please help !

Altera_Forum · ‎11-10-2012

i defined a port in the entity which is : distance : out integer ;

and i also defined a signal : signal echo_width : integer :=0;

i tried to assign multiplication by this way :

distance <= echo_width*(0.02/58);

and i got an error line :

"can't determine definition of operator ""*"" -- found 0 possible definitions"

what is the problem with my assignment and how should i resolve it ?

thank you about your responds.

Altera_Forum · ‎11-10-2012

0.02 is a real, so you need to first cast the integers to reals, then back to integer:

distance <= integer( real(echo_width) * (0.02 / 58.0) );

But, remember that real types are not synthesisable, so I assume this is just testbench code.

Altera_Forum · ‎11-10-2012

first of all - thank you very much for help !

secondly - it is for synthesis , what can i do in this case ?

Altera_Forum · ‎11-10-2012

so if its for synthesis - dont use real types. You can only use reals for set up information, not runtime arithmatic. Have a look into the fixed point packages.

www.vhdl.org/fphdl

Altera_Forum · ‎11-10-2012

is it synthesisable ?

i defined an integer variable called "temp_distance" ,changed echo_width to a variable and used it like that :

temp_distance:=integer(real(echo_width) * (0.02 / 58.0));

distance<=temp_distance;

reminder :

distance : out integer ; (entity port)

echo_width : variable integer;

temp_distance: variable integer;

Altera_Forum · ‎11-10-2012

it doesnt matter - you cannot use reals.

Altera_Forum · ‎11-18-2012

Hi

your output is integer so whatever the real data you get the decimal point digits will be truncated to integer

so you have to find the nearest integer number to the real one

for instance if we have the following operation:

out = num * 0.05;

you can find that 0.05 is nearly 13/256 (0.05078)

so first multiply num by 13 and the division is simply removing the 8 digits from the right the right

note that using integer is the same of std_logic_vector(31 downto 0) which is more flexible

you can try this :

signal num:std_logic_vector(31 downto 0);

signal temp:std_logic_vector(35 downto 0);

signal out1:std_logic_vector(31 downto 0);

num <= x"0000028E"; -- test value 654

temp<=num * x"d"; -- 654 * 13 = 8502

out1 <= "00000000"&temp(31 downto 8);-- 8502 / 256 = 33

this result 33 is near to 654 * 0.05 = 32.7

this what I do to deal with float numbers

did you got it?

Altera_Forum · ‎11-18-2012

yes i did . i will try that...

thank you very much !!!