App Note Test | Electro-Meters

0
RFQ 0

International

Back

Digital Oscilloscopes
Waveform Generators
Multimeters & Data Acquisition
DC Power & Loads
Spectrum Analyzers
RF Signal Generators
Accessories
Applications
Support

RIGOL – Innovation or nothing Page | 1

Introduction

Embedded design and especially design

work utilizing low speed serial signaling is one of

the fastest growing areas of digital electronics

design. The need to communicate between

modules, FPGAs, and processors within a wide

array of consumer and industrial electronics is

increasing at an astounding rate. Customized

communication protocol and bus usage is critical

to design efficiency and time to market, but

comes with the risk of being sometimes difficult

to analyze and debug. The most common sources

and types of problems when using low speed

serial data in an embedded application include

timing, noise, signal quality, and data. We will

recommend debug tips and features available in

modern oscilloscopes that will make debugging

these complex systems faster and easier.

Types of Errors

Timing

Timing is critical in any serial data system,

but finding the system timing limitations related to

components, transmission length, processing time,

and other variables can be difficult. Let’s start with a

simple 16 bit DAC circuit. First, make sure you

understand the data and timing specifications for the

protocol in use. Does it sample data right on the clock

edge? How far off can the clock and data be when we

still expect good data? In other words: do we have a

clock sync error budget defined? Once we understand

these timing requirements then we can

experimentally verify both the Tx and Rx hardware

subsystems. Now we can analyze the system level

timing delays and the overall accuracy of the

conversions because we can make direct

measurements of both the logic and analog channels

in a time correlated fashion. We will also be able to

simultaneously view the decoded bit patterns

numerically on an oscilloscope that comes in well

below your budget limits.

Here is a simple example of measuring a bit

on channel 2 (blue) that is driving the DAC output

that is creating the Sine wave on channel 1 (yellow).

Meeting Embedded

Design Challenges

with Mixed Signal

Oscilloscopes

RIGOL – Innovation or nothing Page | 2

Utilizing parallel bus decoding (Figure 1) we

can get a quick look at the transitions of this single

line. But this doesn’t give us all the information we

need since the DAC is utilizing a number of data lines

to set its output level. Getting more complete data

requires a different approach. Let’s move all the DAC

lines (Figure 2) over to the MSO’s digital inputs. Now

we can see how the digital lines really coordinate

with the DAC output. To investigate further we can

simplify the decoding to show Hex values (Figure 3)

and zoom in so we can view the decoded data.

Now we can use the zoom feature to clearly

see the relationship between the bit and DAC

transitions (Figure 4). For the zoom we have turned

on analog channel 2 in blue that is on the DAC clock.

Zoomed in by a factor of 500x from 50 usec per

division to 100 nsec per division allows us to see that

the bit transitions are occurring 100 nsec before the

clock transition. The clock transitions in under 5 nsec

and the DAC output starts changing in sync with the

clock. We can also utilize the scope cursors to make

the timing in the transitions clearer and well defined

(Figure 5).

Verifying timing in mixed signal systems can

be made easier with the right tools. Select a modern

scope with the correct set of channels and options to

make sure you can easily view what you need on the

display when you need it. From digital buses to

processing delays, get the full picture of the device’s

operation and delve into details as needed to verify

timing issues on your device.

We can also trigger on the digital patterns

instead of the analog signal (Figure 6). Triggering on

a digital pattern can be critical for debugging when

there is a problem. There isn’t always a good way to

track events from the analog side of a system. When

using a digital trigger method make sure to set the

additional trigger parameters. These may include

start bits or even address and data for some

protocols. Even for a simple parallel bus like this you

need to define and arrange the channels in the bus

for the results to be easiest to interpret.

Accurate timing of Low Speed Serial signals is

critical to system stability. Therefore, making sure

your measurement tools are up to the task of precise

and easy triggering, monitoring, and analyzing of

Figure 1: DAC output and input bit on a RIGOL MSO5354

Digital Oscilloscope

Figure 2: DAC output and 8 bit input bus on a RIGOL

MSO5354 Digital Oscilloscope

Figure 3: DAC output and 8 bit input bus with Hex decode on

a RIGOL MSO5354 Digital Oscilloscope

Figure 4: DAC output and 8 bit input bus zoomed in on a

RIGOL MSO5354 Digital Oscilloscope

RIGOL – Innovation or nothing Page | 3

your waveforms is vital to improved R&D efficiency

and ultimately time to market.

