Chapter 4_2 Signal Integrity in Digital Design

Transcription

1 Aspectes elèctrics del disseny digital Chapter 4_2 Signal Integrity in Digital Design

2 + ρ V G G L A B L L B G G A + + ρ ρ eflexion At any discontinuity in the signal path there is a reflexion. Also at the ends (generator and load) in case they are not adapted. eflexion coefficient varies between + and -. It is zero when impedance is adapted (no change)

3 eflexion: An example () 6Ω Ω V A ρ A ρ B G G L L + + B Ω

4 eflexion: An example (2) (V) (A) (B) (t p )

5 eflexion: Simulation (3) 5. (V) (A) (B) DATA: l cm Ω t ns/m t r.2 ns t r /t 2 cm n 32 elem (ns)

6 OSC. 5Ω LINE P.G. 5Ω t r < ns TD 2τ V V 2 esolution: x min t t r 25 Ω ;L' τ ;C' τ / V2 V

7 Line Terminations () C (a) 2 (c) C V C T C (b) (d)

8 Clamping Diodes Clamping diodes to absorb reflexions D V clamp D clips above V clamp + V D D 2 D 2 clips below -V D Internal diodes are sometimes unable to absorb the current External diodes must be fast enough. Usually Schottky type

9 Coupled lines () L C L C L L m C m L m C m L m I C L 2 /2 I C /2 I C /2I L 2 L C 2 C 2 -/2 I L L 2

10 Coupled lines (2) In most of practical situations coupling is week. In that case, line Impedance changes only slightly k k m c m 2 m 2 magnetic copupling electrostatic coupling

11 Crosstalk () x Backward crosstalk: SAME POLAITY /2 I C +/2I L /2 I C /2I L backward or forward or near end far end Forward crosstalk: BOTH POLAITIES The two induced currents, capacitive and inductive, add with sign to produce two types ot Crosstalk: Backward and Forward Backward or near end Crosstalk happens at the passive line end corresponding to active line generator end. It has always the same polarity as the active line signal. Forward or far end Crosstalk happens at the passive line end corresponding to the load end of active line. It may have any polarity or even cancel.

12 Crosstalk (2) FOWAD V i V f l V f2 V f V t t ( t ) dd dd r f l par dvi () t dt In forward crosstalk, both interfering and interference signals travel at the same speed and direction, thus interference signal amplitude increases while propagating, adding in phase. This means that induced signal time span is independent of coupled length, depending only on interferent signal edge duration. On the contrary, amplitude depends on both dv/dt and coupled length.

13 Crosstalk (3) V i V b V b V b l t par t l V t di BACKWAD di 2t par dvi ( t) dt In backward crosstalk, interfering and interference signals travel at the same speed and opposite direction, thus interference signal time span increases while propagating. This means that induced signal amplitude is independent of coupled length, depending only on interferent signal dv/dt. On the contrary, duration depends on coupled length.

14 Crosstalk (4) k k m c m 2 m 2 k k m c m m SYMMETIC LINE TO DECEASE COSSTALK: DECEASE MUTUAL COUPLING (L m & C m ) INCEASE DOWN COUPLING (L & C ) MULTIPLY BY (+ρ) AT LINE ENDS!

15 Switching Noise () inductive noise caused by simultaneous switching of currents Depending on: V c N L eq di dt Geometry (L eq ) and distribution (I) of SUPPLYs Geometry (L eq ) and distribution (I) of DATA lines Simultaneity factor (N)

16 Switching Noise (2) Delays False change Alimentació di di st active N th active Distortion Extra delay Latch L st inactive glitch input Crosstalk

17 Switching Noise (3) NOISE COUPLING CAUSE Switching Supply lines Inductive Crosstalk Signal lines Inductive and Capacitive Crostalk and Switching Noise are similar and sometimes difficult to separate

18 How to decrease Switching Noise: Decrease N: Multiple clocks V c N L eq di dt Decrease di: Good Supply distribution Increase dt: Slow down switching Decrease L eq : Geometry