Slide 1
Binary values occupy voltage ranges
A digital circuit represents zero and one using permitted voltage ranges. The physical signal can vary without necessarily changing its logical meaning.
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This tolerance helps a signal survive small disturbances. It is a deliberate property of the electrical design, not evidence that digital signals are immune to noise.
Slide 2
Outputs and inputs make different promises
An output guarantees voltages within specified limits. A receiving input guarantees correct interpretation across its own low and high ranges.
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The gap between these guarantees provides room for disturbance. Connecting devices with incompatible voltage ranges can remove that protection even when both are described as digital.
Slide 3
Noise margin is spare room
A noise margin measures how far a guaranteed output can move toward an input threshold before the logical interpretation becomes uncertain.
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There are separate high-state and low-state margins, and they need not be equal. Engineers check the limiting case under relevant supply, temperature and manufacturing conditions.
Slide 4
Some voltages have no guaranteed meaning
The region between valid low and high input ranges is not a third intended logic value. A circuit specification may leave its interpretation undefined.
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Signals pass through this region while switching, which is expected. The problem is treating a slow or unsettled transition as a stable, valid input at the wrong moment.
Slide 5
A gate restores a valid level
Within its operating conditions, a logic gate converts acceptable input ranges into controlled output ranges. This prevents small analog errors from simply accumulating forever.
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The restoration takes a finite propagation time. Noise tolerance and timing constraints therefore work together: a correct voltage that arrives too late can still cause a system error.
Sources
- Digital Integrated Circuits: Inverter Basics
Massachusetts Institute of Technology
- Computation Structures: CMOS Technology
MIT OpenCourseWare