What is propagation delay in digital circuits and why is it important?

Propagation delay is the time taken for a change at the input to appear at the output of a logic gate or circuit. It includes tpLH (low-to-high) and tpHL (high-to-low) delays, which may differ. Propagation delay is crucial because it limits maximum operating frequency (faster clocks than gate delays cause setup/hold violations), affects power consumption (shorter delays often mean higher power), and accumulates in cascaded logic determining critical path. Modern high-speed design requires careful analysis of propagation delays.