Noise

One of the most common issues in correct

serial data measurements is the handling of system

noise. Noise in these measurements can come from a

number of sources including poor grounding,

bandwidth issues, crosstalk, electromagnetic

immunity (EMI) problems. Sometimes the problem is

in the device, but improved probing and

measurement techniques can also improve the

results significantly without changing the device

under test. A good first step is always to make sure

we are using best measurement practices.

Here is a decoded I2C bus segment using a

5000 series oscilloscope (Figure 7). In the first

example we have extremely poor grounding on our

probes. Because the scope’s ground is tied directly to

power ground signals that need to float or simply use

a different or noisy ground plane can cause results

like this. It is also possible for high current draw

through ground in local power to create ground loops

that can cause noise to be injected in your system.

We solve these problems in order from easy

to difficult. First, we can look at our probe

connections. Normally, we would use the alligator

clip ground strap that connects on the probe to make

a ground connection. Assuming we are doing that

correctly and still having a problem we may need to

use the ground spring instead. The ground spring

connects closer to the probe tip and significantly

reduces the loop area of the connection. This can

significantly improve noise and signal quality

(Figure 8) especially for high speed signals or signals

sensitive to capacitance or coupled voltages. All

RIGOL probes come with both the standard ground

strap and the ground spring for these types of

measurements.

If ground noise is still an issue, try isolating

your device from ground. The scope operates best

grounded to AC power ground via the plug. If the rest

of the device or system being tested can be isolated

from ground this eliminates ground loops. If ground

noise is still an issue you may consider a differential

probe like the RP1100D (Figure 9) which enables

measurements without reference to ground on the

Figure 5: DAC output and 8 bit input bus zoomed in with

cursors shown on a RIGOL MSO5354 Digital Oscilloscope

Figure 6: DAC output and 8 bit input bus triggered by a

digital pattern on a RIGOL MSO5354 Digital Oscilloscope

Figure 7: I2C clock and data with noise from poor grounding

in a color graded display using a RIGOL MSO5354 Digital

Oscilloscope

Figure 8: I2C clock and data with ground noise improved in a

color graded display using a RIGOL MSO5354 Digital

Oscilloscope

RIGOL – Innovation or nothing Page | 4

scope. Differential measurements may be the only

way to clearly view some low speed serial data such

as a LVDS bus (Low Voltage Differential Signaling).

Buses like this purposely move the reference line to

maximize bandwidth and increase communication

distances, but it may require true differential probing

or the use of multiple channels of your scope together

to view the signal correctly. RIGOL has several

different probe types for these measurements

including the RP1000D series differential probes

typically used for high voltage floating applications

and the RP7150 1.5 GHz differential probe (Figure

10) for high speed data applications.

Now that we have improved our signal to

noise ratio by decreasing noise injected from the

ground, we can turn our attention to bandwidth

filtering. High frequency noise can also enter your

measurements via channel to channel cross talk or

other high frequency sources nearby or within your

device. One way to address this is to utilize the

channel bandwidth limits (Figure 11). Every RIGOL

scope channel can limit the bandwidth to the ADC. A

20 MHz limit is pretty standard. Some scopes will

have higher options as well.

Additionally, there are a few acquisition mode

and triggering settings that can improve performance

in the face of noise. Many trigger types have a menu

item allowing you to turn on noise rejection for the

triggering scheme. The 5000 series even includes

HFR and LFR (high and low frequency rejection) as

options in how to couple the signal triggered on. The

5000 series comes with High Res or High Resolution

mode (Figure 12). This feature uses extra

oversampling that is being done behind the scenes on

many measurements to provide an average that

results in less noise. This is best to use if you are able

to set the sampling to take at least 200 samples per

time division. This will average rather than reject

high frequency signals, so be sure to understand your

potential error sources and how they may interact

with your measurement setup. Finally, the 5000

series scope also has an NRJ (noise reject) feature

directly within the trigger menu. This removes noise

that appears in bursts and can be set in time rather

than frequency.

