Chapter 1. Tools Utilized
57
Table 5.1: std logic values
Character
Value
’U’
uninitialized
’X’
strong drive, unknown logic value
’0’
strong drive, zero
’1’
strong drive, one
’Z’
high impedance
’W’
weak drive, unknown logic value
’L’
weak drive, logic zero
’H’
weak drive, logic one
’-’
don’t care
The introduction of logic synthesis for HDLs pushed HDLs from the background into the
foreground of digital design. Synthesis tools compiled HDL source files (written in a con-
strained format called RTL) into a manufacturable netlist description in terms of gates
and transistors. A circuit design from a skilled engineer, using labor-intensive schematic-
capture/hand-layout, would almost always outperform its logically-synthesized equiva-
lent, but the productivity advantage held by synthesis soon displaced digital schematic
capture to exactly those areas that were problematic for RTL synthesis: extremely high-
speed, low-power, or asynchronous circuitry.
5.1.1
The IEEE standard
The IEEE Standard 1076 defines the VHSIC Hardware Description Language or VHDL.
It was originally developed under contract F33615-83-C-1003 from the United States Air
Force awarded in 1983 to a team with Intermetrics, Inc. as language experts and prime
contractor, with Texas Instruments as chip design experts and IBM as computer system
design experts. The language has undergone numerous revisions and has a variety of
sub-standards associated with it that augment or extend it in important ways.
The IEEE 1164 standard uses ”multi-valued” signals, where a signal’s drive strength
(none, weak or strong) and unknown values are also considered. As a result a signal of
type std logic can acquire 9 values according to table
his system promoted a useful set of logic values that typical CMOS logic design could
utilize in the vast majority of modeling situations. The ’Z’ literal makes tri-state buffer
logic easy. The ’H’ and ’L’ weak drives permit wired-AND and wired-OR logic. Ad-
ditionally, the ’U’ state is the default value for all object declarations so that during