1. technology and computing

Continental Automated Buildings Association
Information Series
IS 2009-44
How to Design a Solar-Powered
Computing Device
www.caba.org
How to Design a Solar-Powered
Computing Device
Reprint Date: March 2009
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White Paper
Intel® Atom™
Processor Z510
Solar-Powered
Applications
How to Design a Solar-Powered
Computing Device
Improved technology and lower solar panel
costs will spark an explosion of embedded
solar-powered products
Going beyond useful gadgets powered by the sun, solar-powered computing devices
are just over the horizon. Imagine network routers and surveillance devices soaking
up the sun and running networking, video and security software. Free of power and
Ethernet cables, these embedded systems can be deployed in the field quickly
and cheaply.
These opportunities are upon us because the economics and technologies surrounding
solar are making great strides. The cost of solar panels is coming down rapidly as
production grows, and the power consumption of new processors is decreasing as
technology advances. Clearly not just any CPU can be used in a solar application, but
some of the latest power-optimized processors are up to the task. This is the case
with the Intel® Atom™ processor, which consumes 2.0 watts activei ,1,2 and as little
as 0.1 watts in a Deep Sleep state.
This white paper describes different types of embedded solar-powered computing
devices and provides design suggestions for Intel Atom processor-based platforms.
It covers hardware and software practices for developing ultra-low power devices,
as well as open source software available to designers.
White Paper How to Design a Solar-Powered Computing Device
Table of contents
When Solar Makes Sense....................................................................................................................................................................................................... 3
Device Opportunities............................................................................................................................................................................................................... 3
Design Requirements............................................................................................................................................................................................................... 4
Satisfying the Design Requirements.............................................................................................................................................................................. 4
System Example: Surveillance Sensor........................................................................................................................................................................... 5
Designing a Solar-Powered Computing Device......................................................................................................................................................... 6
Challenge 1: Voltage regulation................................................................................................................................................................................. 6
Challenge 2: Source voltage......................................................................................................................................................................................... 6
Challenge 3: Power management............................................................................................................................................................................. 6
Truly Untethered Embedded Devices............................................................................................................................................................................ 7
2
How to Design a Solar-Powered Computing Device White Paper
When Solar Makes Sense
Most embedded computing devices, such as PLCs, ATMs, and
networking appliances, are tethered and have a continuous
source of power. However, there are times when it may be more
convenient, or even essential, to use devices that aren’t 100
percent reliant on wired network connections and a steady stream
of power. These cases include setting up and using infrastructure
after natural disasters, during remote military operations or when
power outages are frequent, as listed in Table 1. In these scenarios,
computing devices must operate in settings that are less stable
than standard industrial and enterprise infrastructure.
Device Opportunities
Solar power is not likely to enable new categories of embedded
computing devices. Instead, existing device types will leverage
solar power and wireless connectivity to advance energy sustain­
ability and ease of deployment. These attributes open the door
to slightly different usage models in the following common
equipment segments:
• Surveillance system sensors: Today, it’s hard to go anywhere
without seeing surveillance cameras monitoring public and
business premises. These devices are often situated in difficultto-reach places, like the tops of buildings, surrounding walls and
tall trees. In these cases, solar devices are easier to deploy than
tethered sensors and can be simply repositioned at a later time
as needed.
• Data acquisition: Remote data collection is routinely used
to study a host of disciplines, including meteorology, geology
and astronomy. For example, seismograms taken at different
locations pinpoint earthquakes, and remote sensors help identify
areas rich in oil and gas reserves. This data can be acquired using a
solar-powered sensor board which offers users, such as academics,
businesses and government agencies, more placement options and
data processing capability.
• Femtocells and picocells: Boosting cell phone reception within
buildings and homes, femtocells and picocells create an intermediary network that improves coverage and connects customers to
the service provider network. Solar power enables small business
and consumers to deploy these devices in sunlit areas.
• Network routers: Network access is a way of life, and solarpowered routers can make it even more so. These devices
will enable coverage in areas that were previously too inconvenient to reach, such as trains, decimated regions or desolate
military campgrounds.
In all these examples, the solar-powered computing devices rely
on wireless technology to communicate with the rest of the world.
