USBee DX Mixed Signal Oscilloscope, Logic Analyzer and Bus Analyzer

This section details the operation of the Logic Analyzer and Oscilloscope application that comes with the USBee DX, also known as a Mixed Signal Oscilloscope, or MSO.  Below you see the application screen after startup.

The USBee DX Mixed Signal Oscilloscope functions as a standard Digital Storage Oscilloscope combined with a Digital Logic Analyzer, which is a tool used to measure and display analog and digital signals in a graphical format.  It displays what the analog and digital input signals do over time.  The digital and analog samples are taken at the same time and can be used to debug mixed signal systems.

The USBee DX Logic Analyzer / Oscilloscope application also includes embedded bus decoders and the PacketPresenter which extracts bus data packets from the logic analyzer traces and displays them in an easy to interpret format for faster debugging.

Mixed Signal Oscilloscope/Logic Analyzer Specifications

[1] Maximum sample rate depends on your PC hardware CPU speed, USB 2.0 bus utilization and number of channels selected.   

For the fastest possible sample rates, follow these simple steps:

• Disconnect all other USB devices not needed from the PC

• Do not run other applications while capturing or generating samples.

[2] Maximum buffer size depends on your PC available RAM at the time the application is started.  Each sample requires 4 bytes of RAM (16 bits for the 16 digital lines and 8 bits each for the 2 analog channels)

Features

Setup Configuration

The MSO can capture 16 channels of digital and 2 channels of analog at the same time.  All of the captured data is streamed over the USB bus to your PC to be stored in the RAM of the PC.  In order to optimize the sample bandwidth you can choose to see only the channels of interest to you.

The configurations available are as follows:

To select a configuration, click Setup on the menu and select the configuration of your choice.  Below are examples of the application in various modes.

16 Digital–2 Analog Channels  8 Digital–0 Analog Channels

8 Digital–1 Analog Channels    0 Digital–2 Analog Channels

Signal Names

To change the names shown for a signal, click on the signal name and enter a new name.

Pod Status

The MSO display shows a current USBee DX Pod Status by a red or green LED.  When a USBee DX is connected to the computer, the Green LED shows and the list box shows the available Pod ID List for all of the USBee DX’s that are connected.  You can choose which one you want to use.  The others will be unaffected.  If a USBee DX is not connected, the LED will glow red and indicate that there is no pod attached. 

If you run the software with no pod attached, it will run in demonstration mode and simulate data so that you can still see how the software functions.

Acquisition Control

The MSO captures the behavior of the digital and analog signals and displays them as “traces” in the waveform window.  The Acquisition Control section of the display lets you choose how the traces are captured.  Below is the Acquisition Control section of the display.

When the MSO is first started, no acquisition is taking place.  You need to press one of the acquisition buttons to capture data.

The Run button is the Run/Stop control.    This Run mode performs an infinite series of traces, one after the other.  This lets you see frequent updates of what the actual signals are doing in real time.  If you would like to stop the updating, just press the Stop button and the updating will stop.  This run mode is great for signals that repeat over time.

The Single button captures a single trace and stops.  This mode is good for detailed analysis of a single event, rather than one that occurs repeatedly.

The Buffer Size lets you select the size of the Sample Buffer that is used.  For each trace, the buffer is completely filled, and then the waveform is displayed.  You can choose buffers that will capture the information that you want to see, but remember that the larger the buffer, the longer it will take to fill.

You can also choose the Sample Rate that you want samples taken.  You can choose from 1Msps (samples per second) to up to 24 Msps.  The actual maximum sample rate depends on your PC configuration and the number of channels that you are using.  See the table below for maximum sample rates for a given channel setting.

Trigger Control

The Mixed Signal Oscilloscope uses a Trigger mechanism to allow you to capture just the data that you want to see.  You can use either a digital channel trigger or an analog trigger.  You can not use a combination of analog and digital.

For an Analog trigger, you can specify the trigger voltage level (-10V to 10V) by using the slider on the left hand side of the analog waveform display.  A red line that indicates the trigger level will momentarily be shown as you scroll this level.  A small T will also be shown on the right hand side of the screen (in the cursors bar) that shows where this level is set to.

For an analog trigger, the trigger position is where the waveform crossed the Trigger Voltage level that you have set at the specified slope.  To move the trigger voltage level, just move the slider on the left of the waveform.  To change the slope, press the Analog Trigger Slope button.

You can also specify if you want the MSO to trigger on a Rising or Falling Edge.  The following figures show a trace captured on each of the edges.

Analog Trigger Slope = Rising Edge

Analog Trigger Slope = Falling Edge

The Trigger position is placed where the actual signal crosses the trigger voltage with the proper slope.  The USBee DX allows for huge sample buffers, which means that you can capture much more data than can be shown on a single screen.  Therefore you can scroll the waveform back and forthon the display to see what happened before or after the trigger.

