rnewton@mrmngo.intel.com (Richard Newton II ) (04/16/91)
Hi all,
Another person here at Intel saw a problem with the proposed 9-value
logic package; I thought I'd post it, and maybe start a discussion.
Below is an extract from the v2.800 package posted to the net. The
concern is with the don't care state. It used to be '-', now it
is 'D'.
-- v2.300 | wdb | 10/24/90 | Changed '-' to 'D', deleted attributes
-- v2.400 | wdb | 11/12/90 | Fixed subtyped subtypes
-- v2.500 | wdb | 01/02/91 | Change output types of overloaded operators
-- v2.600 | wdb | 01/04/91 | Clean UP
-- v2.700 | wdb | 01/08/91 | Add Conversion functions to Bit
-- v2.800 | wdb | 03/03/91 | Pre-Ballot Cleanup
-- --------------------------------------------------------------------
-- Library IEEE; -- proposed location of this package
-- --------------------------------------------------------------------
PACKAGE logic_system is
-------------------------------------------------------------------
-- Logic State System (unresolved)
-------------------------------------------------------------------
TYPE std_ulogic is ( 'U', -- Unitialized
'X', -- Forcing 0 or 1
'0', -- Forcing 0
'1', -- Forcing 1
'Z', -- High Impedance
'W', -- Weak 0 or 1
'L', -- Weak 0 ( for ECL open emitter )
'H', -- Weak 1 ( for open Drain or Collector )
'D' -- don't care
);
If you had a vector of this type (say, an address) wouldn't the don't
care type get confused with the hex value D? This especially comes
into play when you have a simulator who's waveform display can show
user-defined types (like this one) as hex.
What was wrong with '-' as don't care, anyway?
Richard Newton II rnewton@mrmngo.intel.com
Intel Corp., 2625 Walsh Ave, Mail Stop SC4-69, Santa Clara, CA 95052-8122