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In article <1992Dec22.014551.24293@ole.cdac.com>, george@ole.cdac.com (George Lippincott) writes:
> 
> I am trying to figure out a way in VHDL to merge two bidirectional
> signals into one signal.  This is something that most of the major
> logic simulators let you do but which as far as I can tell is
> impossible in VHDL. 
> 
> This situation comes up when an entity is implemented in such a way 
> that two bidirectional ports connect to the same signal.  What I
> would like to do is have some way to "short" these two signals together.
> Ideally the resolution function would see all of the drivers for
> both signals and calculate a value for both nets.
> 
> This can be done in Verilog like this:
> 		module wire(w,w)
> 		inout w;
> 		endmodule;
> 
> I haven't been able to figure out a way to do the same thing in VHDL.
> If I use the code:
> 		a <= b;
> 		b <= a;
> 
> Then a driver is added to both signals and the value gets stuck at 'U'
> due to the nature of the resolution function in the IEEE 1167 package.
> 
> Does anyone have any idea on how to work around this?
> 

I ran off the following VHDL for joining two "std_logic" resolved signals (hope
you find it some help - N.B. 'U' is sometimes mapped to 'Z' and intermediate
values appear in some deltas):

-- begin

library ieee;

use ieee.std_logic_1164.all;

architecture a of wire is
begin

    process
        variable res: std_ulogic_vector(1 to 2);
	variable drv: std_logic;
    begin

     	w2 <= 'Z';
        w1 <= 'Z';
	
	wait for 0nS;

	if w1 = 'U' then
	    drv    := w2;
        elsif w2 = 'U' then
	    drv    := w1;
	else
	    res(1) := w1;
	    res(2) := w2;
            drv    := resolved(res);
	end if;
	
	if w1 = drv and w2 /= drv then
	    w2 <= drv;
	end if;
	
        if w2 = drv and w1 /= drv then
	    w1 <= drv;
	end if;
	
	wait until w1 /= w2;

end process;
    
end;

entity test_wire is
end;

library ieee;

use ieee.std_logic_1164.all;

architecture a of test_wire is
    signal a,b: std_logic;
    component wire port (w1,w2: inout std_logic);
    end component;
begin

    w1: wire port map (a,b);
    
    a <=    '1' after 1ns, 
                 'L' after 2ns, 
		      'Z' after 3ns,
				       'H' after 14ns,
				          'Z' after 15ns;
    
    b <= 'Z',              'H' after 4ns, 
                               'Z' after 7ns,
			           'U' after 12ns,
				       'H' after 14ns,
				          'Z' after 15ns;
    
end;

-- end

which gives:

      Time(fs) + Cycle     a    b 
----------------------  ---- ---- 
                  0+ 0:  'U'  'U' 
                  0+ 1: *'U' *'Z' 
                  0+ 2: *'U'  'Z' 
            1000000+ 0: *'1'  'Z' 
            1000000+ 1: *'1' *'Z' 
            1000000+ 2:  '1' *'1' 
            2000000+ 0: *'L'  '1' 
            2000000+ 1: *'L' *'Z' 
            2000000+ 2:  'L' *'L' 
            3000000+ 0: *'Z'  'L' 
            3000000+ 1: *'Z' *'Z' 
            4000000+ 0:  'Z' *'H' 
            4000000+ 1: *'Z' *'H' 
            4000000+ 2: *'H'  'H' 
            7000000+ 0:  'H' *'Z' 
            7000000+ 1: *'Z' *'Z' 
           12000000+ 0:  'Z' *'U' 
           12000000+ 1: *'Z' *'U' 
           12000000+ 2:  'Z' *'U' 
           14000000+ 0: *'H' *'H' 
           15000000+ 0: *'Z' *'Z' 

Kev.
-- 
Kevin Cameron          CLSI, Suite 100, 5457 Twin Knolls Rd., MD 21045, USA
kevin@clsi.com         Tel: (USA) (410) 992 5700 (- 5709) x224
uunet!clsi.com!kevin   Fax:       (410) 992 3536
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