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Analysis of Timing Requirements for Flip-Flop Circuit FF2: Set-Up and Hold Times

This document examines the timing analysis for flip-flop FF2 to validate that the set-up time (tSU) and hold time (tH) requirements are satisfied under worst-case conditions. We detail the propagation delays from clock inputs, connectivity, and skew effects. We analyze the equations for tSU and tH, taking into account propagation delays (tPD) from G1 and G2, as well as clock distribution delays. The analysis assumes the worst-case scenario in terms of data transfer and provides insights into potential timing violations due to clock skew.

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Analysis of Timing Requirements for Flip-Flop Circuit FF2: Set-Up and Hold Times

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  1. System Level Timing Analysis

  2. Q Q D D FF2 Required Timing Analyses B FF1 G2 G1 A C CLK TCLK=CLK Period The circuit must be analyzed to show that the set-up (tSU) and hold (tH) times of FF2 are met under worst-case conditions.

  3. Analysis Assumptions • The propagation delay of G1, tPDG1, includes all the delays from CLK to the clock input of FF1: tPDG1 = The connection delay from CLK to G1 input + delay of G1 itself + connection delay from G1 output to FF1 clock input • Similarly for G2 • Similarly, tPDFF1 includes the connection delay from the FF1 output to FF2 input

  4. Set-up Time Analysis tSU@FF2 = TCLK - Slowest Data + Fastest CLK = TCLK - (tPDG1Max + tPDFF1Max) + tPDG2Min tPDDataMax tPDG2Min CLK B C tSU

  5. Hold Time Analysis tH@FF2 = Fastest Data - Slowest CLK = tPDG1Min + tPDFF1Min - tPDG2Max tPDDataMin CLK B C tH tPDG2Max

  6. Hold Time Support Optional delay is matched to the clock distribution delay to make tH = 0 ns.

  7. Clock Skew • If the clock is slower than the data, then the wrong value will be transferred from FF1 to FF2 • tH@FF2 = tPDG1Min + tPDFF1Min - tPDG2Max < 0 If tPDG2Max > tPDG1Min + tPDFF1Min Should latch this Latches this instead CLK tPDDATA B tPDCLK C tH < 0

  8. Using Minimum Delays • In the tSU and tH equations, the minimum delays are questionable • Minimum delays are supplied by some vendors but not all. • If no guaranteed minimum delay value exists, then the correct value to use is 0 ns.

  9. Minimum Delays and Skew • Given that tPDMin=0 should be assumed unless guaranteed otherwise, we have tH@FF2 = tPDG1Min + tPDFF1Min - tPDG2Max < 0 = 0 + 0 – tPDG2Max < 0 which is true for any tPDG2Max > 0.  Circuits should be more carefully examined for skew effects now that parts are getting faster.

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