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Keysight Technologies
Logic Analysis Fundamentals
Application Note
Introduction
Today, a wide range of end products, such as mobile devices, radar systems, and industrial controls are found with a combination of
serial and parallel bus structures. Internal FPGA signals are almost exclusively parallel bus in nature. This application note exam-
ines the basics of parallel bus measurements, including functional and timing verification and debug and tracing system crashes in
search of a root cause.
Synchronous versus asynchronous capture in logic analyzers
Before looking at specific measurement examples, it is helpful to consider the difference between synchronous and asynchronous
capture and the benefits and limitations of each.
Synchronous (state) capture means that the measurement system in the logic analyzer determines the logic value of digital parallel
buses or control lines when there is an associated valid clock, such as a rising edge on a system clock line that is probed, as shown
in Figure 1. Intermediate unsettled bus values in between valid clock edges are completely ignored by the analyzer. The bus values
stored into analyzer memory represent the "states" of the bus, either state machine values or data flow. The primary purpose of
such measurements is to determine if the basic functionality of the system is correct. Does the state machine move through the
proper sequence of states considering the inputs to the system? For synchronous designs, this approach is often the most insight-
ful, although it does require the user to specify an input clock signal to the logic analyzer. Portable logic analyzers, such as the
16850 Series, can trace buses running as fast as 1400 Mbps state data rates.
Synchronous and asynchronous capture, combined with the right triggering, is the key to efficient
digital system debug
Threshold
VInput Output (0 or 1)
+ Latch
_
VOutput
Comparator
External DUT Clock
Data AA OC 61 B3
Clock
Clock Data
1 AA
2 OC
3 B3
Figure 1. State (Synchronous) capture
03 | Keysight | Logic Analysis Fundamentals - Application Note
Synchronous versus asynchronous capture in logic analyzers (continued)
In contrast, asynchronous (timing) capture means that the measurement
system samples the value of a bus or individual digital lines "asynchronously"
from the system under test or "not in sync" with the system, as shown in Figure
2. The measurement clock is generated by the logic analyzer rather than the
target system. Portable logic analyzers are available that offer timing capture
with deep memory at rates as high as 2.5 GHz with full channel count. Typically,
sampling happens faster than the target system clock rate ideally 4x to even
10x the system clock rate. This allows the designer to see the general activity
on the bus when sampling around the rate of four times the system clock rate,
and to see the "timing" characteristics of the signals involved when sampling
closer to a rate of 10 times the system clock rate. It also allows the designer
to observe buses and signals that are running asynchronously to each other
such buses and signals are also referred to as being in multiple clock domains.
But one drawback is that such capture records both valid and invalid values
of buses and signals as they transition to settled values for each clock edge.
Channel-to-channel skew between individual bits of a bus can sometimes make
it difficult to see what the final, settled value is on a bus for a particular clock.
But the timing mode of the logic analyzer can also enable the designer to see
the skew that is a problem in the design
A classic tool for digital debug is a logic analyzer, but for lower count channel
needs, and where synchronous capture is not needed, a mixed-signal oscil-
loscope with digital inputs has found popularity. The logic analyzer is the best
choice for higher complexity target systems where wider digital buses are
implemented, such as I/Q vector modulator designs used in LTE communica-
tions systems and radar systems. Logic analyzers are also best for systems
where a long period of target activity must be captured to validate the design,
such as digital video based systems where designers need to see one or more
video frames. Logic analyzers have the channel count, memory depth, and
sample rate combination to accomplish this. A mixed-signal oscilloscope may
be ideal when the analog capture of signals is the primary objective, but digital
inputs are helpful to provide more complex triggering or capture and analyze
what is happening on more narrow parallel buses.
VThreshold
VInput + Output (0 or 1)
_ Latch
VOutput
Comparator
Internal DUT Clock
VThreshold
VInput
Internet
analyzer clock
VOutput
Figure 2. Timing (asynchronous) capture
04 | Keysight | Logic Analysis Fundamentals - Application Note
Functional verification with synchronous capture
When a digital design physical prototype is being turned on, some designers
first want to know if the correct functionality is occurring in the system through
a variety of synchronous, state measurements. If something doesn't look right,
they'll then move to asynchronous, timing mode measurements to see if they
can figure out what the problem is. Interestingly, other designers would rather
start with asynchronous, timing mode measurements right away to see the
edge placement in signals of interest, and then they will move to state mea-
surements at the end to verify functionality. Both approaches provide valuable
insight into digital system behavior.
