Hazard in an Asynchronous sequential circuit Sequential circuit, it may cause the circuit to go to the wrong stable state. However, if a momentary incorrect signal is fed back in an asynchronous
Removal of hazards requires the addition of redundant gates to the circuit.ĭesign associated with synchronous sequential circuits, hazards are of noĬoncern, since momentary erroneous signals are not generally troublesome. Two minterms that may produce a hazard with a product term common to both. In general, hazard s in combinational circuits can be removed by cove ring any The extra gate in the circuit generates the product term x 1x 3. Hazard-free circuit obtained by such a configuration is shown in figure below. Two minterms with another product term that overlaps both groupings. The remedy for eliminating a hazard is to enclose the Implemented in gate 1 and minterm 101 is covered by the product term Minterm 111 is covered by the product term Results in a different product term covering the two minterm. The hazard exists because the change in input
The change in x2 from 1 to 0 moves the circuitįrom minterm 111 to minterm 101. The first case is referred to as static 1-hazard and the second case as static 0-hazard.Ī third type of hazard, known as dynamic hazard,Ĭauses the output to change three or more times when it should change from 1 to If however, the circuit is implemented instead in product-of-sums form namely, This type of implementation may cause the output to go to 0 when it should remain a 1. The two circuits shown in Fig implement the Boolean function in sum-of-products form: In the inverter may cause the output of gate 1 to change to 0 before the output Propagation delay through the inverter is taken into consideration. However, the output may momentarily go to 0 if the Then the output of gate 1 changes to 0 and that of gate 2 changes to 1, This causes the output of gate 1 10 be 1, that of gate 2 Single variable produces a momentary change in output when no change in output When they occur inĪsynchronous sequential circuits hazards may result in a transition to a wrongĪ hazard is a condition in which a change in a Where they may cause a temporary false output value. That may appear at the output of a circuit because different pathsĮxhibit different propagation delays. Hazards are unwanted switching transients InĪddition, there is one more phenomenon called a hazard that may cause the
COMBINATIONAL VS SEQUENTIAL LOGIC FREE
With only one input changing at any time and must be free of critical races. The circuit must be operated in fundamental mode Taken to conform to certain restrictions and precautions to ensure that theĬircuits operate properly. In designing asynchronous sequential circuits, care must be
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