AR# 11308: Virtex/-E/-II/-II Pro, Spartan-II/-IIE, Spartan-3 - Is there hysteresis on the clock/data inputs? What is the value?
Virtex/-E/-II/-II Pro, Spartan-II/-IIE, Spartan-3 - Is there hysteresis on the clock/data inputs? What is the value?
Is there hysteresis on Virtex, Virtex-E, Virtex-II, Virtex-II Pro inputs?
(Hysteresis refers to the required change in input voltage level that is needed to produce a change in the logic.)
If an input voltage moves through the transition point (~1/2 VCCO) and a little beyond, the logic changes from a 0 to a 1. The input voltage must move back through the center and below by more than the "hysteresis spec" for the logic to change from a 1 back to a 0.
A small amount of hysteresis makes the part more robust and more resistant against SI problems (ground bounce, noise).
Yes. A Virtex-E, Virtex-II, Virtex-II Pro or Spartan-IIE, Spartan-3 single-ended I/O (LVTTL, LVCMOS) clocks/data input has ~100 mV of hysteresis.
In Virtex or Spartan-II, hysteresis for LVTTL is ~150 mV; for LVCMOS, it is ~100 mV.
NOTE: Hysteresis does not apply to differential I/O inputs (inputs with differential Op Amp) such as HSTL and LVDS. These inputs use a comparator to guarantee the Vih/Vil level; the hysteresis on these inputs is very small (less than 10 mV).