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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 |


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