Therefore, the devices must have enough computing capacity to run
real-world applications, service IP stacks and USB ports and process
security functions such as WEP and encryption. Additional usage
models are listed in Table 2.
Table 1. Usage Models for Solar-powered
Computing Devices
Table 2. Solar-powered Computing Device Opportunities
Setting
Possible scenario
Requirement
Device
Emergencies
Emergencies
Phone systems lose
power during tsunami
or hurricane
Deploy radio-based
phone network for
relief workers
Network
Routers
• Relief organizations
Remote
Operations
No communications
infrastructure exists for
military or industrial (oil
exploration) teams
Quickly install a new
network in the field
Unreliable
Power Grid
Electrical service is
spotty or non-existent
in rural locations or
emerging countries
Operate networks
and PCs regardless
of energy situation
• Law enforcement
Remote operations/
unreliable power grid
•T
ransportation: Train/bus
passengers
•A
d-hoc military
installations
•H
ouseholds and
businesses without
power access or backup
Surveillance
Sensors
•S
earch and recovery
operations
• Crowd control
Data
Acquisition
•E
nvironmental
conditions monitoring
•A
udio and textual
reporting
Femtocells
and Picocells
• Imaging for military
•S
ecurity for homes
and businesses
• Oil drilling
• Agricultural sampling
• Environmental protection
• Small businesses
• Households
3
White Paper How to Design a Solar-Powered Computing Device
Design Requirements
Table 3. Peripheral and Computing Requirements
Besides more stringent control of power consumption, the design
requirements for a solar-powered computing device are nearly
the same as other small form factor networked devices. All these
devices generally operate without fans, use standard peripherals
and interfaces, and run networking and security applications, as
shown in Table 3. Although most space-constrained devices are
low power, solar devices are different because power consumption
directly impacts the size of solar panels and backup batteries, and
consequently overall product cost.
Device
Peripheral
requirements
Computing functions
requirements
Generic
Requirements
•U
SB wireless adapter
• Networking stack
Designers can minimize power consumption by aggressively
pursuing power management. This is normally accomplished with
a combination of hardware and software techniques, which will be
discussed in more detail in the “Challenge 3: Power Management”
section. The basic idea is to keep the device in a sleep state for as
much time as possible, and only wake up the device when it is needed.
– WEP
– VPN
– Encryption
Surveillance
Sensors
1. Employs a low-power computing system: This two-chip
computing platform has a combined thermal design power
(TDP) under 3 watts1 (0.65W processor and 2.3W chipset),
and features embedded lifecycle support up to seven years.
Using the Deeper Sleep processor state, also called C6, the
TDP of the processor drops to 0.1 watts1.
2. E
nables a small form factor design: This platform can be
implemented with a board that measures (14 cm x 12 cm),
or slightly smaller than a mini-ITX board (17 cm x 17 cm).
•U
SB camera
• Image processing
Data
Acquisition
•S
erial link (RS232)
for sensor interface
• Data processing
Femtocells
and Picocells
• PCI Express* links for
connecting to radios
and transmitters (e.g.,
CDMA, WiMAX)
•P
rotocol conversion
(e.g., CDMA to IP)
Equipment makers typically find maintaining software code
for general-purpose processors, like the Intel Atom processor, is
easier than for application-specific hardware. This is because Intel®
processors are supported by a broad ecosystem offering a wide
range of mature development tools. Developers also benefit from
an extensive Intel tool chain comprising compilers, performance
analyzer and software libraries. And since the Intel Atom proces­
sor maintains Intel® Core™2 Duo processor-based instruction set
compatibility, it can run the breadth of x86 code written over the
past few decades.
3. S
upports standard interfaces and peripherals support:
Designer can use standards based components such as USB
2.0, PCI Express*, DDR2 SDRAM memory, IDE FLASH and other
interfaces supported by commonly used super I/O chips.
4. E
xecutes standard networking and security software:
Since many networking, wireless and security applications are
built for Intel® architecture-based PCs, they work seamlessly on
the Intel Atom processor, thereby lowering equipment manufacturers’ development risk. Networking and security software is
available from the open source community, free of charge.
