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