Logic Delay Times
Output Switching Times:
- Low to High or Rise Time
- High to Low or Fall Time
Definition of Rise Time
Rise time is the difference between the time when the signal crosses a low threshold to the time when the signal crosses the high threshold. It can be absolute or percent.
Absolute Rise Time
In absolute rise time, the low and high thresholds are fixed voltage levels around the mid voltage level.
Percent Rise Time
In percent rise time, the low and high thresholds are percent levels, and are usually either 10% and 90% respectively or 20% and 80% respectively. The percent levels are converted to absolute voltage levels at the time of measurement by calculating percentages from the difference between the starting voltage level and the final settled voltage level.
Definition of Fall Time
Fall time is the difference between the time when the signal crosses a high threshold to the time when the signal crosses the low threshold. It can be absolute or percent.
Absolute Fall Time
In absolute fall time, the low and high thresholds are fixed voltage levels around the mid voltage level.
Percent Fall Time
In percent fall time, the low and high thresholds are percent levels, and are usually either 10% and 90% respectively or 20% and 80% respectively. The percent levels are converted to absolute voltage levels at the time of measurement by calculating percentages from the difference between the starting voltage level and the final settled voltage level.
Propagation delay is the average transition delay time for the signal to propagate from input to output. The operating speed is inversely proportional to the propagation delay.
Measurement of Propagation delay
The propagation delay, or gate delay, is the length of time starting from when the input to a logic gate becomes stable and valid, to the time that the output of that logic gate is stable and valid. Often this refers to the time required for the output to reach from 10% to 90% of its final output level when the input changes. Reducing gate delays in digital circuits allows them to process data at a faster rate and improve overall performance.
Propagation delay increases with operating temperature, marginal supply voltage as well as an increased output load capacitance. The latter is the largest contributor to the increase of propagation delay. If the output of a logic gate is connected to a long trace or used to drive many other gates (high fan-out) the propagation delay increases substantially
A full adder has an overall gate delay of 3 logic gates from the inputs A and B to the carry output Cout shown above
The difference in propagation delays of logic elements is the major contributor to glitches in asynchronous circuits as a result of race conditions.
A race condition or race hazard is a flaw in an electronic system or process whereby the output and/or result of the process is unexpectedly and critically dependent on the sequence or timing of other events. The term originates with the idea of two signals racing each other to influence the output first.
Note: More on http://en.wikipedia.org/wiki/Race_condition