Don't Panic an Introductory Guide to Debugging Video Applications and Beyond

Don't Panic an Introductory Guide to Debugging Video Applications and Beyond
TECHNICAL ARTICLE
Joe Triggs
Applications Manager,
Analog Devices, Inc.
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DON’T PANIC: AN
INTRODUCTORY GUIDE
TO DEBUGGING VIDEO
APPLICATIONS AND BEYOND
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Issue Observed
Abstract
The effective identification and debugging of semiconductor
issues are critical for the technical teams involved in system
design and delivery. The challenging combination of increased
system complexity and decreased time to market can result
in extreme pressure on resolving issues in the shortest time
possible—what engineer hasn’t been asked to deliver their fix
yesterday? At times of such intense pressure, it can be difficult to
follow a logical and structured approach in debugging an issue.
Conversely though, following a logical and structured approach
in debugging an issue is the very key to a timely resolution.
It is clear that a comprehensive debug framework could save
engineers significant time and frustration in debugging complex
semiconductor issues. Using illustrative examples, this article
describes such a framework. Although video products are used as
a lens to examine how semiconductor issues can manifest and be
resolved, the framework outlined here should be considered as
generic and applicable to many semiconductors and problems.
Software Debug
Pursue static settings
Hardware Debug
Correct part specification
Schematic benchmark vs. reference
Layout benchmark vs. reference
Query Knowledge Bases
Measure and Characterize the Problem
Confirm supplies, XTAL …
Check setup and hold times
Introduction
Define reproducibility rate
The front cover of The Hitchhiker's Guide to the Galaxy famously called
out a calming message to those who were lucky enough to possess a
copy: Don’t Panic.1 Douglas Adams famously wrote that “despite its many
glaring (and occasionally fatal) inaccuracies, The Hitchhiker’s Guide to the
Galaxy itself has outsold The Encyclopedia Galactica because it is slightly
cheaper, and because it has the words ‘DON’T PANIC’ in large, friendly
letters on the cover.”
We’ve all been there, though. The demo system is overdue for delivery,
the marketing department is on the phone looking for updates, and a
small group of engineers in the lab pores over a board that refuses to
act as it was intended to. It is at times like this that an intergalactic
traveler would reach for the Guide. Engineers have many alternatives to
the Guide—the Internet, The Art of Electronics, or even one of Dilbert’s
many insightful cartoons. Such an exalted list is now augmented by this
article—a faultfinding recipe that engineers can use to alleviate the
panic when it occurs.
Define the input that causes the issue
Define the output that causes the issue
How is the part configured?
What are the influencers?
Does it occur on the eval board or application system?
Single platform or all platforms?
Share traces, videos, snapshots …
Formal Failure Analysis
Most Importantly, DON’T PANIC!
Figure 1. Generic framework for a systematic approach to the debug effort.
Visit analog.com
Dont Panic: An Introductory Guide to Debugging Video Applications and Beyond
Start with Software
Every engineer has their own biases toward starting a debug with either
software or hardware. Temporarily suppressing these biases (although
the author does have a hardware background), software can often be the
best place to start a debug given the ability to change complex elements
reasonably quickly and the sticky nature of hardware (for example, the
lead time involved in nonbill of materials changes ).
Silicon vendors invest significant time and resources prior to releasing
products to define optimal configuration settings that work across a
range of operation conditions, such as process variations, temperature,
and voltage. Modern semiconductor devices such as HDMI receivers rely
heavily on the use of optimal configuration settings to ensure their stable
and robust operation. Although extra settings may need to be added to
the core configuration settings to address application specific issues (for
example, input muxing or color space conversion), the core configuration
settings must be maintained without adjustment.
When confronted with an issue, the configuration settings being employed
in the application must be examined as a priority. If the configuration
settings being employed do not match those recommended by the
silicon vendor, the next step must be to change those settings before
immediately retesting. The impact of incorrect settings can range from
the slight (for example, blurred video due to filters being disabled) to the
serious (for example, complete absence of video or failing compliance).
During software debug, it can be helpful to isolate a complex software driver
with many interactions (for example, interrupt responses or scheduled tasks)
from the basic I2C configurations required for the system components. If
everything works okay with the static configuration, the issue may be in the
increased number of interactions incorporated into the software. If there are
still issues, hardware debug is the next step.
