What are the advantages and disadvantages of dynamic logic over static logic in IC design?

1 Dynamic logic basics

Dynamic logic circuits use capacitors to store logic values temporarily. A capacitor is a device that can store electric charge, and its voltage represents the logic value. A dynamic logic circuit has two phases: precharge and evaluation. In the precharge phase, the capacitor is charged to a high voltage, corresponding to logic 1. In the evaluation phase, the capacitor may discharge through a network of transistors, depending on the input signals, corresponding to logic 0. Dynamic logic circuits can implement complex functions with fewer transistors than static logic circuits.

2 Static logic basics

Static logic circuits use transistors to maintain logic values indefinitely. A transistor is a device that can act as a switch, allowing or blocking the flow of current. A static logic circuit has a direct path from the power supply to the output for both logic 0 and logic 1. Static logic circuits can implement simple functions with fewer capacitors than dynamic logic circuits.

3 Speed advantage of dynamic logic

One of the main advantages of dynamic logic over static logic is speed. Dynamic logic circuits can operate faster than static logic circuits because they have less parasitic capacitance and resistance. Parasitic capacitance and resistance are unwanted effects that slow down the signal transitions and increase the power dissipation. Dynamic logic circuits have less parasitic capacitance and resistance because they have fewer transistors and wires. Dynamic logic circuits can also achieve higher fan-out, which means they can drive more downstream circuits without degrading the signal quality.

4 Power disadvantage of dynamic logic

One of the main disadvantages of dynamic logic over static logic is power consumption. Dynamic logic circuits consume more power than static logic circuits because they have more switching activity and leakage current. Switching activity is the amount of charge and discharge cycles that occur in a circuit. Leakage current is the amount of current that flows through a transistor even when it is supposed to be off. Dynamic logic circuits have more switching activity and leakage current because they have to precharge and evaluate the capacitors every cycle, and because they have less noise margin, which means they are more sensitive to voltage variations.

5 Noise disadvantage of dynamic logic

Another disadvantage of dynamic logic over static logic is noise susceptibility. Noise is any unwanted signal that interferes with the correct operation of a circuit. Noise can be caused by external sources, such as electromagnetic interference, or by internal sources, such as crosstalk or glitches. Dynamic logic circuits are more susceptible to noise than static logic circuits because they have lower noise immunity and higher noise generation. Noise immunity is the ability of a circuit to reject noise without changing its output. Noise generation is the amount of noise that a circuit produces and propagates to other circuits. Dynamic logic circuits have lower noise immunity and higher noise generation because they have smaller voltage swings, which means they are closer to the threshold voltage of the transistors, and because they have more feedback loops, which can amplify noise.

6 Complexity disadvantage of dynamic logic

A final disadvantage of dynamic logic over static logic is complexity. Dynamic logic circuits are more complex than static logic circuits because they require more design and verification efforts. Dynamic logic circuits require more design efforts because they have more timing constraints, which means they have to meet certain deadlines for the precharge and evaluation phases, and because they have more layout constraints, which means they have to avoid unwanted coupling and interference between the capacitors and the transistors. Dynamic logic circuits require more verification efforts because they have more functional and reliability issues, such as charge sharing, charge leakage, clock skew, and noise sensitivity.

7 Here’s what else to consider

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