A method of synthesis of multilevel combinatorial circuits with usage of chains of feedbacks (method M5)
In some cases when the requirement of simultaneous creation of output
signals of the combinatorial circuit is not critical, it is possible to
apply methods of synthesis of multilevel combinatorial circuits. Main
disadvantage of methods of synthesis of multilevel combinatorial
circuits is possible(probable) significant distinction of time of
creation of values of output signals. However the indicated
disadvantage is easily eliminated by programming of register outputs
PLD. Advantage of multilevel circuits, in comparison with methods M2 -
M4, consists in smaller cost of implementation, the PLD is less used
macrocells.
The Feature of method M5 is that in it already realized functions or
parts of functions are widely used, and also their inversions, as
factors - functions for implementation of other functions. Method M5 in
many respects is similar to method M3 and also includes three stages.
Differences of the first stage consist that is fulfilled not separate,
but joint minimization of a set of functions ?Yê. From two yi functions
and i at execution necessary and sufficient conditions of
implementation in set Y* the function which is most effective as the
factor - function at implementation of other functions is selected. At
the second stage it agrees a method of synthesis M2 the subset of parts
of functions ?B1..., BT is formed. Difference consists that if some
realized function (a part of the function) or its inversion is the
factor - function for other functions the intermediate function is
introduced and appropriate conversion of representation of Booleen
Function Systems Y* is fulfilled. In method M5 possibility of input of
intermediate functions also is stipulated with the purpose of
simplification of implementation of Booleen Function System Y*. At the
third stage of synthesis of a part of functions are united on OR with
help PAL of the last level in complete correspondence with method M3.
To positive qualities of method M5 concern more complete usage of
architectural possibilities PLD:
To disadvantages of method M4, in comparison with method M3, concerns:
Thus, method M5 may be recommended for implementation of complicated Booleen Function Systems when speed is not critical, and it is necessary to minimize cost of implementation.