Part to Play
Once software has been ruled out as the possible source for the issues
being experienced, hardware is the next area for analysis. It might
sound unlikely and elementary but the first stage of any hardware debug
should be to confirm that the parts on the PCB are actually correct.
Silicon vendors typically develop several models of each product with
each model differentiated from the others in varying degrees.
Hardware Is, Well, Hard
Getting a complex hardware design correct the first time isn’t easy. A good
design process incorporating upfront planning, schematic entry, layout,
and layout simulation is a solid foundation that maximizes the likelihood
of success. Even with a good process in place, however, errors can still
happen. Once the correct part has been confirmed on the PCB, the next
step is to confirm the hardware.
Speaking Schematics
Most silicon vendors provide reference schematics and/or schematic
guidelines to assist in getting hardware designs correct first time
around. If your system is not behaving as desired, review it against the
aforementioned references to ensure that no significant differences
exist beyond those required by the application; for example, input
and output connections may differ between a reference system and
a specific application.
Careful attention should be paid to the power supply, ensuring that the
filtering and decoupling are implemented in accordance with the
recommendations—failure to do so could result in noise coupling
from a digital switching supply (for example, a digital core supply) to
a sensitive analog supply (for example, a PLL supply). Unused pins
must be carefully handled to ensure that damage is not caused to the
part or that interference is not induced into the system (for example,
unterminated outputs could oscillate uncontrollably).
Layout Lament
If the schematic is okay, the next step for analysis is the layout itself.
Layout induced issues can range from basic component placement
problems through to complex coupling issues. Video products usually carry
recommendations to place key external circuit components (for example,
external loop filers or crystal oscillators) on the same side of the board as,
and close to, the video part itself. Failure to layout external circuit
components carefully, and in accordance with the recommendations,
could result in unpredictable behaviour from the circuit.
1
2
3
4
5
6
7
8
ADV7281
Top View
(Not to Scale)
24 AIN3
23 Diag 2
22 Diag 1
21 AVDD
20 VREF N
19 VREF P
18 AIN2
17 AIN1
DGND
DVDDIO
DVDD
DGND
INTRQ
GP 02
GP 01
GP 00
1
2
3
4
5
6
7
8
ADV7281–M
Top View
(Not to Scale)
DOP 9
DON 10
Clk P 11
Clk N 12
MVDD 13
XTALP 14
XTALN 15
PVDD 16
DGND
DVDDIO
DVDD
DGND
P7
P6
P5
P4
32 PwrDwn
31 SClk
30 SData
29 ALSB
28 Reset
27 AIN6
26 AIN5
25 Diag 2
One common differentiator between models is in speed grade. A simple
mistake in selecting from an ordering guide could result in a model
that does not perform to the required specification; for example, an
ADV7802BSTZ-150 video decoder operates at frequencies of up to
150 MHz, whereas an ADV7802BSTZ-80 video decoder only operates
at frequencies of up to 80 MHz. Another differentiator could be in
feature set differences between models. For example, ordering a nonHDCP supporting HDMI receiver by mistake (for example, ADV7611BSWZ
vs. ADV7611BSWZ-P) could result in a system that does not support video
from consumer video sources. Models of the same product may also differ
in pinout despite sharing the same package. For example, two models of
the same product with different output interfaces may have slightly offset
I2C interfaces as illustrated in Figure 1. A minor ordering oversight could
result in an incorrect part and a nonfunctional hardware design.
32 LLC
31 PwrDwn
30 SClk
29 SData
28 ALSB
27 Reset
26 INTRQ
25 AIN4
2
P3 9
P2 10
P1 11
P0 12
DVDD 13
XTALP 14
XTALN 15
PVDD 16
ADV7281WBCPZ
ADV7281WBCPZ-M
Figure 1. Product comparison.
24 AIN4
23 AIN3
22 Diag 1
21 AVDD
20 VREF N
19 VREF P
18 AIN2
17 AIN1
Visit analog.com Differential circuits (for example, HDMI, MHL, MIPI, and APIX), if not
correctly designed and implemented, can be particularly susceptible to
layout induced issues. Failure to follow recommendations relating to such
technologies can result in degraded performance, leading to functional or
compliance issues. Discovery of functional or compliance issues should
trigger a comprehensive review of the basic principles of differential layout:
Have the differential traces been kept short and on the same side of the
board (avoiding vias) as the video part? Has a solid ground plane been
used underneath the traces, have intrapair and interpair spacings been
kept consistent? Has the surrounding copper ground fill been kept far
enough away from the differential traces?
