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System Level Timing Analysis

System Level Timing Analysis. 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 (t SU ) and hold (t H ) times of FF2 are met under worst-case conditions. Analysis Assumptions.

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System Level Timing Analysis

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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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