I'm still trying to get used to some of the quirks of VHDL and I'm having a bit of an issue. First off, I understand that shift operators like rol, ror, ssl, srl, etc. are not synthesizeable. The purpose of this lab is to use a golden model to check against a synthesizeable version of the same thing in a testbench.
Now, the purpose of this program is to convert thermometer code into a 3-bit binary number. So, in other words, thermometer code "00000001" = "001", "00000011" = "010", "00000111" = "011", etc. I'm basically trying to count the number of 1's in the string from right to left. There will be no case where a '0' is placed between the string of 1's, so the vector "00011101" is invalid and will never occur.
I've devised a non-synthesizeable (and so far, non-compile-able) algorithm that I can't figure out how to get working. Basically, the idea is to read the thermometer code, shift it right and increment a counter until the thermometer code equals zero, and then assign the counter value to the 3-bit std_logic_vector. Below is the code I've done so-far.
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity therm2bin_g is
port(therm : inout std_logic_vector(6 downto 0); -- thermometer code
bin : out std_logic_vector(2 downto 0); -- binary code
i : integer range 0 to 7);
end therm2bin_g;
architecture behavioral_g of therm2bin_g is
begin
golden : process(therm)
begin
while(therm /= "00000000") loop
therm <= therm srl 1;
i = i + 1;
end loop;
bin <= std_logic'(to_unsigned(i,3));
end process golden;
behavioral_g;
here's a version that is synthesisable. the while loop is replaced by a for loop. srl is implemented explicitly:
entity therm2bin_g is
port(therm : inout std_logic_vector(6 downto 0); -- thermometer code
bin : out std_logic_vector(2 downto 0); -- binary code
i : out integer range 0 to 7);
end therm2bin_g;
architecture behavioral_g of therm2bin_g is
begin
golden : process(therm)
variable i_internal: integer range 0 to 7;
begin
i_internal:=0;
for idx in 0 to therm'length loop
if therm/="0000000" then
therm<='0' & therm(therm'left downto 1);
i_internal := i_internal + 1;
end if;
end loop;
bin<=std_logic_vector(to_unsigned(i_internal,bin'length));
i<=i_internal;
end process golden;
end behavioral_g;