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Altera_Forum's avatar
Altera_Forum
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13 years ago

Negative hold time

Hi

i started reading a data sheet and found the external device setup/hold time specified

the specified setup time is positive but the holds time is specified as negative

after the launch clock the time for which the data needs to be stable is defined as a hold time

please explain me why the hold time should be negatively defined for external device

does it mean there is a phase difference between clocks

1 Reply

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    13 years ago

    setup/hold time is always stated as positive with respect to register. But when viewed from pins (and that is practically what you handle) then it go shift such that either can be positive or negative. The shift of timing window is due to delay differences of clock and data within device from register to its pins.