# C to FPGA Tools (Impulse C and others) and necessary trig IP

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• Subject
• Author
• Posted on I have been presented with a c program to implement on an fpga.
I am investigating the possible processes/tools I could employ rather
than a straight rewrite in VHDL.

Current candidates are:

Impulse C
Handel-C
Xilinx System Generator

The algorithm is littered with sin cos sqrt & divides. So I expect that
I require some of the xilinx IP cores that come with my ISE tool.
(That's why XSG is getting a look in)

Q. How do I implement this algorithm's cos functions (for example) in
Impulse C so that it is represented in the resultant HW? Re: C to FPGA Tools (Impulse C and others) and necessary trig IP Hi Pete,
There's a guy here on CAF who's something to do with FpgaC.
http://fpgac.sourceforge.net/
Perhaps you could ask him to help implement the COS function, he seems to be
at a loose end! ;-)

Back in the real world, I think you're gonna have to code this in an HDL. I
suggest doing some research on CORDIC algorithms.
Alternatively, For SIN/COS you might consider the Sunderland algorithm and
the sine-phase difference algorithm.
For square roots, I like this:-
http://lib.tkk.fi/Diss/2005/isbn9512275279/article3.pdf
HTH.
Cheers, Syms. Re: C to FPGA Tools (Impulse C and others) and necessary trig IP Often, for sin and cos you can get by with a lookup table.  Depending on how
much accuracy you actually need your lookup table may not even need to be all
that large, especially since you only need from 0 to 90 degrees in the table.

Phil Re: C to FPGA Tools (Impulse C and others) and necessary trig IP Yep, the algorithms I mentioned dramatically reduce the size of the lookup
table for a given accuracy.
Cheers, Syms. Re: C to FPGA Tools (Impulse C and others) and necessary trig IP Actually it would be a fun project, and down the line of where
we are headed long term anyway. The FpgaC project has
a way to go to be fully featured, but it seems that this project
could be done as a series of user written functions with little
trouble.

As far as being a loose end, I'll take that as a complement :)
Life is very disappointing when you sit around a wait for things
to happen. Stirring things up a little is sometimes required to
shake the cobwebs off that are holding everyone stuck in place. Re: C to FPGA Tools (Impulse C and others) and necessary trig IP
I think you are taking the wrong approach.  I'm sure you can get the C code
running.  But in reality if you have to implement sin/cos & sqrts, then the
C code will be far from optimal, and you might need a vertex to do the job
of a Spartan

Simon  Re: C to FPGA Tools (Impulse C and others) and necessary trig IP Actually, I find it not that different in many cases from similar
VHDL/Verilog
and have the advantage of being able to tweek the output in FpgaC when
I don't like it. Especially when I do a boolean design, it goes right
thru FpgaC
as boolean logic equations to the LUT. The C may look a bit ugly that
way, but
you certainly have nearly exact control over the output. For example:

[jbass@fastbox tests]\$ cat example1.c
int a:1,b:1,c:1;
#pragma inputport (a,a9)
#pragma inputport (b,a10)

int sum_of_products:1;
#pragma outputport (sum_of_products,a11)

main()
{

c = (sum_of_products = (a&b) + c);
}

produces:

[jbass@fastbox tests]\$ fpgac -target cnf example1.c
[jbass@fastbox tests]\$ cat example1.cnf
_example1_Running^CLK = VCC;
_a^CLK = port(_a,"a9");
_b^CLK = port(_b,"a10");
_c^(CLK*_example1_main__state_C1) = (~_c*_b*_a)+(_c*~_a)+(_c*~_b);
port(_sum_of_products,"a11")^(CLK*_example1_main__state_C1) =
(~_c*_b*_a)+(_c*~_a)+(_c*~_b);
_example1_main__state_C1 = ~_example1_Running;

which in xnf/edif is a bit more verbose (so I'll just take the meat of
it):

SYM, _sum_of_products-OBUF, OBUF
PIN, I, I, _sum_of_products
PIN, O, O, sum_of_products
END
EXT, sum_of_products, O,, LOC=_a11
SYM, FFin-_sum_of_products, EQN, EQN=((~I0*I1*I2)+(I0*~I2)+(I0*~I1))
PIN, I2, I, _a
PIN, I1, I, _b
PIN, I0, I, _c
PIN, O, O, FFin-_sum_of_products
END
SYM, _sum_of_products, DFF
PIN, D, I, FFin-_sum_of_products
PIN, C, I, CLK
PIN, CE, I, _example1_main__state_C1
PIN, Q, O, _sum_of_products
END Re: C to FPGA Tools (Impulse C and others) and necessary trig IP
This is also true.. but that is NOT what a C programmer calls C code ;-)
If you are going to write Boolean equations in C then you might as well use
one of the "official" languages as others can help.
my two cents :-)

Simon

P.S. I didn't see any sin functions :-) code
the
job Re: C to FPGA Tools (Impulse C and others) and necessary trig IP Sorry ... but the arithmetic functions in every synthesis tool are
reduced to
the boolean equations too ... FpgaC included. My point is that C based
HLL's, or even the HLL versions of them, can do just as good a job at
both
the boolean's and the arithmetics, as you will see in VHDL doing
arithmetics.

Nope .... but didn't you get the point that better arithmetics are on
the list
of development projects for FpgaC?  And that working with someone to
implement them in a useful way would just add to the value of FpgaC?

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