Know your limits

By Patrick Carrier

“A man’s got to know his limitations” … true, and so does a digital bus.  Clint Eastwood’s quote to conclude the movie Magnum Force in 1973 also applies to digital busses.  Of course, back then, signal edge rates were slow enough that most people didn’t have to care too much about signal integrity.  And now, in sharp contrast, we’ve progressed to the point where we try to validate busses down to bit error rate (BER) levels of 10^-15.  And things are moving so fast now that you can’t rely on measurement to know your margins, or your limits.  Even parallel busses like DDR3 require simulation in order to appropriately understand the margins of the bus.  Probing at the pin on such busses yields little more than very ugly waveforms, so simulation allows a unique perspective into the limits of the bus. 

For SERDES busses, this is especially true.  With receiver equalization becoming very common, it is very important to know what the signal looks like inside the IC.  And, in order to ensure the bus will work to the required performance standards, you have to run simulations that predict performance for trillions of bits, and include effects of deterministic and random jitter.

You can read about this in greater detail in my recent article in New Electronics magazine on page 25:

Leave a Reply

This article first appeared on the Siemens Digital Industries Software blog at