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Codiode/Problems/Sequential Logic

Retiming: Moving a Register to Balance Pipeline Stages

HardLogic CircuitTiming Diagram

To balance a critical path, a register initially placed after 12 units of combinational logic is retimed backward by 5 units. The logic is now split into two balanced 7-unit stages: Stage 1 computes A AND B, and Stage 2 computes Q_MID XOR C. Two positive-edge-triggered flip-flops are used: Q_MID captures the output of Stage 1, and OUT captures the output of Stage 2. Five signals are shown: CLK (clock, period 2), inputs A, B, C, and outputs Q_MID, OUT (both initially 0). Trace Q_MID and OUT across all 12 time steps.

Constraints

  • Both Q_MID and OUT update strictly on the LOW-to-HIGH transitions of CLK.
  • OUT evaluates Q_MID XOR C using the stable value of Q_MID present immediately before the clock edge.

Topics

Flip-FlopsTimingRetimingPipelining

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