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A Static Hazard is defined when a single variable change at the
input causes a momentary change in another variable [the output]. A
Dynamic Hazard occurs when a change in the input causes multiple
changes in the output [i.e. from 1 to 0 and back to 1]. In either case of
a Static or Dynamic hazard the product produced is an unanticipated
glitch [the hazard]. The resulting glitches in the circuit may or may not
induce additional problems ~ other then increased issues due to switching
noise. However; a problem may occur if the signal with the glitch is a
clock line. A glitch on the clock line to a flip flop will cause device
to clock in data when it wasn't meant to. A glitch on a clock line of a
counter will also have a negative effect. In any case, good design
practice means designing out these hazards. |
If Static Hazards are removed from the design, Dynamic Hazards will not occur. A Karnaugh map [K-map] is the easiest way to eliminate a Static Hazard or glitch. These timing hazards will develop as random or intermittent circuit failures. The type of circuit failure will depend on the signals used in the AND / OR gate circuit, and perhaps how often they change state. Another method to eliminate timing hazards from effecting an IC down the line is to re-clock the final output signals. Re-clocking the signal does not eliminate the glitch, but stops it from causing circuit failure. Re-clocking the signal seems to be common for designers unsure of why the glitch occurs, or how to stop the glitch from developing. Solving the problem via a K-map results in an additional AND gate, re-clocking requires an additional flip flop.
A K-map for each combinatorial logic function which has an output should
be used. Redundant prime implicants should be added to the K-Map
(circuit) [shown in RED above], which will guarantee that all single-bit
input changes are covered. Multi-level functions will be reduced to
"two-level" functions, and analyzed by the K-map approach. The procedure
for designing a static-hazard-free network is a straightforward
application. The key is to place the function in such a form that the
transient output function guarantees that every set of adjacent 1's in
the K-map are covered by a term, and that no terms contain both a
variable and its complement. The former condition eliminates 1-hazards
and the latter eliminates 0-hazards. Dynamic hazards happen because of
multiple paths in a multilevel network, each with its own asymmetric
delay. Circuits which contain multiple paths of the same signal should be
re-clocked before the signal is used by a circuit.
Static 0 hazards occur in 'Product-Of-Sums' [POS] implementations, but do
not occur in 'Sum-Of-Products' [SOP] implementations. Static 1 hazards
occur in SOP implementations, do not occur in POS implementations.
How to avoid logic hazards: develop a Karnaugh map [K-map].
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