The power supply is another element of the layout that can induce
significant issues if poorly implemented. As outlined in the schematic
section, while power supply filtering is important, the supporting layout is
equally so. Key things to check for are that low inductance power supply
planes have been used wherever feasible and that decoupling capacitors
have been carefully located such that decoupling can be achieved right at
the pin. If all the basics look okay, then more subtle elements may need
to be examined, such as whether stitching capacitors have been used to
reduce current return paths.
Investigate All Available Knowledge Bases
If the hardware and software have been eliminated as possible sources
of the issues being observed, it may be valuable to check with other
engineers and knowledge bases. While some semiconductor problems are
novel, many others are observed on a regular basis by technical specialists
or experienced engineers.
While this has traditionally happened on a local level with engineers
within the same company working together to identify solutions, many
semiconductor companies now provide Web-based collaboration
tools such as the Analog Devices EngineerZone Community to broaden
the mindshare.
There are many benefits to using such tools—they provide rapid access
to technical specialists, they do not discriminate between customers, and
they provide access to a unique knowledge base, which can result in up
to 30% of people suffering issues finding answers to their queries without
actually asking a question.
Characterize the Problem
If a solution to the problem is not obvious from either local assistance or
from Web-based collaboration tools, the time has arrived to characterize
the problem. A detailed analysis of the issue at hand will assist in delivering
a faster solution to the problem once support has been sought.
If the hardware and software checks completed during the initial
investigation stages were not conclusive, then the ecosystem around
the part should be examined to ensure that it is to specification. The first
element of the platform to examine is the mechanical fixing of the parts to
the printed circuit board; was the printed circuit board electrically tested;
do all visible solder joints look robust; do all nonvisible solder joints seem
robust when viewed using an alternate means of inspection; for example,
an x-ray. The reliability of mechanical assembly is always improving, but
low volume prototype production runs can sometimes result in issues.
If, following investigation, the board is confirmed to be reliable, the next
step should involve correlating the hardware behavior to the original
design specifications. Are all aspects of the platform operating as per
each part’s data sheet requirements (for example, power supply level
and stability or crystal frequency)? For example, if a single voltage supply
is not within specification or incorrect supplies have been connected
together, the part may not operate as expected (the output may suffer
increased noise, compliance performance may be compromised). When
checking supply levels, it is important to probe as close to the device pin
as possible, thus ruling out any possible voltage drop across the board.
In addition to the power supply, other common sources of issues are the
crystal circuit (for example, stability, load capacitance requirement, or
negative resistance), termination circuits (device selection and layout),
and voltage reference circuits.
Another key aspect of the design is the margin available for setup and
hold time on each chip-to-chip connection. Manifesting on the display
as anything from sparkles (that is, pixel errors appearing as occasional
white dots), through green lines, to complete picture loss, timing errors
can be detrimental to the performance of a system across a range of
temperatures. Matching the data invalid times of the transmitter with the
setup and hold times required of the receiver while incorporating all PCB
specific information (for example, trace length or series resistors) is vital
to delivering a stable system.
Report Your Issue
If the hardware debug is successful and everything is to specification,
the next step should be to characterize the issue as concisely as possible
before seeking any further assistance. Gathering information on the
following topics will assist technical support staff in providing an
informed and timely response.
1. How is the issue reproduced and what is the reproducibility rate of the
issue (for example, 1 in 1000, 1 in 100, 1 in 10, or 10 in 10 attempts)?
Can the issue be recreated in a static manner or does it require
dynamic input (for example, repeated hot plugging of an input)
2. Describe the input used to duplicate the issue. For example, is the
input a standard or application specific source such as a reversing
camera? Does the issue occur with just one or a range of different
sources? What is the resolution/frequency of the input? What is the
color space of the input? What is the color depth of the input? For
LVDS or MIPI type inputs, how many lanes does the input employ?
