Question

I am using a Basys2 board to program a simple string detector to read patterns like “0101”. I am using the Xilinix Design Suite 13.2 for this project. The .ucf file gives the following message.

ERROR:Place:1018 - A clock IOB / clock component pair have been found that are not
placed at an optimal clock IOB clock site pair. 
The clock component <reset_IBUF_BUFG> is placed at site <BUFGMUX_X1Y1>. 
The IO component <reset> is placed at site <IPAD73>.

Here’s what my user constrains file looks like:

NET "x"     LOC = "P11";# input switch
NET "reset" LOC = "K3"; # reset switch
NET "ck"    LOC = "B8"; # clock
NET "b1"    LOC = "A7"; # button 1
NET "b2"    LOC = "M4"; # button 2
NET "z"     LOC = "M5"; # output

One of the solutions I found online was to use this in the ucf:

CLOCK_DEDICATED_ROUTE = FALSE;

But unfortunately this didn’t work for me. I’d appreciate any help with this problem.

Answer 1

The message appears to imply that your reset is being detected as a clock signal. That should be worrying – do you have processes which are sensitive to the edge of the reset signal?

You should (until you really know what you’re doing) only use a couple of forms of process.

---

Asynchronous reset

process (clk, reset)
begin
  if reset = '1' then
  -- async reset stuff
  elsif rising_edge(clk) then
  -- sync stuff
  end if;
end process;

Synchronous reset

process (clk)
begin
  if reset = '1' then
    --sync reset stuff
  else
    -- other sync stuff
  end if;
end process;

---

Also, be careful – you still need to release the reset signal to these processes synchronously.

Here’s a good read on various reset strategies:

http://www.xilinx.com/support/documentation/white_papers/wp272.pdf

Finally, using CLOCK_DEDICATED_ROUTE = FALSE; is another no-no for almost every situation.