Civil Engineering Reference
In-Depth Information
lies in its fanin cone‚ and gate is any successor of node Let the skews
at the flip-flops and be and respectively. The logic signal separation
constraint under a clock period
P
is stated mathematically as follows:
The left hand side of this equation represents the arrival time of the signal
from the current wave at the output of the successor of This arrival implies
that the value at is no longer important for the current wave. The right
hand side is the arrival time at the output of during the next wave. Simply
stated‚ the inequality says that it is essential for the inputs of to be stable
for as long as its output influences the current wave. In case has multiple
predecessors and successors‚ the relationship (9.25) must hold between each
predecessor-successor pair.
Earlier in this chapter‚ several techniques for combined sizing and skew were
described. None of them explicitly considers the logic signal separation con-
straints in the form in which they were published. However‚ it is relatively
easy to incorporate them into the formulations. For the linear programming
formulation‚ the output arrival time at each gate is available as a variable‚ and
it is an easy matter to add constraints of the form (9.25) at For
the convex programming formulation‚ this constraint can be checked during the
application of CPM‚ and any violation can be flagged as a constraint violation
to be reconciled by altering the sizing and/or skew solution.
9.7
DELIBERATE SKEWS FOR PEAK CURRENT REDUCTION
In synchronous circuits‚ all of the flip-flops switch at the latching edge of the
clock. This simultaneous switching of flip-flops requires a large switching cur-
rent‚ which in turn creates a significant voltage drop on the power distribution
network‚ creating the need for multiple pins to distribute the power supply.
In [VHBM96] deliberate clock skews are used to reduce the dynamic transient
current drawn from the supply pins. This reduction in the transient current
reduces the number of supply pins required by the chip‚ and therefore‚ its
packaging cost.
The objective of introducing skews at the flip-flops is to ensure that all of
them do not switch at the same time and hence‚ do not draw current at the
