XST: Problem with simple logical operators

[grubby23]

0 down vote favorite


Hi,

I have a very simple operator problem in VHDL. I try to compare some inputs with logical operators but get an error message...

entity test is
port (
paddr : in std_logic_vector(15 downto 0);
psel : in std_logic;
penable : in std_logic;
pwrite : in std_logic
);
end entity test;

signal wrfifo_full : std_logic;

process (paddr, psel, penable, pwrite, wrfifo_full) is
begin
if (((paddr(8 downto 2) = "1000000")) and (psel and penable) and (pwrite and not(wrfifo_full))) then
dt_fifo_wr_i <= '1';
else
dt_fifo_wr_i <= '0';
end if;

end process;

Unfortuantely, I get then the following error message:

if (((paddr(8 downto 2) = "1000000")) and (psel and penable) and (pwrite and not(wrfifo_full))) then | ncvhdl_p: *E,OPTYMM (hdl/vhdl/test.vhd,523|43): operator argument type mismatch 87[4.3.3.2] 93[4.3.2.2] [7.2]

Anyway sees the problem?

Cheers

 

[shnain]

Hello,

It's better to do the comparison as follows:

signal wrfifo_full : std_logic;
signal wrfifo_write_allowed : std_logic;  

...

wrfifo_write_allowed <= psel AND penable AND pwrite AND NOT(wrfifo_full); 

...

process (paddr, wrfifo_write_allowed) is
begin
if (((paddr(8 downto 2) = "1000000")) and (wrfifo_write_allowed = '1') ) then
dt_fifo_wr_i <= '1';
else
dt_fifo_wr_i <= '0';
end if;

end process;

 

[grubby23]

THat makes so much more sense, thank you very much for your help!

 

[TrickyDicky]

To explain the problem properly:

an "if-then-else" tree requires all comparisons to result in a boolen. For you, you tried to "and" together a boolean : (paddr(8 downto 2) = "1000000") with std_logics (psel, pwrite etc). Unless you write a custom "and" function to do this (that also outputs a boolean), it cannot check the if statement. This is why the error was "operator type missmatch".

 

[Dave_PL]

Thats true but only prior to VHDL2k8. In VHDL2k8 you can write explicitly with "??" operator in the if expression.

 

[TrickyDicky]

True. But you'll be hard pressed to find software that supports it currently