The feedback solution is simple, straightforward
and inexpensive. To provide the necessary hysteresis
behavior, the result value is fed back with a
weight of one less than the threshold as
shown in Figure 10. The rounded gate symbol will
represent a gate with the hysteresis behavior.
This hysteresis behavior may be provided by feedback
internal to the gate or by some inherent behavior
of the gate implementation approach.
Figure 10. Threshold gate with weighted feedback
of threshold -1.
Beginning in a NULL state with all inputs NULL
and asserting a NULL result value, the gate will
not assert a DATA result value until its input
data set is complete, which in this case is four
DATAs. With the feedback weight of three, the
gate will not transition to a NULL result value
until all of its input values are NULL. As long
as at least one input value remains DATA, the
gate meets its threshold and will continue asserting
a DATA result value. It does not fall below its
threshold and transition to NULL output until
all inputs are NULL. This behavior is shown in
Figure 11. Hysteresis behavior of NULL Convention
The gate enforces the completeness of input
criteria for both data in relation to NULL
and for NULL in relation to DATA. A
threshold 1 gate does not require the feedback
connection because its completeness of input criteria
is inherently enforced for both DATA and NULL.