5. Implements power management features: Power
management is accessible using standards-based Advanced
Configuration and Power Interfaceii (ACPI) and Linux* utilities
and kernels. ACPI defines common interfaces for hardware
recognition, computing board and device configuration and
power management.
4
• Motion detection
• Data compression
Satisfying the Design Requirements
The classes of solar-powered computing devices discussed thus far
can be based on a generic Intel® Atom™ processor-based platform,
as shown in Figure 1. This platform satisfies the following five
design requirements:
• Security
Intel®
Atom™
Processor
Z510
DDR2 400/533
400/533 MHz
FSB
FLASH
(x2)
(x1)
PCI Express* x1
Intel® SCH
US15W
SMBus
(x8)
LPC
USB 2.0
WiFi 802.11 a/b/g
WiMax
PCI Express* x1
PCI Express* x1
IDE Channel
(PATA only)
USB ports
(memory down)
FWH
SIO
Figure 1. Generic Intel® Atom™ Processor-based Platform
How to Design a Solar-Powered Computing Device White Paper
Developers benefit from using one platform for both development
and deployment based on the same Intel architecture that today
supports the majority of the one billion PC users who access the
Internet. Furthermore, developers of software can write their appli­
cations on a standard Intel architecture PC and then drop their code
onto the target platform with high confidence that it will perform
well with minimal tweaking required.
System Example: Surveillance Sensor
Intel constructed a solar-powered surveillance sensor using
an Intel Atom processor-based board, as shown in Figure 2.
The chipset interfaces to a USB wireless adapter, USB camera,
FLASH memory and a console that supports development and
device configuration. The design uses FLASH memory instead
of a hard disk drive to save power and increase reliability.
The board has a voltage regulator module (VRM) that is powered
by an off-board voltage regulator connected to the solar panel.
The solar panel in this design is 10 inches x 10 inches and delivers
5 watts. The voltage regulator also charges the back up battery,
which powers the board when there’s insufficient sunlight to
keep the board running.
Upon initialization, the processor sets up the USB camera and
USB wireless adapter. It runs the IP networking stack and starts
communicating with the access node (e.g., wireless router). The
board then acquires images from the camera and executes
applications such as motion detection and image recognition
and compression. The device sends messages and preprocessed
images to the access node using virtual private network (VPN)
technology and Wired Equivalent Privacy (WEP) encryption.
During normal operation, the board consumes approximately
2.5 amps of current at 5 volts. More current is needed at start up,
and the current draw reaches 1.2A. When the processor is in sleep
mode, only about 0.2A is required. Using this data and knowing the
percentage of time the board is in normal operation, designers can
determine capacity requirements for the battery, as shown in Table
4. There are two ratings on every battery: volts and amp-hours (AH).
Based on calculations, a 6 amp-hour, 12V battery can sustain the
board for 19 hours, assuming it’s in normal operation just 5 percent
of the time. However, battery backup time drops down to 2.4 hours
if the board never enters sleep mode. Developers should conduct
a full characterization of the battery backup system across various
use conditions and manufacturing lots to measure the robustness
of the design.
Table 4. Battery Hours
Device Board
Intel® Atom™ Processor
Intel® SCH US15W Chipset
USB
PATA
LPC
Super I/O
Normal operation
(@ 2.5A)
Sleep mode
(@ 0.2A)
Hoursa based on
6 amp-hours at 5V
100%
0%
2.4 hours
50%
50%
4.4 hours
25%
75%
7.7 hours
5%
95%
19.0 hours
a
4 GB IDE Flash
VRM
Exclusive of board start up
USB Camera
USB Wireless Adapter
Voltage Regulator
and Charger
Battery
Backup
Serial LCD Console
Solar Panel
LPC: Low pin count bus
Figure 2. Surveillance System Sensor Implementation
5
White Paper How to Design a Solar-Powered Computing Device
~5V to circuit board
~12.5V charge
Vsolar up to 25V, 1.2A
3055
3055
5K
5K
Tip29
220µF
Battery
GND
Tip29
200µF
Tantalum
GND
Figure 3. Step-down Voltage Output Schematic
Designing a Solar-Powered Computing Device
Compared to other small form factor embedded designs, it’s
no surprise that solar-powered devices pose additional voltage
regulation and power management challenges. Designers need
to integrate a step-down voltage output circuit and a battery
backup scheme and use processor sleep states to conserve
energy. This section discusses these design aspects.