3. Describe the output used to duplicate the issue. For example, is the
output a standard or application specific sink? Does the issue occur
with one or a range of sinks? What is the resolution/frequency of the
output? What is the color space of the output? What is the color depth
of the output? For LVDS or MIPI type outputs, how many lanes does
the output employ?
4. Describe the configuration of the part required to duplicate the issue
for example, what input(s) are being employed—analog or digital?
What signal path through the part is being used—component
processor, scaler, deinterlacer, on-screen display? What output(s) are
being used—analog or digital? What are the settings being employed
and how have they been adjusted from those recommended?
5. Characterize the influencers. For example, does the issue change
with temperature? Does the issue change with voltage? Is the
issue independent of cable length or are the parts being connected
on a PCB?
Note: When applying either heat or cold to the device, it is strongly
recommended to use either a chamber or forced air equipment.
Freezer spray and heat guns are to be avoided if at all possible due to
the unpredictable nature of their output.
6. Can the issue be replicated on the semiconductor device’s evaluation
board when the same inputs and outputs are connected with the DUT
configured in as similar a manner as possible? Does the issue manifest
in the same, or in a slightly different, manner?
Note: This question is vitally important for two key reasons; first, it
indicates very quickly to the technical support staff whether the
problem is related to the semiconductor device and supporting
material (for example, documentation or recommended configuration
settings) or if the problem is related to something application specific;
second, it may facilitate the technical support staff with a very fast
3
means of reproducing the problem if the problem can be reproduced
on the evaluation board.
7. Does the issue happen on all platforms or on a single platform? An
ABA swap may be required to confirm this.
Note: An ABA swap is an effective tool for semiconductor packages,
which can be removed from, and remounted to, a PCB with minimal
levels of rework—for example, this is suitable for LQFP and LFCSP
packages but less so for BGA packages unless there is scope to reball
the device.
8. The purpose of an ABA swap is to confirm whether the failure follows
a suspect system, or a suspect device. First, reproduce the issue on
the suspect system and confirm a second system that does not show
the issue. Remove the parts from both systems, swap and remount.
Confirm if the issue remains on the suspect platform or if the issue
moved to the second system with the suspect part.
9. Capture scope traces, pictures or, if more insightful, videos to
demonstrate the issue (a video is significantly more useful if the issue
is dynamic in nature, for example, if there are sparkles or random
noise).
Once all of this research has been completed and the results are at
hand, it is time to return to the Web-based collaboration tools to initiate a
conversation with the technical support specialists.
Go Formal
After exploiting all possible avenues of exploration, the physical condition
of the semiconductor device may itself be the final stage of investigation.
To progress with this stage, the device must normally be returned to the
semiconductor supplier through the formal failure analysis channel—this
can usually be achieved by contacting either the semiconductor supplier’s
local sales office or the sales office of the distributor.
The failure analysis process employs tools such as repeat production
testing and bench evaluation to check for failure modes such as
electrical overstress, electrostatic discharge damage, manufacturing
defects, production test escapes, and application related issues. Should
the failure analysis process be unsuccessful in reproducing or confirming
the issue, the part may be returned categorized as no trouble found (NTF)
for further debug.
Conclusion
The sage words, “DON’T PANIC,” can seem antagonistic when offered
at the wrong instant. But the intention of the advice is the key to
unlocking so many issues that are commonly experienced by engineers
when trying to design and deliver complex systems featuring multiple
semiconductor devices. Following a structured and reasoned approach
to the debug effort (for example, reviewing reference schematics and
layouts, recommended settings, or confirming hardware operation as
being to specification) increases the possibility of finding the root
cause of any issue.
Background
Analog Devices video products constitute one of many categories of
products offered by the long established semiconductor vendor.
Analog Devices video products are supported by product experts
through the EngineerZone community.
References
1
Douglas Adams. The Hitchhiker’s Guide to the Galaxy.
New York, Pocket Books, 1981.
About the Author
Joe Triggs is the applications manager for the Digital Video
Products Group at Analog Devices (Limerick, Ireland). He has
worked at Analog Devices since 2007 and his team supports
automotive and consumer video products ranging from single ADC
video decoders to complex video signal processors. He earned
his primary degree (B.Eng.) from the University College of Cork in
2002 before continuing to complete his M.Eng. through research at
the University of Limerick in 2004. He also completed his M.B.A. at
the University of Limerick’s Kemmy Business School in 2012.
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