I'm trying to decide if the internal programmable output termination (UNTUNED_50) of the spartan XC6SLX45-2CSG324I if sufficient for our design, or an external termination is needed.

The traces from the fpga pin (driver) to the IC (receiver) are relative long ~30cm, so reflections will occur and be a problem without source termination.

I have looked at the IBIS model spartan6 IBIS for the driver LVCMOS18_S_8_LR_33 the output termination resistance is calculated to: min=41, typ=50, max=64. Which I would consider good.

But looking in the datasheet ds162 on page 5, the UNTUNED_50 has very wide range 21 to 96 ohm, which would give significant ringing.

So two questions:

  1. Why is there so big difference between the IBIS model, and the datasheet ?
  2. When should I generally not use the internal output termination ?

1 Answer 1


The datasheet values are across the recommended operating conditions (-40C to +100C Tj for industrial grade); IBIS models are usually at 25C and do not account for large variations in temperature.

This is a common issue in models, as the typical (not guaranteed) values are used.

If you expect to see a wide range of temperatures, use the values from the datasheet, as these are the only values that are guaranteed (if you have problems and used the IBIS values, Xilinx will not help you as those values are not guaranteed).

If you have a need for precise output terminations, then do not use the internal terminators.

  • \$\begingroup\$ According to the IBIS model the temperature range is 0 to 85 Celsius. Which would be a ok range. The IBIS model for the 2.5V is a lot worse than 1.8V, so could the big variation come from all supported voltages. ? \$\endgroup\$
    – JakobJ
    Jan 16, 2017 at 10:31
  • 1
    \$\begingroup\$ As the guaranteed range is across all operating conditions, then supported voltages will also have an effect. \$\endgroup\$ Jan 16, 2017 at 10:33
  • \$\begingroup\$ Ok, so when using 1.8V LVCMOS and temperature range 0 to 85 Celsius, are my assumptions correct, that the output driver resistance is between 41 to 64 ohm ? \$\endgroup\$
    – JakobJ
    Jan 16, 2017 at 10:37

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