
This loop should compile fine in Synplify.

Andre Bonin wrote:
> Hey all, I'me trying to convert a C algorithm to Verilog using Quartus > II Web edition. > > The following for loop doesn't compile because it says its not of > constant loop time. > > What i really need is to be able to calculate the loop time "on the fly". > > Can VHDL or Verilog do this? or is this a limitation? > > > Thanks > Error: Verilog HDL For Statement error at XXXXXXXX.v(49): must use only > constant expressions in terminating conditions > > ---- Error at while loop - > integer X = 0; > always > begin > while( X < 30 ) > begin > X = X + 1; > end > end
Ken McElvain wrote:

> This loop should compile fine in Synplify.
I'me using quartus2. What is synplify?
> > Andre Bonin wrote: > >> Hey all, I'me trying to convert a C algorithm to Verilog using Quartus >> II Web edition. >> >> The following for loop doesn't compile because it says its not of >> constant loop time. >> >> What i really need is to be able to calculate the loop time "on the fly". >> >> Can VHDL or Verilog do this? or is this a limitation? >> >> >> Thanks >> Error: Verilog HDL For Statement error at XXXXXXXX.v(49): must use >> only constant expressions in terminating conditions >> >> ---- Error at while loop - >> integer X = 0; >> always >> begin >> while( X < 30 ) >> begin >> X = X + 1; >> end >> end > >