Hello,
I have a little problem, it is about real values in VHDL, and the way Quartus is managing them.
I am describing a simple DSP function (filter) in VHDL to implement on a Stratix device. As you probabely know, these functions need to shift input values in a register bank (declared as "signal" in VHDL), applying coefficients (reals) to them. These signals must therefor be of real type.
Quartus returns this error on compilation : <snip> Error (10414): VHDL error at iir_butterworth_3.vhd(49), at object "X": a real cannot be non-constant </snip> ("X" is such a register) Of course the filter coefficients (reals) are hardcoded as constants in a package, but these shift registers containing reals *are not* constants, they of course have to be signals to enable shifting.
I have read about obscure floating point units implemented on Stratix devices, so I suppose it should be possible to manage real signals (floating/fixed point representation ?) in it.
Does anyone have an idea about how I can use non-constant reals to create a DSP core on Stratix ? I think it is only a synthesis tool issue.

Here is the code (no comment on the casts, I *AM* aware this is not the most elegant)
A, B, C and D are the filter coefficients.
This description simulates PERFECTLY with Modelsim...

architecture testing of iir_butterworth_3 is

type register_bank is array (0 to 2) of real;
signal X : register_bank;
signal Y : register_bank;

begin

DSP : process(clock, reset, data_in)

begin

if reset = '1' then -- initialise registers to 0
for i in 0 to 2 loop
X(i) <= 0.0;
Y(i) <= 0.0;
end loop;

elsif clock'event AND clock = '1' then
X(0) <= real(conv_integer(data_in));
X(1 to 2) <= X(0 to 1); shift X's

Y(0) <= ( real(conv_integer(data_in)) + 3.0*X(0) + 3.0*X(1) + X(2) - B*Y(0) - C*Y(1) - D*Y(2)) / A;
Y(1 to 2) <= Y(0 to 1); shift Y's
end if;

end process DSP;

data_out <= conv_std_logic_vector(integer(Y(0)), data_width);

end architecture;


Thanks in advance for the help
Regards,


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- Erik Verhagen (div: AB-BI) -
- CERN, Geneva -
- European Organisation for Nuclear Research -
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