Countermeasures for Hazards

Countermeasures for hazards

In the case of multiple-input combinational logic consisting of OR, AND and other gates, a slight difference in the timing of input signal changes causes a brief whisker-like pulse called a hazard.
Using the circuit shown in below figure, let’s see how a hazard occurs because of a difference in signal delays. Suppose that a rising signal transition occurs simultaneously at A and B. The signal applied to B reaches the AND gate via an inverter. Since the signal that enters the AND gate from B is delayed by an inverter, the AND gate receives input signals A and C at different timings, potentially producing a High pulse at the Y output.

Countermeasures for hazards
combinational logic should not be designed to produce a desired output value from simultaneous changes in its inputs. Using a flip-flop to adjust the output timing also helps eliminate a hazard.

In addition to a signal timing difference, a slowly changing input can be a cause of a hazard.
Hazards due to a slowly changing input can be prevented by using a logic gate with a Schmitt-trigger input.

Usage Considerations of CMOS Logic ICs

Handling of Unused Input Pins
Input Rise and Fall Time Specifications
Multiple Outputs from a General-Purpose CMOS Logic IC Come Into Conflict (Short-Circuiting)
Connecting a Load Capacitance to a CMOS Output Pin
Calculating the Operating Supply Current and Power Dissipation
Level Shifting Using an Input-Tolerant Function
Example of Application of the Power-Down Protection Function (Partial Power-Down)
Input-Tolerant and Output Power-Down Protection Functions Available with Each Series
Types of Noise to be Noted
Countermeasures for Reducing Switching Noise
Countermeasures for Signal Reflection
Countermeasures for Crosstalk
Countermeasures for Metastability
Countermeasures for Latch-Up
Countermeasures for ESD Protection


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