Challenge 1: Voltage regulation
As with most board designs, the voltage regulator module (VRM) on
the solar-powered device does most of the heavy lifting for supplying
the necessary board voltages. For the Intel Atom processor, these
voltages are VCC (processor core), VCCA (phase lock loop supply) and VCCP
(front side bus AGTL+ termination voltage). The VRM requires at least
5V at 1 amp from the battery, which is charged by a 24V solar panel.
The battery backup stabilizes the platform because it powers the
VRM and provides a large amount of capacitance which is needed
at start up. The battery can drive the VRM using a step-down
voltage output circuit similar to the one illustrated in Figure 3.
Here, the battery voltage is stepped down to 5V to supply the
VRM on the circuit board. Likewise, the solar panel voltage sources
an intermediate 12V step to charge the backup battery. The solar
panel may supply as much as 1.2A at 25V.
A significant limitation of the simplified schematic shown in
Figure 3 is its full board battery charging. A production system
would normally deploy a trickle charge scheme to prolong the
battery’s useful life.
Challenge 2: Source voltage
The VRM does most of the work as long as the battery has
sufficient charge. As mentioned earlier, designers must also
account for the additional current draw and power demands
when the board boots up.
An additional circuit (not shown here) is needed to prevent the
board from attempting to boot up when neither the solar panel
nor the backup battery can supply sufficient power. For example,
6
suppose the battery runs down when there’s no sunlight; the board
will stop running. Later, when the sun begins to charge the solar
panel, the board could try to reboot continuously even though
there’s not enough power in the system to maintain it. Likewise,
the battery never has a chance to recharge because power is
incessantly wasted by failed reboot attempts. Therefore, it’s
necessary to deploy a safeguard that permits the board to
reboot only after there’s enough available energy to sustain
normal operation.
Challenge 3: Power management
Optimizing the system for minimum power consumption is usually
done as a combination of software (operating system) and hard­
ware elements. Most modern operating systems (OS) operate on
buffers associated with the ACPI specification that instruct the
processor to transition between various power-saving states. The
sleep state control logic in an ACPI-enabled processor assumes
the core(s) implements different power-saving states (also termed
sleep states) called C0 to Cn. When developing code for a solarpowered device, software developers should proactively control
the power state of the processor as opposed to leaving it up to
the OS.
The following describes ACPI and open source efforts available
to assist developers.
•ACPI: This is an open industry specification co-developed by
Hewlett-Packard, Intel, Microsoft and Toshiba. ACPI establishes
industry-standard interfaces for OS-directed configuration and
power management on laptops, desktops, servers and embedded
devices. It advances the existing collection of power management
BIOS code, Advanced Power Management (APM) application
programming interfaces (APIs), PNPBIOS APIs and Multiprocessor
Specification (MPS) tables into a well-defined power management
and configuration interface specification. The specification enables
new power management technology to evolve independently in
operating systems and hardware while ensuring that they continue
to work together.
How to Design a Solar-Powered Computing Device White Paper
remotely develop for target hardware, so it’s not necessary to
have hardware in hand (e.g., headless development environment).
The workgroup holds regular conference calls and posts platform
guidelines on its Web site. For more information, visit www.linuxfoundation.org/en/Mobile_Linux.
C0 – Active
C1
C2
C4/C6
Idle States
Scheduler idle
Break
Figure 4. ACPI-based Power State Management
Figure 4 illustrates the basic mechanisms used by a traditional
ACPI software layer to control the sleep states of the processor.
When the core is active, the processor always runs at C0. When
the core is idle, the application transitions the processor to a
sleep state that balances the overhead of entering and exiting
the state and the corresponding power consumption. Thus, C1
represents the power state with the least power savings; however, it can be switched on and off almost immediately. In contrast,
the Deep Sleep states (C4 and C6) consume negligible power, but
the time to enter into these states and respond to activity (back
to C0) is quite long. Note: The Deeper Sleep state (C6) is similar
to the Deep Sleep state (C4), except it further reduces core voltage levels.
