Hazards and delay padding

Description

This applets demonstrates the two basic circuits that result in a dynamic hazards.

First, the circuit on the top left demonstrates the basic structure responsible for a dynamic 1-hazard (0-1-0 transition). The idea here is to feed the AND gate both with the input value directly, and with a delayed and inverted variant of the input value. Therefore, the stable output value of the circuit is always 0. However, after a rising-edge on the input signal A, the bottom input of the AND gate has the value 1, while the output of the last inverter on the delay-path is also still 1. Therefore, the circuit creates a short 1 pulse after the rising edge of the A input. Again, the actual gate-delays are set to very large values in order to demonstrate the effect in real-time simulation with glow-mode.

Obviously, the second circuit on the top right uses the same basic structure. However, due to the use of an OR gate instead of the AND gate, the default value of the circuit output is 1. A dynamic 0-hazard (1-0-1) is created on the output after each falling edge on the input.

The third circuit (bottom) demonstrates the use of gate-delays (and hazards) to generate specific output waveforms, a technique called delay padding. Here, the duration of the 0-1-0 pulse on the output depends on the number of inverters in the delay-line. When using n gates in the delay-line, the output pulse duration is n-times the propagation delay of a single inverter.

Due to the increasing difficulties to operate clocked circuits at very high speed in the GHz-range and with possibly dozens of different clock signals, research in asynchronous circuits has become popular (again). In such circuits, delay-padding techniques like the one presented in the applet are central to control the timing dependecies between different parts of the whole systems. Elaborate logic synthesis algorithms are used to guarantee the correct timing when taking manufacturing tolerances into account. For a good introduction to asynchronous circuit design see (Proceedings of the IEEE, special issue on asynchronous circuits and systems, 02-1999).

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