To further isolate and locate sources of noise

within your system you may want to focus on EMC or

Figure 9: RP1100D 100 MHz Differential Probe

Figure 10: RP7150 1.5 GHz Differential Probe

Figure 11: I2C clock data with reduced noise using

bandwidth limit on a RIGOL MSO5354 Digital Oscilloscope

Figure 12: I2C clock and data using high resolution mode on

a RIGOL MSO5354 Digital Oscilloscope

RIGOL – Innovation or nothing Page | 5

EMI related issues. To further investigate these error

sources please download the EMC Precompliance app

note for use with the Rigol DSA815 Spectrum

Analyzer at http://www.rigolna.com/EMC

Noise is always a concern when working with

low speed serial signals. By definition these signals

continue to go to higher speeds, more advanced

encoding, farther transmission distances, and lower

voltage and power levels. All of these trends make

hardware more susceptible to noise. Making careful

measurements that limit or eliminate adding noise to

our system then enables us to focus on noise in the

system that may still cause long term design issues.

Signal Quality

Monitoring and improving the quality of low

speed serial signals is a critical part of the debugging

process. Issues like impedance mismatches,

bandwidth, and loading errors can all effect the

quality of signals even when noise isn’t present. Now

that we are looking more closely at the exact nature

of these signals it is important to verify the way we

are using our oscilloscope for these tests. For signal

quality tests we will be using the analog channels

because they provide the best look at what is actually

occurring with our signals. This requires some

additional forethought. To clearly see data

transitions, we should definitely use a sampling rate

that is as high as possible. Sampling at 5x the bit rate

of the digital bus should be considered the minimum

because of the high frequency components that we

need to visualize. Sampling at 10 times the bit rate

should enable us to see any of the issues. But when

we decode the signal the scope likely uses a subset of

the full memory data to handle the decode analysis.

This can be important because you don’t necessarily

want to decode being performed at too high of a rate.

That can mask problems you will find when a more

nominal receiver is used to decode the data. On

RIGOL scopes the decode is done on 1 Mpts of

memory spread across the acquisition. By setting the

memory depth and the time per division the user can

determine whether they want the decode to be done

directly from the analog points or from a subset.

Decoding is also shown across the display region. To

capture more decoded bytes than you can view on

the display use the event table function (Figure 13).

RIGOL – Innovation or nothing Page | 6

You can also export the table results to a text file

from the event table menu for record keeping or

offline timing analysis.

Here is an example table of how the memory

depth, time per division, and sample rate effect the

actual decode sample rate on a MSO5000 Series.

Based on your serial data speeds and the receiver you

will eventually use to collect the serial data you can

optimize your serial decode rate.

Sample

rate (per

Memory

Depth

(pts)

Analog

Sample

Rate

(per s)

Time Per

Horizontal

Division of

Display

Decode

Sample

Rate on

Analog

Channel

(per s)

