Synthesis Error in XST

[Nisheeth]

hello group
I have been using Xilinx ISE + Modelsim for last 2
months.During this time i made small projects and did simulation. I
tried to carry out next logical step after simulation this
time..translate,map,P&R.
Though the following code compiled w/o error and simulation was ok but
gave error in translate step.

Code
---

entity result_out is
port (
pe_row : in std_logic_vector(7 downto 0);
ws_row : in std_logic_vector(7 downto 0);
clock : in std_logic;
pe_moving_row: out std_logic_vecotr(7 downto 0)
);
end entity;

architecture behavioural of result_out is
begin
process(clock,pe_row,ws_row)
begin
if clock='1' and clock'event then
pe_moving_row<=pe_row + ws_row;
else
pe_moving_row<=(others=>'Z');
end if;
end process;
end architecture;

-----

Error-> Signal pe_moving_row cannot be synthesized, bad synchronous
description.

After hit and trial i found removing clock'event makes the thing
work...but i have no clue why....can anyonet tell me why ?

A bit info about how this block will be used...
16 "result_out" blocks will be connected in parallel and at a time
only 1 "result_out" will output data on "pe_moving_row"
bus...depending on to which block clock is supplied...

Regards
Nisheeth

 

[Tim Good]

Hi,

Your sample VHDL is asking the synthesis tool to create logic which does
the following:

ONLY at the instant of a positive edge transition of the clock (and for
only this instant) set the output pe_moving_row to the addition of
pe_row and ws_row and for all remaining time set it to be tristate.
This is simply not possible!

If you are trying to add the two numbers together then this is purely
combinational and you dont need a process block:

architecture RTL of result_out is
begin
pe_moving_row<=pe_row + ws_row;
end RTL;

If you are asking for something with a tri-state output then you will
need an extra signal, "OutputEnable". Note to infer latch behavior the
"clock if statement" does not have an else :


architecture RTL of result_out is
process(clock,pe_row,ws_row,output_enable)
begin
if clock='1' and clock'event then
if output_enable = '1' then
pe_moving_row<=pe_row + ws_row;
else
pe_moving_row<=(others=>'Z');
end if;
end if;
end process;
end architecture;

Hope this helps,

Tim

 

[Nisheeth]

hi tim
thanks ...i believe i understood the mistake i m making.


nisheeth