If I have an 8-bit high-speed interface say at 1GHz talking to memory and I want to do interface testing, I can send patterns like AA/55/00/FF.

When I send a pattern like 55 i.e 01010101, every even bit sees a rise transition.

In the case of AA - 10101010, every odd bit sees a rise transition.

In short, I can exercise maximum toggle per bit to see rise/fall delay effects per bit using a pattern like A5 or 5A.

From an electrical perspective does the use of FF and 00 patterns give any additional benefit?


2 Answers 2


Both patterns are necessary.

AA/55 causes no net bus current in the transitions, pairs of lines are effectively differential, so there's no ground current in either driver or driven ICs. Alternating FF/00 causes maximum ground pin current, so is needed to test ground bounce.

If the tracks run parallel, then AA/55 excites them in odd mode, with capacitance to ground and the mid line between tracks, making them effectively CPWG traces, lower impedance than microstrip. FF/00 excites them in common mode, so capacitance to ground only, and an even higher impedance than equivalent microstrip. It's worth testing signal integrity with both extremes of effective impedance.

  • \$\begingroup\$ FF & 00 will have a maximum ground pin current when the current bus state if FF and the next pattern is 00 and vice versa? \$\endgroup\$
    – user22348
    Commented Oct 15, 2020 at 5:43
  • 2
    \$\begingroup\$ @user22348 Yes, it's the alternation between them \$\endgroup\$
    – Neil_UK
    Commented Oct 15, 2020 at 6:03
  • \$\begingroup\$ @Niel_UK, Thank you. \$\endgroup\$
    – user22348
    Commented Oct 15, 2020 at 6:16

From an electrical perspective AAh and 55h are preferred to test a worst case cross-talk in addition to delay effects. Testing other patterns is also beneficial for instance FFh to 00h will consider thermal stress better than AAh to 55h.


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