Ideal for

Decoding this

Speed (bits

per s) based

on 5x

oversampling

on Analog

Channels

12.5K

100K

1 sec

100K

20K

125K

10M

1 sec

100K

20K

250K

25M

1 sec

80K

16K

625K

50M

1 sec

100K

20K

1.25M

100M

10M

1 sec

100K

20K

2.5M

200M

20M

1 sec

100K

20K

12.5K

100K

1 msec

100K

20K

125K

10K

1 msec

200K

1.25M

100K

10M

1 msec

10M

12.5M

100M

1 msec

100M

20M

125M

10M

1 msec

100M

20M

250M

25M

1 msec

80M

16M

500M

50M

1 msec

80M

16M

100M

1 msec

80M

16M

200M

1 msec

40M

12.5M

100M

1 usec

100M

20M

125M

10K

1usec

200M

100K

1 usec

1.6G

1 usec

1.6G

10M

1 usec

800M

160M

25M

1 usec

320M

64M

50M

1 usec

160M

32M

100M

1 usec

80M

16M

200M

1 usec

40M

Now that we have set and verified our

sampling times for best analog and decoding results,

we also want to set our display up for optimal

triggering conditions. When triggering on the rising

edge of an analog signal make sure to keep the trigger

level at least 1 division away from the signal low

state. This separation allows for consistent triggering

action without any false triggers. When visualizing

digital signals with the analog channels use more

screen real estate when possible. Using about 2

vertical divisions and about ½ to 1 horizontal

division per decode character will allow you to see

any major overshoot or impedance issues as well as

Figure 13: I2C clock and data event table view using a

RIGOL MSO5354 Digital Oscilloscope

Table of memory depth, time per division, sample rate,

decode sample rate, and optional decode speed based on

sample rate on a RIGOL MSO5000 Series Digital Oscilloscope

RIGOL – Innovation or nothing Page | 7

some of the other types of error we will be looking at.

Here is the setup (Figure 14) I prefer to monitor

decoded data on a bus like RS232.

On a more complex bus like I2C we view both

clock and data lines on the screen. The timing

correlation between multiline buses is, of course,

vital to successful decoding. Making critical

measurements on the screen like risetime and

overshoot for each line makes reliability tests simple

to setup. We can view the measurements in max/min

or using standard deviation notation for more

advanced statistical testing (Figure 15).

In addition to the risetime and overshoot for

the 4 serial bus lines you can also see the jitter in the

clock when compared to the data transition. This test

device appears to have the clock transition occur

about 1.6 microseconds of jitter on the clock when

viewed, as above, in reference to the triggering data

transition. In (Figure 16) we zoom in on the data

transition to get a more accurate measurement of the

risetime and overshoot.

Signal quality encompasses many of the types

of issues you find on LSS buses. Efficient debug

means making the most of your embedded analysis

capabilities to find signal discrepancies that can lead

you to design changes as early as possible in your

design process. A mixed signal oscilloscope is the

perfect tool for measuring signal quality (Figure 17)

from risetime variance to ASCII packet data.

Data

The key to any Low Speed Serial application

is the ability to quickly and easily look at the data

being transmitted. This means adding the capability

to do embedded decoding on your oscilloscope.

Decoding affects both the triggering and display on

the scope. It adds a decoded bus display to the

instrument’s screen. You can decode values as ASCII

or as hex, octal, or binary data depending on what

you want to look at. You can also trigger on these

values to make sure you are looking at the packets of

most interest to you (Figure 18). In addition to

triggering on these signals with the decode specific

trigger you are also able to trigger on any type of

signal with a zone trigger which allows you to not

only to trigger on any type of signal but also exclude

any unwanted noise or data from a signal. These are

Figure 14: RS232 optional decoded data view settings using

a RIGOL MSO5354 Digital Oscilloscope

Figure 15: I2C clock and data min/max signal measurements

using a RIGOL MSO5354 Digital Oscilloscope

Figure 16: I2C zoom in on data risetime measurement using

a RIGOL MSO5354 Digital Oscilloscope

Figure 17: I2C data measurements with standard deviation

using a RIGOL MSO5354 Digital Oscilloscope

RIGOL – Innovation or nothing Page | 8

created by simply drawing a rectangle on the screen

of the instrument (Figure 19).

If you are interested in timing between

packets or evaluating more than one or two

consecutive packets of data, use the event table mode

(Figure 20) to generate a list of data packets on a

wider time base. Additionally, with using an event

table you are easily able to move around on the

decoded signal by stopping the instrument from

sampling and then select your desired trigger point in

the packet’s submenu on the event table then press

the Jump To button to move to the desired trigger

point on the captured signal (Figure 21).

You can always utilize the zoom function to

see the signals and data from an individual packet in

that series (Figure 22).

Another way to view this signal is by using

the search and navigation menu. This allows you to

view multiple trigger points and easily move around

on the signal when the scope has stopped scanning.

All of the trigger points are represented on the search

and navigation menu and they correspond to the

white triangles at the top of the screen. The trigger

point that is highlighted on the table corresponds

with the red triangle at the top of the screen (Figure

23).

