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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

§   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 |