# Unit delay simulation, Why and When

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Unit delay simulation is a technique for modeling the behavior of digital circuits in a discrete time domain. It is based on the assumption that each logic gate has a unit delay, which means that it takes one time step for the output to change after the input changes. Unit delay simulation can be used to verify the functionality and timing of a circuit design, as well as to detect and debug any glitches or hazards that may occur due to signal propagation delays.

To perform unit delay simulation, we need to represent the circuit as a network of nodes and edges, where each node corresponds to a logic gate or an input/output port, and each edge corresponds to a wire or a connection. Each node has a current value and a next value, which are updated at each time step according to the logic function of the node and the values of its predecessors. Each edge also has a value, which is equal to the current value of its source node. The simulation proceeds by iterating over the following steps:

1. Initialize the values of all nodes and edges according to the initial conditions of the circuit.
2. For each node, evaluate its next value based on its logic function and the values of its predecessors.
3. For each node, assign its next value to its current value, and propagate the value to its successors.
4. Increment the time step and repeat from step 2 until the desired number of time steps is reached or a steady state is achieved.

Unit delay simulation is a simple and intuitive way to model digital circuits, but it also has some limitations and drawbacks. One of them is that it assumes that all logic gates have the same delay, which is not realistic in practice. Different types of gates may have different delays depending on their complexity, size, technology, etc. Moreover, the delay of a gate may vary depending on the operating conditions, such as temperature, voltage, load, etc. Therefore, unit delay simulation may not accurately reflect the actual performance and behavior of a circuit in real-world scenarios.

Another limitation of unit delay simulation is that it does not account for any physical effects that may affect the signal transmission along the wires, such as capacitance, resistance, inductance, noise, crosstalk, etc. These effects may cause distortions, attenuations, reflections, or interference in the signal waveforms, which may affect the timing and reliability of the circuit. Therefore, unit delay simulation may not capture all the possible sources of errors or failures that may occur in a circuit.

To overcome these limitations, more advanced simulation techniques have been developed, such as gate-level simulation, switch-level simulation, transistor-level simulation, etc. These techniques use more detailed and accurate models of the logic gates and the wires, which take into account their physical characteristics and parameters. However, these techniques also require more computational resources and time to perform the simulation, which may not be feasible for large or complex circuits.

Therefore, unit delay simulation is still useful as a first-order approximation or a quick check of a circuit design, especially in the early stages of development or for simple or small circuits. It can help to verify the functionality and logic of a circuit, as well as to identify any obvious timing issues or glitches. However, unit delay simulation should not be relied upon as the sole or final verification method for a circuit design, as it may not reflect all the aspects and factors that may affect the performance and behavior of a circuit in reality.