I’m the kind of engineer that doesn’t like to build more than ten to twenty prototypes before I have something working. I’m joking of course… the first or second prototype should be very close to a working production system. However, when designing anything high speed, I can pretty much guarantee that without simulation (and special layout tools) you’ll be building more than two prototypes! The problem is simulating the circuits with tools designed for circuit accuracy. That is, SPICE (Simulation Program with Integrated Circuit Emphasis) was designed to accurately simulate how a circuit will perform given a defined input stimulus over relatively short periods of time.
This works great for many designs where you are looking at the transient or steady-state response of a circuit. Now imagine trying to prove to your boss that your bit error rate (BER) is better than 10-12. That means only 1 bit error in 1012 bits. Which translates to simulating at least 100 seconds to catch a single bit error – in practice you’d simulate a great deal more such as an entire day of network traffic – a whopping 8.64x1014 bits. If the simulation takes 1 millisecond per bit, you’d be waiting over 27,000 years. I’m not that patient!
Figure 1. Shows an IBIS-AMI model simulation using the impulse response mode (one of two modes) that simulates long run lengths and can predict BER.
IBIS (Input/output Buffer Information Specification) models on the other hand are behavioral using I-V and V-t look-up tables that make simulations extremely fast. The issue with IBIS modeling is (or was) the inability to simulate pre-emphasis, de-emphasis and receive equalization. This falls back to the realm of SPICE due to the analog nature of the system. So to fix this, the IBIS 5.0 specification introduced the Algorithmic Modeling Interface or AMI. This is nothing more than an API for the IBIS model that allows the addition of a dynamic link library (DLL) which is common to many operating systems.
The AMI library is also behavioral in nature emulating the function of an equalizer or repeater with de-emphasis, such as the DS80PCI800, without exposing how it was done – which often is a trade secret. It can also run amazingly fast and allows IC vendors, such as TI, to provide accurate modeling of these systems without waiting for another ice age.
So, if you’ve been tasked with simulating the power consumption of your system during its steady state operation… go with SPICE models. If you’re going for the settling time of an equalizer in your channel or worse, the system bit error rate… IBIS-AMI is your friend. Till next time…