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C-comp and output risetime

Question asked by david.royle on Jun 23, 2011
Latest reply on Feb 7, 2020 by weston_beal

We have discovered that with Hyperlynx 8.0 the output rising edge is dependent on the value of c_comp. From what I understand this is wrong.

We are modeling an Altera Arria IIGX part (LVDS) and if we set c_comp = 0pf then its simulated rising edge matches precisely the measurement made with 12GHz scope (200ps).

However, the Altera IBIS model calls out a 7pf c_ comp which slows the simulated rising edge down to 400ps. In other words it looks like Hyperlynx is double counting c_comp,  its effect has already taken into account in the V-T curve. I thought it was well understood that c_comp needs to be visible from the outside looking in, but invisible from the inside looking out. I have verified that 8.0 causes this behavior with other LVDS drivers as well (DS90LV049). Is this a bug in this particular version?

I have attached a doc that does a reasonable job of describing why c_comp value should not alter launch waveform.