To view how decoded segments, differ over

time or compare between triggered events when

other signals might be affecting the results the best

analysis method is often to use record mode. RIGOL’s

record mode enables you to capture thousands of

frames around a trigger event. Once captured, you

can use pass/fail or a trace difference analysis mode

to visualize changes from frame to frame (Figure

24).

These recordings can be stored and played

back as a movie as well, but the analysis features let

you search for failures or outliers while also viewing

decoded data for comparison (Figure 25).

Data errors as well as the debugging process

are always closely tied to the protocol and its

specifications. To be efficient with your test

equipment make sure you are utilizing the best

analysis method to easily view the data you need to

see without extraneous results getting in your way.

Figure 18: RS232 optimal decoded data view settings using a

RIGOL MSO5354 Digital Oscilloscope

Figure 19: A zone trigger is added to exclude part of the

RS232 signal.

Figure 20: RS232 event table capture of multiple

transmissions using a RIGOL MSO5354 Digital Oscilloscope

Figure 21: RS232 Event used to move around on the signal

by using the Jump To soft key.

RIGOL – Innovation or nothing Page | 9

Keys to Look Out For

Proper Oversampling & Bandwidth

As discussed, proper sampling is critical to

making correct measurements as well as completely

debugging your low speed serial signal. A good rule

of thumb for analog signals is 5x the bandwidth of the

signal you want to measure. This limits your risetime

error to about 2%. To view the best detail on high

frequency signal components set up your scope to

achieve 5-10X over sampling as well. When digital

signals this means sampling 5 times in the width of

one bit. When sampling on digital lines or for

sampling that will be used for decoding oversampling

is less important but set up your measurement device

so it is as similar as the LSS receiver you will

ultimately be using. This gives you the best chance to

focus on material errors that will cause problems

down the road.

Grounding, Noise, and Differential Signaling

Proper probing and understanding the use of

differential vs. ground referenced signals is

important to debugging. If your data lines are not

ground referenced make sure to understand the

impact of ground loops and ground coupled noise on

your measurements. Use proper probe techniques

and advanced noise cancelling features on the scope

to limit noise sources. If necessary, add differential

probes to your measurement system to improve

measurement quality.

How to Best View Low Speed Serial Signals

There are a number of methods for analyzing,

viewing, and evaluating LSS bus activity on a modern

oscilloscope. The best way differs depending on

whether you want to look at a single bit transition for

noise, speed, or synchronization; whether you want

to look at a complete packet of data; or if you want to

compare packets and packet timing over a longer

time period. Make sure your bench tools allow you to

see everything you need and familiarize yourself with

features like zoom, record mode, search and

navigation, event tables, deep memory, and

automatic measurements as well as how they interact

and how best to transition between them when

considering your test plan. Ideally, an oscilloscope

empowers you to view all the results you need and

Figure 22: RS232 packet view using zoom mode on a RIGOL

MSO5354 Digital Oscilloscope

Figure 24: I2C with pass/fail analysis enabled using a

RIGOL MSO5354 Digital Oscilloscope

Figure 25: I2C transmission record mode in playback using a

RIGOL MSO5354 Digital Oscilloscope

Figure 23: Using Search and Navigation feature to move

around on the RS232 signal.

RIGOL – Innovation or nothing Page | 10

quickly switch modes to acquire additional

information.

Conclusions

Embedded design and debugging of digital

data is a growing test requirement in a broad range

of consumer and industrial applications. Having the

right mixed signal oscilloscope can make viewing,

analyzing, and resolving issues including timing,

noise, signal quality, and data easier and faster. This

improves engineering efficiency and time to market.

RIGOL’s line of UltraVision II enabled oscilloscopes

comes with standard or optional capabilities for the

methods and measurements discussed here and are

powerful benchtop instruments that provide

uncompromising performance at unprecedented

value.

For more information on our oscilloscopes

please go to rigolna.com or contact us directly at

applications@rigoltech.com or call us toll free at 877-

4-RIGOL-1.

RIGOL Technologies USA

8140 SW Nimbus Ave.

Beaverton, OR 97008

877.474.4651

Download the PDF