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You need to test, we're here to help.

16 August 2018

More on Quiet-Low I/O Drivers and Ground Bounce

To configure an I/O driver as a quiet-low line, its output is connected directly to Vss on the die
Figure 1: To configure an I/O driver as a quiet-low line, its
output is connected directly to Vss on the die
Ground bounce can plague digital I/O lines with bit errors and turn your hair grey trying to uncover the cause in the process. But there is a trick you can use to make the analysis a little easier: using a quiet-low I/O driver as a sense line to reveal the existence, and magnitude, of ground bounce in your system.

07 August 2018

About Ground Bounce and How to Measure It

Shown are five I/O drivers within a package driving signal lines on a PC board
Figure 1: Shown are five I/O drivers
within a package driving signal
lines on a PC board
Designing and/or troubleshooting a system with, say, an MCU driving signals across transmission lines, can be an interesting exercise in patience and diligent sleuthing. Perhaps you're seeing an inordinate amount of bit errors at the receive end of I/O lines but having some difficulty nailing down the source. In many cases, the problem is ground bounce, an issue that can be tough to diagnose and cure. Let's begin an examination of the ground-bounce phenomenon by explaining how it arises and then outlining an approach for finding it.

23 July 2018

Feed-Forward Equalization

FFE creates a number of delayed versions of the input signal that are then added back to the signal with proper weights
Figure 1: FFE creates a number of delayed versions of the
input signal that are then added back to the signal with
proper weights
In addition to continuous time linear equalization (CTLE), another means of improving signal quality at the receiver end of a high-speed serial data link is known as feed-forward equalization (FFE). In terms of implementation, FFE is not unlike the pre-emphasis filtering that is done on the transmitter side. An FFE implementation looks for all intents and purposes like a digital finite impulse response (FIR) filter.

17 July 2018

Continuous Time Linear Equalization

A CTLE implementation at the receiver end of a serial-data channel seeks to boost higher frequencies while not boosting noise any more than necessary
Figure 1: A CTLE implementation at the receiver end of a
serial-data channel seeks to boost higher frequencies while
not boosting noise any more than necessary
We've been looking at the broad topic of debugging high-speed serial links, and in that context, we're also touching on ways to improve signal performance on the receiver side. One of those ways is to implement continuous time linear equalization (CTLE).

10 July 2018

Serial-Data Channel Emulation and S Parameters

Higher data rates + "same old" channel media = degraded signal quality at receiver
Figure 1: Higher data rates + "same old" channel media
= degraded signal quality at receiver
Serial data rates have risen but propagation media for the channel remain unchanged, and that results in greater attenuation to the frequencies of interest. We could ignore these losses at lower frequencies, but now that rise times are so much faster, that's not an option. Channel effects now intrude into design margins to the point where eyes deteriorate and bit-error rates become unacceptable.

29 June 2018

The Effects of De-Emphasis on Eye Diagrams

This "at the receiver" eye diagram of a serial-data stream shows the effect a lossy channel has on signal quality
Figure 1: This eye diagram of a serial-data stream as measured
at the receiver shows the effect a lossy channel has
on signal quality
Having discussed what transmit pre-emphasis is all about and the various ways in which it's implemented, it might be useful at this juncture to look at some examples of its application and the salutary effect it can have on the signal's eye diagram at the receiver end of the channel. Recall that there are two variations on pre-emphasis: de-emphasis and pre-shoot, which use different taps of a three-tap finite impulse response (FIR) filter to emphasize the last bit or the first bit of a bit sequence, respectively.

27 June 2018

Introduction to Channel Equalization

Transmit pre-emphasis pre-distorts signals in anticipation of the channel's effects
Figure 1: Transmit pre-emphasis pre-distorts signals
in anticipation of the channel's effects
A number of factors can cause degradation of performance in a high-speed serial data link. Among them are inter-symbol interference (ISI) jitter, attenuation, reflections due to impedance mismatches, and insertion losses, to name a few. But fear not: There are techniques one may use to compensate for these losses known as equalization. We'll review the basics of channel equalization in today's post.