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Custom Solutions, Inc. recognizes that reliability is of great concern to anyone
purchasing or installing a home control system. Therefore, HomeVision,
HomeVision-PC, and HomeVision-Pro were designed from
the ground up to provide many years of reliable operation. During the design process, we
sometimes made decisions that delayed its release or increased its manufacturing cost, but
which were necessary to ensure reliability. Even now, our manufacturing and testing
process takes longer than many of our competitor's because of the stringent procedures we
have in place. We accept these added burdens because they give our customers a highly
reliable product, upon which our reputation rests.
The lead HomeVision designer was previously employed by a major, multi-billion
dollar defense electronics company for over 20 years. While employed there, he was
responsible for the reliability of numerous electronics systems used on military aircraft,
communications satellites, the Space Shuttle, and similar programs. He brought many of the
same reliability concepts to HomeVision's design and manufacture. This article describes
many of these techniques, focusing primarily on the hardware issues. Discussing software
bugs (and how we catch and squash them) would require a whole other article.
Minimizing Electrical and Thermal Stresses
We must minimize stresses on components so they operate continuously for many
years without degradation:
 | Resistors and capacitors operate well below their maximum power and voltage
ratings.
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| Infrared transmitter LED (light emitting diode) operates well below maximum
current, voltage, power, and temperature ratings.
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| High-intensity green LEDs minimize current draw, power dissipation, and
temperature.
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| Low-power CMOS integrated circuits lower power dissipation and temperature.
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| Power regulator rated at 1 Amp or higher. Actual current draw is typically 100mA,
and maximum is 400mA, providing significant margin against part rating.
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| Regulator mounted directly to circuit board with heatsink to minimize
temperature.
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| Power regulator contains over-current and thermal shutdown circuit to protect
against power-to-ground shorts and overheating.
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| Diodes protect against voltage spikes on TW-523 interface to minimize chance of
overstress. |
Robust Electrical Design
The electrical design must ensure the circuitry works 100% of the time,
regardless of component performance drift, temperature, aging, use environment, etc.:
| Microprocessor operates well below maximum allowable clock speed to maximize
circuit timing margins.
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| Memories are faster speed than necessary to guarantee error-free operation.
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| Worst-case circuit analysis performed to ensure circuits function properly over a
wide temperature range even after device aging.
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| High-capacity battery, in conjunction with extra low power RAM, ensures memory
retention after 12 years without power for HomeVision and
HomeVision-PC. The HomeVision-Pro battery
will last at least 4 years without power, and is easily replaceable by the
user.
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| Clock adjustment factor implemented in software to reduce drift of real time
clock.
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| High-performance electrolytic capacitors used (lower-quality ones degrade rapidly
over time).
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| Two infrared receivers achieve exceptional performance. Performance has been
verified using a standard TV remote from over 80 feet away in a "noisy"
environment with large amounts of fluorescent and incandescent light.
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| Undervoltage detector circuitry shuts unit down as soon as power begins to drop
out. This prevents erratic operation that can result when parts operate at low voltages.
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| Precision components in the video oscillator circuitry improve video timing
accuracy.
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| High-performance video switch passes video signal through controller without
degrading picture quality.
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| Extra-strong video output level drives long cables without an external amplifier
(this high level sometimes requires the addition of an attenuator to maximize picture
quality; however, we felt that this was a worthwhile tradeoff to save many of our
customers the cost of an external amplifier). |
Parts Quality
Part quality is critical in preventing failures:
| Parts are chosen from high-quality suppliers we're familiar with, not just
anyone. Parts from new suppliers are thoroughly evaluated and tested before being approved
for use.
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| High-quality terminal blocks provide secure clamping of interface wires.
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| "Dual-leaf" sockets make superior electrical connections with the
integrated circuits than less expensive "single-leaf" sockets.
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Software Design
The software must verify proper hardware operation and use robust control
methods:
| Self-test automatically runs at start-up and tests critical circuitry:
 | EEPROM |
| RAM |
| Processor |
| Peripheral control circuits |
| Serial port chips for
HomeVision-Pro
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| Continuous error checking and recovery methodology ensures reliable operation.
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| Hardware watchdog timer restarts entire unit if software ever locks up or
performs unexpected operations.
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| "Polite" X-10 transmissions won't interfere if other transmissions are
in process.
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| X-10 collision detection and retry to improve transmission reliability.
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| Precise X-10 timing pulses transmitted to TW-523 ensures transmissions always
meet the X-10 specifications (even while HomeVision is receiving an infrared signal and
performing other operations simultaneously). Three-phase transmissions are far more
accurate than in some of our competitors' systems.
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| 32-command X-10 input buffer stores received X-10 signals so HomeVision won't
miss anything.
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| Standard infrared receive signal format uses 16-bit error checking to eliminate
false triggers. Ambient light won't ever accidentally trigger an action.
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| Infrared signal "learning" process measures the signal's carrier
frequency to accurately reproduce the signal. Some of our competitors require you to guess
the frequency yourself and use trial-and-error to verify it.
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| Infrared transmitter can vary the signal's duty cycle, allowing you to increase
or decrease the signal's power output. This can be helpful when using mini-emitters that
sometimes overpower the receiver they're attached to. |
Electromagnetic Interference (EMI) Control
EMI can cause erratic circuit performance and "glitches" if not
properly controlled:
| Multiple bulk decoupling capacitors support current requirements of high-power
devices (to control L di/dt noise and "ground bounce").
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| Numerous high-performance ceramic filter capacitors minimize high-frequency
switching noise.
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| Power distribution system minimizes ground loop areas (to control radiated
emissions) and trace impedances (to control L di/dt noise and "ground bounce").
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| High-performance HC (high speed CMOS) integrated circuits maximize signal noise
margins.
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| Circuit board ground shields around and under sensitive circuitry (particularly
the video and infrared receive circuits).
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| Metal enclosure provides shielding from radiated emission from TV's, stereos,
etc. |
Manufacturing and Testing
We must minimize defects in the manufacturing process and catch any that do
occur with thorough testing:
| Automated board assembly process minimizes chances for human error and increases
product consistency.
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| Electro-Static Discharge (ESD) controls prevent ESD damage to sensitive ICs.
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| All units "run in" for at least 40 hours to stress the hardware and
eliminate any possible defects. All functions are continuously tested, including X-10,
video, infrared, and I/O ports. Units are only shipped if they run PERFECTLY. A single
glitch causes the unit to be rejected (after which the cause must be found and corrected
and the unit run for 100 hours without problems).
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| All units subjected to two complete functional tests (one prior to "run
in", one afterwards). The final test report is included with each HomeVision unit.
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Conclusion
Although this is a long list, it doesn't even include our long-term
product testing or customer Beta testing (That would take another article in itself!). As
you can see, we've put a lot into making HomeVision reliable. We're so confident of
HomeVision's reliability that it comes with a 3-year
warranty (if you've seen the owner's manual that says it's 1-year, ignore
it; it's really 3). Even so, no product is perfect. Hardware failures can occur, and
software bugs are inevitable (just look at Intel's Pentium processor or Microsoft's
Windows operating system - both are great products, but have dozens (and we're being kind)
of known bugs, many of which they have no plans to fix). So if you should ever have a
problem with HomeVision, please let us know and we'll fix it.
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