For a Digital trigger, you can specify the digital states for any of the 16 signals that must be present on the digital lines before it will trigger.  Below shows the trigger settings (to the right of the Signal labels).  This example shows that we want to trigger on a falling edge of Signal 6, which is represented by a high level followed by a low level.  To change the level of any of the trigger settings, just click the level button to change from don’t care to high to low.

The digital trigger condition is made up of up to 4 sequential states of any of the 16 signals.  Each state for a single signal can be high, low or don’t care.  This allows you to trigger on rising edges, falling edges, edges during another signals constant level, or one edge followed by another edge.

The waveforms are shown with a trigger position which represents where the trigger occurred.  This sample point is marked on the waveform display with a Vertical red dotted line and a “T” in the horizontal cursors bar.

You can use the Trigger Position setting to specify how much of the data that is in the sample buffer comes before the actual trigger position.  If you place the Trigger Position all the way to the left, most of the samples taken will be after the trigger sample.  If you place Trigger Position all the way to the right, most of the samples taken will be before the Trigger sample.  This control lets you see what actually happened way before or way after the trigger occurred.

Trigger Position to the Right     Trigger Position to the Left

Waveform Display and Zoom Settings

The Waveform display area is where the measured signal information is shown.  It is displayed with time increasing from left to right and voltage increasing from bottom to top.  The screen is divided into Divisions to help in measuring the waveforms. 

The position of the waveform defaults to show the actual trigger position in the center of the screen after a capture.  However, you can move the display to see what happened before or after the trigger position.

To Scroll the Waveforms in Time left and right, you can use the scroll bar at the bottom of the waveform display (right above all of the controls), or you can simply click and drag the waveform itself with the left mouse button. 

To Scroll the Analog Waveform in Voltage up and down, you can use the scroll bar at the left of the waveform display (one for each channel), or you can simply click and drag the waveform itself by using the colored bar to the immediate left of the actual waveform. 

To change the number of Seconds per Division use the scrollbar at the bottom left of the waveforms.   To change the number of Volts per Division for an analog channel, use the scrollbars at the left of the analog waveforms.  You can also zoom in and out in time by clicking on the waveform.  To zoom in, click the left mouse on the waveform window.  To zoom out in time, click the right mouse button on the waveform window.

The Display section of the screen shows three selections that affect the way the waveform is displayed.

The Wide setting shows the wave using a wider pixel setting.  This makes the wave easier to see.

The Vectors setting draws the waveform as a line between adjacent samples.  With this mode turned off, the samples are shown simply as dots on the display at the sample position. 

The Persist mode does not clear the display and writes one trace on top of the other trace.

The benefits of these display modes can be seen when you are measuring fast signals and want to get more resolution out of the oscilloscope than the maximum sample rate allows.  See the below traces to see the difference.  Each trace is taken of the same signal, but the right one shows much more wave detail over a short time of display updates.

Persist = OFF, Vectors = ON, Wide = ON

Persist = ON, Vectors = OFF, Wide = ON

Measurements and Cursors

The main reason for using an oscilloscope or logic analyzer is to measure the various parts of a waveform.  The USBee DX uses cursors to help in these measurements.

The X1 and X2 Cursors are placed on any horizontal sample time.  This lets you measure the time at a specific location or the time between the two cursors.  To place the X cursors, move the mouse to the gray box just below the waveform.  When you move the mouse in this window, you will see a temporary line that indicates where the cursors will be placed.  Place the X1 cursor by left clicking the mouse at the current location.  Place the X2 cursor by right clicking the mouse at the current location.

The Y1 and Y2 Cursors are placed on any vertical voltage level.  This lets you measure the voltage at a specific location or the difference in voltage between the two cursors.  To place the Y cursors, move the mouse to the gray box just to the right of the scroll bar to the right of the waveform.  When you move the mouse in this window, you will see a temporary line that indicates where the cursors will be placed.  Place the Y1 cursor by left clicking the mouse at the current location.  Place the Y2 cursor by right clicking the mouse at the current location.

In the Measurement window, you will see the various measurements made off of these cursors.

·     X1 Position – time at the X1 cursor relative to the trigger position

·     X2 Position – time at the X2 cursor relative to the trigger position

·     X2-X1 – time difference between X1 and X2 cursors

·     1/(X2-X1) – the frequency or the period between X1 and X2 cursors

·     Y1 Position – voltage  at the Y1 cursor relative to Ground for both CH1 and CH2

·     Y2 Position – voltage  at the Y2 cursor relative to Ground for both CH1 and CH2

·     Y2-Y1 – voltage difference between Y1 and Y2 cursors for both CH1 and CH2

There are also a set of automatic measurements that are made on the analog waveform for each trace.  These are calculated without the use of the cursors.  These are:

·     Max – the maximum voltage of all samples in the current trace for both CH1 and CH2

·     Min – the minimum voltage of all samples in the current trace for both CH1 and CH2