Re: [SI-LIST] : Termination scheme for SSTL2 Class II

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From: D. C. Sessions (si-list@lumbercartel.com)
Date: Tue Feb 27 2001 - 05:30:53 PST


On Monday 26 February 2001 04:36, selvaraj subramanian wrote:

# I am having SSTL2 Class II logic in my design. As per the standard, a series termination of 25ohms and a pull up value of 25ohms to 1.25V at the far end is recommended when the characteristic impedance is 50ohms. I am simulating bidirectional (data) SSTL2 Class II lines. But my simulation using XTK is not showing correct waveforms. But if I use a double termination scheme of 25 ohms series termination and a pull up value of 50ohms to 1.25V at both the ends (near & far), the simulation is showing the expected results.
#
# But my doubt is that if the driver is bidirectional, is it compulsory to use double termination scheme?.
#
# If anybody has worked in this, please let me know which one is correct and if not let me know the correct scheme.

You need to absorb the excess wave energy opposite to each driver.

For a line with only two devices, this can be done with series dampers;
if the line is short and timing noncritical, you can use one resistor midline
with pretty fair success.

For less forgiving two-device connections you can shunt terminate to Vtt
at each end, but the series stub dampers aren't necessary except to limit
current.

The series stub dampers (25 ohm series resistors) are really only necessary
in multidrop environments such as main memory.

-- 
| The race is not always to the swift, nor the battle to the strong. |
| Because the slow, feeble old codgers like me cheat.                |
+--------------- D. C. Sessions <dcs@lumbercartel.com> --------------+

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