BeginnerQuestion 5 of 40Source PDF page 8

What are the four categories of timing paths (reg2reg, in2reg, reg2out, in2out)?

From PDVerse STA Mentor Guide · pdVerse Mentor Guide

Short Answer

These four categories describe where a path begins and ends: reg2reg (flip-flop to flipflop), in2reg (input port to flip-flop), reg2out (flip-flop to output port), and in2out (input port straight through to output port).

Example

A reg2reg path goes CK-to-D between two flops through combinational logic; an in2reg path starts at a chip input pin and ends at a flop's D pin; a reg2out path starts at CK and ends at an output port; an in2out path feeds straight from an input port to an output port with no flops in between.

Why It Matters

Each category needs different constraints to be checked correctly — for instance in2reg and reg2out paths depend on input/output delay values from outside the chip, while reg2reg paths depend purely on the internal clock relationship.

Mentor Note — Common Mistake

Treating all paths as if they were reg2reg — forgetting that in2out feedthrough paths exist means you might miss constraining a path that has no flip-flop at either end.

Visual explanationThe four timing-path categories
The four timing-path categoriesFour rows show input-to-register, register-to-register, register-to-output, and input-to-output paths.in2regFFreg2regFFFFreg2outFFin2outPort → FFFF → FFFF → PortPort → Port

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