Chapter: VLSI Design : Combinational and Sequential Circuit Design

Sequencing Static Circuits

1. Combinational logic: – output depends on current inputs 2. Sequential logic: – output depends on current and previous inputs

SEQUENCING STATIC CIRCUITS

q Combinational logic

– output depends on current inputs

q Sequential logic

– output depends on current and previous inputs

– Requires separating previous, current, future

– Called state or tokens

– Ex: FSM, pipeline

§ If tokens moved through pipeline at constant speed, no sequencing elements would be necessary

§ Ex: fiber-optic cable

§ Light pulses (tokens) are sent down cable

§ Next pulse sent before first reaches end of cable

§ No need for hardware to separate pulses

§ But dispersion sets min time between pulses

§ This is called wave pipelining in circuits

§ In most circuits, dispersion is high

§ Delay fast tokens so they don’t catch slow ones.

§ Use flip-flops to delay fast tokens so they move through exactly one stage each cycle.

§ Inevitably adds some delay to the slow tokens

§ Makes circuit slower than just the logic delay

§ Called sequencing overhead

§ Some people call this clocking overhead

§ But it applies to asynchronous circuits too

§ Inevitable side effect of maintaining sequence

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VLSI Design : Combinational and Sequential Circuit Design : Sequencing Static Circuits |