There are a host of interfaces associated with the main SOC, inside FPGAs, be-
tween chips, or as I/O that can be observed with a logic analyzer for functional
verification.
To begin, let's consider a simple, 8-bit counter circuit, and for this particular
example, the design produces counter data that is valid and settled at the same
time as the rising edge of a clock.
A first look at the counter circuit via synchronous capture
An initial evaluation to determine whether the counter is working properly is
made by connecting eight data input lines of a logic analyzer to the eight data
bit output lines of the circuit. The most common approach is to use "flying lead"
probes that allow independent connections to each signal. Alternatively, one
could have chosen to use a connector on a board such as a Mictor connector.
Another approach is to use Soft Touch probes (without connectors) in which
pads are placed on the board and route signals line through the pads. Then
connectorless probing is attached to the pads via spring pins.
The logic analyzer is placed in a "State" or synchronous capture mode and
clocking is set up to capture data on the rising edge of the clock signal. A bus
name is created by the user in the logic analyzer interface called "Counter," and
it is defined to be driven by the eight data bits brought into the first 8 channels
on pod 1 of the logic analyzer, as shown in Figure 3.
Figure 3. Bus "label" definition
05 | Keysight | Logic Analysis Fundamentals - Application Note
A first look at the counter circuit via synchronous capture (continued)
One easy place to set up a simple trigger is from the Waveform window. The
value hexadecimal E7 can be entered alongside the bus name "Counter" to
define a simple trigger event, as shown in Figure 4.
Figure 4. State (synchronous) capture and
trigger on "Counter = E7 hex"
After pressing "Run," a sequence of hexadecimal values appear in the Waveform
view. They appear to be counting properly, as shown in Figure 1, but another
way of getting a more complete view of this data quickly is to turn on "Chart
Mode." Now, by adjusting the time per division setting, the display shows an en-
tire ramp where data should be going from 00 hex to FF in the form of a ramp,
and then repeating. This chart mode view can be seen in Figure 5, but a clean
ramp is not seen.
F to 0 transition
Figure 5. Chart mode view reveals discontinuities
in counter ramp
06 | Keysight | Logic Analysis Fundamentals - Application Note
A first look at the counter circuit via synchronous capture (continued)
Upon closer inspection, discontinuities are found at the transitions from hex
value F to 0 in the least significant bit of the counter. For example, the counter
has a problem going from hex value DF to E0, EF to F0, and also from FF to 00.
This doesn't yet conclusively mean that there is a functional problem with the
counter circuit, but there could be. Had the counter looked flawless in this
mode, it very likely would have meant that it was functionally correct. But since
the view wasn't flawless, the answer to whether there is a functional problem
won't become obvious without digging deeper.
Timing validation with asynchronous capture
The next step in digging deeper is accomplished through timing validation with
asynchronous capture. This should sort out whether there is a functional is-
sue, a timing issue, or both. The logic analyzer is set to "Timing Mode," where
samples are taken asynchronous to the counter circuit clock, and at a sample
rate much faster than the counter clock rate. The goal now is to look at both
the value and timing of digital signals to verify whether the fundamental timing
relationships are correct between clock and data signals.
In this mode, it is critical to sample and view the clock signal as well as the data
signal. An additional label is defined called "Clock," as shown in Figure 6, and
the proper logic analyzer clock input line is selected that has been physically
attached to the counter circuit clock signal.
Figure 6. "Clock" label added for timing
(asynchronous) mode capture
The next step is to look carefully in the asynchronous timing mode at one of the
counter value transitions where a problem was just seen in the synchronous
state mode. A trigger can be set up for the value FF to catch the transition
from FF to 00. The simplest trigger setup is just to enter the value "FF" into the
simple trigger menu next to the Counter bus in the Waveform Window.
An asynchronous capture with this trigger can be seen in Figure 7. The trigger
event is just left of the screen and the trace has been zoomed in at the point
where the bus is transitioning from "EF hex." In this mode, with the resolution
of the logic analyzer sampling circuit, one can easily view what is happening on
each line of the device under test. Data is supposed to be settled and valid on
the rising edge of the clock line. Looking closely at the value of the counter bits
in the vicinity of the clock rising edge, one has to check to see if basic setup and
hold requirements between clock and data are being met.
