Q4VLSI Design
Question
2 marks
Q.4. What are the advantages and disadvantages of dynamic logic? [2]
Answer
- Advantages of dynamic logic: smaller transistor count and layout area per gate (since only the pull-down or pull-up network plus a precharge/clock transistor is needed, not a full complementary static structure), faster switching speed (lower input capacitance and no short-circuit current during switching), and lower power dissipation in many cases since only precharge/evaluate switching activity draws current rather than any static path.
- Disadvantages of dynamic logic: charge leakage and charge-sharing problems (the dynamically-stored charge on the output node can leak away or be redistributed through parasitic capacitances of adjacent nodes if the gate is not evaluated soon enough, corrupting the stored logic value), clocking overhead (requiring precharge and evaluate clock phases, adding design and timing complexity), increased noise sensitivity (a dynamic node has no active driver holding its value during the evaluate phase, making it more vulnerable to noise/coupling), and difficulty cascading multiple dynamic gates directly (requiring special logic families such as domino logic to avoid race/glitch problems between cascaded dynamic stages).