11.9 LogicGate Modeling
Verilog has a set of builtin logic models and you may also define your own models.
11.9.1 Builtin Logic Models
Verilog's builtin logic models are the following primitives [Verilog LRM7]:
You may use these primitives as you use modules. For example:
module primitive; nand (strong0, strong1) #2.2 Nand_1(n001, n004, n005), Nand_2(n003, n001, n005, n002); nand (n006, n005, n002); endmodule
This module models three NAND
gates (Figure 11.2). The first gate (line
3) is a twoinput gate named Nand_1
; the second gate (line
4) is a threeinput gate named Nand_2
; the third gate (line
5) is unnamed. The first two gates have strong drive strengths [Verilog
LRM3.4] (these are the defaults anyway) and 2.2 ns delay; the third
gate takes the default values for drive strength (strong) and delay (zero).
The first port of a primitive gate is always the output port. The remaining
ports for a primitive gate (any number of them) are the input ports.
FIGURE 11.2 An example schematic (drawn with Capilano's DesignWorks) to illustrate the use of Verilog primitive gates. 
Table 11.5
shows the definition of the and
gate primitive (I use lowercase
'and'
as the name of the Verilog primitive, rather than 'AND'
, since Verilog is casesensitive). Notice that if one input to the primitive
'and'
gate is zero, the output is zero, no matter what the
other input is.
11.9.2 UserDefined Primitives
We can define primitive
gates (a userdefined primitive or UDP) using a truthtable
specification [Verilog LRM8]. The first port of a UDP must be an output
port, and this must be the only o utput
port (we may
not use vector or inout
ports):
primitive Adder(Sum, InA, InB); output Sum; input Ina, InB; table // inputs : output 00 : 0; 01 : 1; 10 : 1; 11 : 0; endtable endprimitive
We may only specify the values
'0'
, '1'
, and 'x'
as inputs in
a UDP truth table. Any 'z'
input is treated as an 'x'
. If there is no entry in a UDP truth table that exactly matches a set of
inputs, the output is 'x'
(unknown).
We can construct a UDP model for sequential logic by including a state in the UDP truthtable definition. The state goes between an input and an output in the table and the output then represents the next state. The following sequential UDP model also illustrates the use of shorthand notation in a UDP truth table:
primitive DLatch(Q, Clock, Data); output Q; reg Q; input Clock, Data; table //inputs : present state : output (next state) 1 0 : ? : 0; // ? represents 0,1, or x (input or present state). 1 1 : b : 1; // b represents 0 or 1 (input or present state). 1 1 : x : 1; // Could have combined this with previous line. 0 ? : ? : ; //  represents no change in an output. endtable endprimitive
Be careful not to confuse
the '?'
in a UDP table (shorthand for '0'
, '1'
, or 'x'
) with the '?'
in a constant that represents
an extension to 'z'
(Section 11.2.4) or the '?'
in a case
statement that represents don't care values (Section 11.8.1).
For sequential UDP models that
need to detect edge transitions on inputs, there is another special truthtable
notation (ab)
that represents a change in logic value from
a
to b
. For example, (01)
represents
a rising edge. There are also shorthand notations for various edges:
primitive DFlipFlop(Q, Clock, Data); output Q; reg Q; input Clock, Data; table //inputs : present state : output (next state) r 0 : ? : 0 ; // rising edge, next state = output = 0 r 1 : ? : 1 ; // rising edge, next state = output = 1 (0x) 0 : 0 : 0 ; // rising edge, next state = output = 0 (0x) 1 : 1 : 1 ; // rising edge, next state = output = 1 (?0) ? : ? :  ; // falling edge, no change in output ? (??) : ? :  ; // no clock edge, no change in output endtable endprimitive