07 | Keysight | Logic Analysis Fundamentals - Application Note
Timing validation with asynchronous capture (continued)
Notice the counter value
has not settled by the
rising clock edge.
Figure 7. Close up timing view of "Clock"
signal and "Counter" 8-bit bus
Is the bus settled to its next value by the rising clock edge? Was it settled for
the setup time specified prior to the clock edge, and did it hold its value for the
time specified after rising clock edge? Looking at the trace at the clock's rising
edge where the counter bus should have transitioned from FF to 00, one can
see that there is something drastically wrong. At this point the data bus has not
yet settled to an 00 value. In fact, it becomes clear that it has settled by the
point of the falling clock edge! A mistake was has been found in the circuit tim-
ing. Markers are placed on the falling edge of clock (M1), at the start of settled
bus value 00 (M2), and at the end of settled bus value 00 (M3). Simple timing
measurements show the amount of setup time (M1-M2) and hold time (M3-M1),
relative to the falling edge of clock.
A quick check to see if the design has indeed been accidentally configured to
settle data on the falling edge of clock would be to place the logic analyzer
back into the synchronous state mode, change the clock definition to "falling
edge," and take a trace. Doing that, one sees the trace in Figure 8, a perfectly
repeating 00 to FF ramp as desired. So this circuit is "functionally" correct, but
it has a fundamental timing issue that would need to be fixed in the design.
Figure 8. State mode capture with logic analyzer
clocking set to falling edge of clock
08 | Keysight | Logic Analysis Fundamentals - Application Note
High-speed timing capture around the trigger point
It is possible to add timing capture, called "Timing Zoom," with an even higher
sample rate, which is positioned near the main logic analyzer trigger point. This
capture can happen in conjunction with the standard state or timing capture
described previously. This option is especially helpful when looking at a state
capture where there are "confusing" state results on screen. It is helpful to view
the same data bits, but with high-speed timing capture, time correlated to the
state capture, to attempt to unravel the mystery.
Take, for example, the same counter circuit where issues were seen in the tim-
ing between clock and data bits. A joint state and high-speed timing capture is
shown in Figure 9. The details behind an improper FF to F0 hex state transition
can be better understood through careful analysis of the clock-to-data tim-
ing seen in the high-speed timing capture. In this case, 12.5 GHz rate, 80 ps
spaced timing samples reveal the setup/hold issue seen previously. However,
when using Timing Zoom, the resolution is much higher than the standard, deep
memory timing trace captured earlier, allowing the user to analyze the clock
to data setup/hold time. Timing Zoom does not capture signals far from the
trigger point but instead clearly depicts a small window of high-speed timing
data near the trigger. In contrast, conventional deep memory timing capture,
coupled with high-speed sampling must be used to view activity far from the
trigger point.
Figure 9. Simultaneous state and high-speed timing
(Timing Zoom) capture reveals setup/hold issue
09 | Keysight | Logic Analysis Fundamentals - Application Note
Helpful triggers a "timeout" trigger example
Often it is difficult to pinpoint a problem in a design. Setting the right kind of
trigger can be crucial to getting to the root cause of a design flaw. One of the
most important trigger types is called a "timeout trigger." A timeout trigger
makes the logic analyzer watch for a repeating, expected target system behav-
ior and then trigger if that behavior doesn't happen within a certain expected
time period. This capability is especially helpful when a target system has a
data bus lock up or "hang" to a fixed data value while the clock continues to run.
Consider a case where the previously-mentioned counter circuit worked prop-
erly for a period of time and then exhibited erroneous behavior in which large
deviations occurred from the ideal "ramp" and it never made an FF to 00 transi-
tion again.
In this case, it is known how often the FF to 00 transition should occur (every
8 sec) so the timeout trigger is set to look within a period of time slightly
greater than that (10 sec) and it watches to make sure there is an occurrence
of counter bus value 00 within that time. This trigger setup can be seen in
Figure 10.
