Advanced digital design【reading notes】

[ Hazard]

If a circuit has a hazard it could exhibit a glitch under certain conditions. There are two types of hazards, static and dynamic. The term static refers to a circuit in which the output should not change under the application of certain inputs, but does.

Static hazards are caused by differential ­propagation delays on reconvergent fanout paths.

Static hazards can be eliminated by introducing redundant cubes in the cover of the output expression (the added cubes are called a "hazard cover").

When a signal changes value at the input of a circuit, a static I-hazard could occur at the output of the circuit if three conditions are satisfied: (1) an output remains asserted (i.e., its value is 1 before and after the input signal changes value), (2) the cube that is asserted in the SOP expression of the output by the initial value of the signal is different from the cube that is asserted by the final value of the signal, and (3) the cubes that are asserted by the initial and final values of the signal are not covered by the same prime implicant.

 

[continuos-assignment]

Continuous-assignment statements are limited to modeling level-sensitive behavior- combinational logic and transparent latches. They cannot model an element that has edge-sensitive behavior, such as a flip-flop.

[filter]

Digital filters are usually designed to have two important characteristics: causality and linear phase. A filter is causal if its impulse response is 0 before the impulse is applied. FIR filters are widely used in practical applications because they can be designed to have a linear phase characteristic, which ensures that the filter's output signal is a time-shifted (delayed), but undistorted copy of its input signal On the other hand, an I1R filter having a linear phase characteristic cannot be causal. Noncausal filters cannot be realized in hardware. Another distinction between the two types of filters is that FIR filters cannot accumulate roundoff error; I1R filters can accumulate roundoff error as the output is successively passed through the filter.