USBee AX Mixed Signal Oscilloscope, Logic Analyzer and More
This section details the operation of the Logic Analyzer
application that comes with the USBee AX.
Below you see the application screen.
Logic Analyzer Specifications
|
Digital
Channels
|
8
|
|
Maximum
Digital
Sample
Rate [1]
|
24
Msps
|
|
Internal
Clocking
|
Yes
|
|
External Clocking |
Yes |
|
Trigger Levels |
4 |
|
Trigger Qualifiers |
Rising
Edge, Falling Edge, High,Low |
|
Number of Samples
[2] |
1 million samples up to
PC RAM |
|
Sample Rates
[1] |
1Msps to 24 Msps |
|
Trigger Prestore |
Yes |
|
Trigger Poststore |
Yes |
|
Sample Clock Output |
Yes |
|
Maximum Input Voltage |
+5.5V |
|
Input Low Level |
< 0.8V |
|
Input High Level |
> 1.4V |
|
Cursors
|
Trigger position, X and
O
|
|
Measurements
|
Hex value, Period, Frequency
|
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Features
Pod Status
The Logic Analyzer display shows a list with the available Pod ID List for all of the USBee Ax’s
that are connected to your PC. You can
choose which one you want to use. The
others will be unaffected. If a USBee AX
is not connected, the list box will read Demo to 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 Logic Analyzer captures the behavior of digital signals
and displays them as a “trace” 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.
The Acquire button
starts and stops a capture. When the logic
analyzer is first started, the Acquire button is not pressed and is waiting for
you to start a capture. The Acquire
button captures a single trace and stops.
This mode is good for detailed analysis of a single event.
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. This uses an Internal Clock at that sample rate you choose. You can choose from 1 Msps (samples per
second) to up to 24 Msps. The actual
maximum sample rate depends on your PC configuration. You can run the menu item Setup | Sample Rate
Test to determine the maximum sample rate for your system.
The USBee AX can also use an External Clock as the sample clock via the CLK line. This is selected by the radio button that
reads “CLK signal is an input…”. You can
also then choose which Sampling Edge
that the samples will be taken on: the rising or falling edge of the external
clock using the pushbutton. In external
timing mode, you can also use the External
TRG signal to qualify the capture of the traces. Set this toggle pushbutton to the state of
TRG that you want samples to be taken.
Samples occurring during invalid TRG times will not be stored.
The Status Box
on the display will show red when the unit is not acquiring samples, flash blue
when it is waiting for a trigger, and glow green when the trigger condition has
been met. It will glow red again when
the capture is completed.
Trigger Settings
The Logic Analyzer uses a Trigger mechanism to allow you to
capture just the data that you want to see.
You can specify the digital states for any of the 8 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 0, 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 Trigger position is placed where the actual signal
crosses the trigger voltage with the proper slope. The USBee AX 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 forth on the display to see what happened before or after the
trigger.
The digital trigger condition is made up of up to 4
sequential states of any of the 8 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.
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 left and right arrows highlighted
above, click and drag the Overview Bar (right under the Display Control title),
or you can simply click and drag the waveform itself.
To change the zoom ratio for the time, click the Zoom In or
Zoom Out buttons. 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.
Measurements and Cursors
The main reason for using a
logic analyzer is to measure the various parts of a waveform. The USBee AX uses cursors to help in these
measurements.
The X and O 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
and O cursors, move the mouse to the white 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 X cursor by left clicking the mouse
at the current location. Place the O
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. To change the selected relative cursor, click
the T,X or O buttons next to the “Timeline Relative To” text.
· X
Position – time at the X1 cursor relative to the selected cursor
·
O
Position – time at the X2 cursor relative to the selected cursor
·
X to O - difference between X and O cursors
There are also a set of automatic measurements that are
made on the waveform by moving the mouse pointer over the waves with the
following modes turned on. These are
calculated without the use of the cursors.
These are:
·
Width
– the width of a pulse
·
Frequency
– the frequency of an periodic cycle
·
Period
– the period of periodic cycle
·
Byte –
the hex value of all 8 signals at a given sample
List
Display
You can also display the captured data in a list format
that details each sample that was taken by breaking it down by digital value (0
or 1), sample number, 8-bit Hex representation and sample time relative to the
chosen cursor.
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