Figure 10. Timeout trigger setting
10 | Keysight | Logic Analysis Fundamentals - Application Note
Helpful triggers a "timeout" trigger example (continued)
A logic analyzer capture of the counter circuit experiencing the anomaly can be
seen in Figure 11. Normal operation can be seen on the left side of screen, while
the right side of the screen shows a transition into abnormal operation. The bus
transitions from FF to 80 instead of from FF to 00. A trigger happens 10 sec
after a bus value of 00 occurred and another 00 bus value did not occur within
10 sec. A listing window shows this abnormal transition at Marker 1 after a
search for bus value 80. This trigger and capture is very useful because the
designer can look back in time from the trigger point to see exactly what hap-
pened prior to a failure or a bus-locking crash. Figure 11 also shows an oscil-
loscope synchronized to the logic analyzer trace and imported to the screen to
get an analog view and further insight into failing counter bit 7.
Figure 11. Trace capture using timeout trigger to trap when the Counter bus doesn't have
value "00 hex" occurring within a 10-sec period
11 | Keysight | Logic Analysis Fundamentals - Application Note
Trigger on a symptom, but view with high speed timing capture in deep memory
Sometimes a digital system will encounter a malfunction that can trigger a logic
analyzer, yet the root cause of failure is hidden in a deep memory trace capture.
In such a case, it is important to have high enough speed timing capture to ob-
serve carefully both the timing of the related signals as well as the logic in the
signals to help sort out where the real problem is.
Take, for example, a system where a state machine is driving the communica-
tion with an external device, and something goes wrong and the communication
ceases. It is common that there will be some kind of a flag if something's gone
wrong, such as a timeout signal. The logic analyzer can be easily set up to trig-
ger on the rising edge of a timeout signal, but in the vicinity of the trigger point,
the state machine has already stopped running, so there is no activity to see
as shown in Figure 12. And the high-speed Timing Zoom capture is not deep
enough to see any activity.
Because the 2.5 GHz timing analyzer captures with deep memory, it is now
possible to look back in time carefully and observe what went wrong. A first
step is to right-click on screen, select "Zoom Out Full" and get a high-level
picture of everything that happened leading up to the trigger point, as seen in
Figure 13. This shows that the state machine had activity, as some acknowledge
signals (Acks) were coming from the other device, but then the Acks stopped
coming, and sometime later the state machine stopped.
Time out
flag trigger
Error: state
machine locked
Cause of
failure not found
around trigger
Figure 12. This high-speed Timing Zoom capture is not deep enough to see any timing
activity. Designers need to be able to look farther back in time
12 | Keysight | Logic Analysis Fundamentals - Application Note
See the error of Acks stopping at
the 500 us before the trigger
And find the error of
the state machine stopping
Figure 13. See a high-level view of of everything that
happened leading to the trigger point
By drawing a box around the state machine activity, easily zoom in and see the
specific operation when things began going wrong. By zooming in, one can see
the detailed timing between the last state of the state machine cycle, where it
was looking for an acknowledge, and when the final acknowledge came. Zoom
in again, and the detailed timing can also be seen between the clock rising edge
and the individual bits of the state machine, as shown in Figure 14.
2.5 GHz, 400 ps resolu-
tion timing
capture in deep
memory allows you to
sort out timing versus
functional issues
Figure 14. Zoom in to see the specific
operation when things began going wrong
13 | Keysight | Logic Analysis Fundamentals - Application Note
Summary
Despite the changes in digital system architecture, including transitions to
serial bus protocol-oriented bus structures, many designs today still employ
basic parallel bus architectures. Often, such buses must be analyzed to either
validate a design or track down a defect. Knowing how to use synchronous and
asynchronous capture modes, as well as powerful triggering, can significantly
influence the speed of moving a design past the debug stage and into the
market. Additionally, knowing how to use fast timing capture with deep memory
is important, especially when triggering on a symptom but the root cause is
hidden in the deep capture.
A variety of logic analyzer options are available that offer different timing
speeds with deep memory. Capabilities in the new 16850 Series of portable
logic analyzers provide a 2.5 GHz timing capture with up to 128 M sample depth
to help a the designer's debug process. Fast state capture is also now possible,
with up to a 1400 Mbps rate to track digital system operation, also helpful for
debug.
For even higher performance requirements, Keysight Technologies, Inc. U4154A
modular logic analyzer modules for AXIe-based modular frames are available.
These modular logic analyzers with state capture up to 4 GHz extend this capa-
bility into areas like high speed memory.
14 | Keysight | Logic Analysis Fundamentals - Application Note
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