Correlating Low to High-Speed Events in Complex Embedded Systems

Figure 1: A challenge when testing embedded systems
is to correlate events in a low-speed interface like SPI
to events in a high-speed interface like PCIe.

A common requirement when testing embedded systems is to measure the timing between signals with low data rates and those with high data rates. Looking at the functional block diagram of our typical deeply embedded system in Figure 1, we see low-speed serial interfaces like SPI and I2C along with high-speed serial links, like PCIe (often serving as the high-speed serial PHY in our diagram).

Take for example testing the initialization of the system. When power is first turned on, the ROM bios and flash memory initialize program elements that are required by the embedded system’s microprocessor. Once the initialization is complete, the microprocessor has to notify the motherboard via PCIe that it is active and ready to receive data via the high-speed serial bus. This all has to happen within 200 milliseconds. 

How to Use Memory Properly

In a recent post, we addressed setting sample rate for serial data acquisition, but let’s look again at how time per division (time/div), memory length and sample rate all interact, and what you can do to optimize your use of oscilloscope capture memory when setting up your timebase.

Figure 1: Sample rate as a function of time/div for three different memory lengths.
Longer memory extends the range of time/div settings that support the highest sample rate.

TDME Primer: Selecting Sample Rate for Serial Bus Analysis

Figure 1. Sample rate of only four sample points per bit
decodes correctly and lengthens serial bus acquisition.

Teledyne LeCroy supports trigger, decode, measure/graph, and eye diagram (TDME) software options for over 20 serial data standards, and the list is growing. This series will address practical tips for using TDME software successfully, and showcase some examples of applying TDME capabilities to real-world problems.

Given the wide range of protocols supported, you might be curious about how to best choose the oscilloscope sampling rate for a given standard when acquiring serial data signals. The optimal sample rate is determined by three principal factors: 

1) the bandwidth of the signal being digitized by the oscilloscope’s analog-to-digital converter (ADC);

2) the desired duration of the acquisition;

3) what you are going to do with the acquisition.

A Real-World FFT Example

Figure 1: Shown at top right is the output of a
5V switch-mode power supply acquired
with an RP4030 active voltage-rail probe

Performing a fast Fourier transfer (FFT) on an oscilloscope can be likened to driving a car. Just as there are two dominant strains of power-train transmissions, there are two dominant approaches to transferring signals acquired on an oscilloscope from the time domain to the frequency domain. There's the stickshift approach, in which the FFT parameters are set manually, and the automatic approach, in which you let the oscilloscope make decisions for you.

Getting The Most Out Of Your Oscilloscope: Setup

Figure 1: Choosing a effective sample rate is key
to seeing the finer details of a waveform

Today's real-time digital oscilloscopes are so packed with bells and whistles (or "features," if you prefer) that you can forget how to use many of them. In fact, you might not even realize some exist! But they're all there for a reason, and they're all useful, maybe even more so than you know. To that end, we'll take a tour of a typical Teledyne LeCroy oscilloscope's features and give you some pointers as to how, and when, you can best take advantage of them.

Dynamic Range, Signal Integrity, and ESD Pulses

Figure 1: Use your oscilloscope's full vertical range to
take full advantage of the ADC's resolution

Having considered the impact of sampling rate on ESD pulse measurements, let's now turn our attention to dynamic range and signal integrity. No matter what oscilloscope you use, it's important to make the most of the instrument's full vertical range to achieve maximum accuracy.

How Does Sampling Rate Affect ESD Pulse Measurements?

Figure 1: Characterization of an ESD pulse's rise time
depends largely on the oscilloscope's sampling rate

In continuing our look at ESD/EMC pulse measurements, it would be useful to consider how sampling rate figures into the equation. What sort of sampling rate makes sense to use for capturing an ESD pulse? The answer to that question depends primarily on your pulse's rise time.

Taking Best Advantage of Oscilloscopes' Long Memory

Figure 1: Maintaining the maximum sample rate over more
timebase settings is possible with long memory

Two very important considerations when choosing a digital oscilloscope are the length of the acquisition memory and the amount of RAM available for processing of the raw data. Note that acquisition memory and RAM are not the same things, but they are still both important. The amount of acquisition memory often determines the fidelity with which an oscilloscope can record a signal. But that's only the first step; it's the instrument's processing horsepower is the key to finding signal abnormalities and characterizing circuit performance.

Why High Oscilloscope Sampling Rates Matter

Figure 1: Here is an example of aliasing that results from
sampling a signal at less than the Nyquist rate of 2f_max

A key to accurate measurements with an oscilloscope is to ensure that the instrument maintains a high sampling rate. This applies to most measurements; conversely, for many measurements, accuracy may suffer as sample rate decreases. In the worst case, some signal components may be "aliased," meaning that the true signal shape is corrupted by the addition of bogus signal components that arise from undersampling of real signal components.

Oscilloscope Basics: Choosing an Oscilloscope

Figure 1: An oscilloscope such as
Teledyne LeCroy's HDO6054-MS
serves a very broad range of
applications

Choosing an oscilloscope might seem to be a challenging task, but it doesn't have to be. Rather, it's a more-or-less logical process based on your measurement needs. Having said that, if the application for the instrument is "general lab work," the decision can become trickier.

Applying Multi-Stage, Multi-Rate Digital Filtering

Figure 1: The input signal shows both the desired 63-kHz signal
along with a 60-Hz component. Zoom trace Z1 shows the
60-Hz component in detail.

A while back, we posted some basics on how to apply digital filters to sort out signals with undesirable elements riding on top of them, i.e. a square wave that's being corrupted by a sinusoidal signal creeping in from somewhere in your system design. Now, let's look at how to extend the range of cutoff frequencies for digital filters, allowing them to be used even more effectively.

Back to Basics: Random Interleaved Sampling

Figure 1: This image illustrates the
general principle underlying RIS.

Modern oscilloscopes come with all kinds of bells and whistles, and users might be tempted to invoke them for all sorts of situations. But not every whiz-bang feature of an oscilloscope is applicable all the time. Rather, some features are great in the right applications but disastrous in others.

Oscilloscope Basics: Sampling Rate

In a recent overview post on oscilloscope banner specifications,
one of the topics covered is sampling rate. Let's do a somewhat deeper dive on that topic and look at what sampling rate means to oscilloscope users.

An Overview of Oscilloscope Banner Specs

"Banner specs" is a term that oscilloscope makers use often. If you've ever met with one of the vendors' salespeople, you're likely to have heard it. But what are banner specs and what do they mean to you?

The Making of 12-Bit Scope Hardware

In many applications, the accuracy of a true 12-bit oscilloscope is not only desirable, but necessary. Going forward, this will become the case more and more often. When choosing one, it's a good idea to peek under the covers and gain a little insight into how the instrument operates. Having discussed in an earlier post the advantages of oscilloscopes with 12-bit vertical resolution, let's look at the ways in which that resolution is accomplished in hardware.

Oscilloscope Basics: Oscilloscope Bandwidth

Among the most important basic specifications of a digital oscilloscope is its bandwidth. Knowing a bit about bandwidth and the influences on the specification can be very helpful in selecting the right oscilloscope for your application. This post will cover some fundamental aspects of oscilloscope bandwidth.