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13.2 ARCHITECTURES CENTERED AROUND NONREGISTERED PLDs 623

ROM Inputs ROM Outputs
/ \
Sanity
PS NS
/ —-* ^
IB 's '4 '3 '2 '1 >o Y 6 Y T 3 Y T 2 T Y 1
A B c s T u* V DA D B DC M N* p Q
0 0 0 0 X X X 0 0 0 0 0 0 0
0 0 0 1 X X X 0 0 1 0 0 0 0
0 0 1 X 0 X 0 0 0 1 0 0 0 0
0 0 1 X X X 1 0 0 0 1 0 0 0
0 0 1 X 1 X 0 1 0 1 0 0 0 0
1 0 1 0 0 X X 0 0 0 0 0 0 0
1 0 1 0 1 X X 1 1 1 0 0 0 0
1 0 1 1 X X X 1 1 0 0 0 0 0
1 1 0 X X 1 X 1 1 0 0 0 0 1
1 1 0 X X 0 X 1 0 0 0 0 0 1
1 1 1 X X X X 0 1 0 0 0 1 0
0 1 0 X 0 X X 1 1 0 1 0 0 0
0 1 0 X 1 X X 0 1 0 1 0 0 0
MiT
1 0 0 X X X 0 1 0 1 0 1 0 0
1 0 0 X X X 1 1 0 1 0 0 0 0
Synchronous Inputs Outputs
S(H), T(H), U(L), V(H) M(H), N(L), P(H), Q(H) X = Irrelevant input
* Indicates an active low input or output
FIGURE 13.10
Design of the NS and output logic for a fictitious FSM by using a ROM. (a) State diagram, (b) Coll-
apsed ROM program table constructed directly from the state diagram in (a).

The ROM program table is constructed directly from the state diagram and is given in
Fig. 13.10b. As can be seen, this table is a collapsed canonical truth table involving only
7
15 rows. Fully expanded, this table would require 2 = 128 rows, which is unnecessary for
programming purposes. Remember that the irrelevant input X is to inputs as the don't care
0 is to outputs. Thus, all input data for state 011 (not shown) take X's and all output data
relative to this state take don't cares.
There are other features of this ROM program table that are noteworthy. Active low
inputs to a ROM can be dealt with by complementing the logic values in columns of those
active low inputs, or by placing inverters on these inputs to the ROM and not complementing
the columns. Similarly, active low outputs can be handled by either complementing their
columns or by placing an inverter on these outputs, but not both actions. The input U(L) and
the output N(L) are represented by using an asterisk in the ROM I/O table to indicate that
one of the two actions just stated is necessary to accommodate their active low logic level.
For this example, only the U input column is complemented in the ROM program table of
Fig. 13.10b. This eliminates the need to use an inverter on the input to produce U(L). The
active low output N(L) will be issued from an output holding register as discussed later in
this section. Section 7.5 provides a review of this subject.
One other feature of the ROM program table, mentioned earlier, is important to remem-
ber: The outputs are always given relative to the present state, never the next state. The