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Differential Threshold Voltages used by DDRx Batch Simulation

Question asked by dfurni01 on Dec 17, 2014

I've been using the DDRx Batch Simulation tool (HyperLynx 8.1.1) to simulate an implementation of DDR3 memory, and I noticed that the tool seems to be ignoring the differential thresholds defined in the IBIS model for the Micron memory I'm using, and is using 0V instead.

 

For single-ended signals (address, control, command, data), it seems to be using the correct VIH(AC) and VIL(AC) thresholds (VTTref +/- 160mV in this case) for determining setup and hold times. But for differential signals (clock and data strobes), it seems to be using Vdiff = 0 rather than VIH,diff(AC) and VIL,diff(AC) (+/- 320mV for this part) for determing setup and hold times. In my simulations, there is ~50 ps between Vdiff = 0 and Vdiff = +/- 320mV, so all setup and hold times reported in the results spreadsheets are off by ~50 ps. This is causing some of my data signal setup times to be reported as failures under worst-case conditions, when manual inspection of the waveform files shows that they are actually passing when using the correct differential voltage thresholds for the DQS signals.

 

Is this behavior expected? I checked the "Use all four receiver AC/DC thresholds" option in the DDRx Batch-Mode Wizard setup, and I assumed that they would be used for differential as well as single-ended signals.

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