The power management capability of the Intel Atom processor
entails more capability than presented here, and a full description
is available in the datasheetiii. In Deeper Sleep (C6), the Intel Atom
processor Z510∆ consumes less than one-eighth the power1 of
the Active (C0) state.
ACPI also enables device drivers to power down peripherals
when idle during normal operation. For example, a driver for
the Intel® 82541ER Gigabit Ethernet Controller goes into Smart
Power Down mode when no signal is detected on the wire. The
Ethernet controller supports power-down states without software
assistance, which frees application developers from being responsible for every system-level power management mechanism.
•Mobile Linux*: The Mobile Linux workgroup has as its mission
to accelerate adoption of Linux on next-generation mobile
handsets and other converged voice/data portable devices,
and to provide a mobile profile for the Linux Standard Base
(LSB). One advantage of this approach is that developers can
•Mobile Linux Internet Project: Moblin.org is an open source
community for sharing software technologies, ideas, projects,
code and applications to create an untethered computing
experience across Mobile Internet Devices (MIDs), Netbooks and
embedded devices. The computing hardware is based on Intel®
Atom™ Processor Technology for use in low power, small footprint,
wireless-enabled solutions. The Moblin Core Linux Stack, an
integrated open source software stack, serves as a starting
point for developing applications for these devices. For more
information, visit www.moblin.org.
•LessWatts: This open source project aims to improve the power
efficiency of the Linux operating system and applications.
LessWatts is about creating a community around saving power
on Linux, bringing developers, users and system administrators
together to share software, optimizations, tips and tricks. For
example, there’s information about WiFi power-saving modes
(Power Save Poll, PS-Poll) that enable the WiFi adapter to notify
the access point when it powers down the radio to save power.
While the radio is powered off, the access point stores any
network packets for the device and sends them after the adapter
powers back up. Other discussions on the Web site include Wake
on LAN (WOL), which allows a master system to send a magic
packet over Ethernet to wake up the solar-powered device.
However, WOL keeps the network card active so it consumes
power even when the processor is in a sleep state. For more
information, visit www.lesswatts.org.
Truly Untethered Embedded Devices
Before the Intel Atom processor, it wasn’t really practical to employ
an Intel architecture processor in a solar-powered application.
However, the revolutionary performance per watt and power
management features of the Intel Atom processor have led to
tremendous advances in reducing power consumption. And the
open source community is sharing best known methods and
creating standards to help realize even greater power savings.
These capabilities are available to equipment makers seeking to
bring the convenience of untethered operation (no power and
network cables) to embedded applications.
7
Intel processor numbers are not a measure of performance. Processor numbers differentiate features within each processor family, not across different
processor families. See www.intel.com/products/processor_number for details.
i
Power consumption numbers are the thermal design power (TDP) for a 1.1 GHz Intel ® Atom™ Processor Z510. Please see disclaimers numbers 1 and 2.
ii
ACPI Specification at http://www.acpi.info/spec.htm
iii
Please download the Intel ® Atom™ Processor Z510 datasheet for the most current product specifications at http://download.intel.com/design/chipsets/
embedded/datashts/319535.pdf.
1 Intel may make changes to specifications and product descriptions at any time, without notice. Designers must not rely on the absence or characteristics
of any features or instructions marked “reserved” or “undefined.” Intel reserves these for future definition and shall have no responsibility whatsoever for
conflicts or incompatibilities arising from future changes to them. The information here is subject to change without notice. Do not finalize a design with
this information. The products described in this document may contain design defects or errors known as errata which may cause the product to deviate
from published specifications. Current characterized errata are available on request. Contact your local Intel sales office or your distributor to obtain the
latest specifications and before placing your product order. Copies of documents which have an order number and are referenced in this document, or
other Intel literature, may be obtained by calling 1-800-548-4725, or by visiting www.intel.com.
2
Performance tests and ratings are measured using specific computer systems and/or components and reflect approximate performance of Intel ® products
as measured by those tests. Any difference in system hardware or software design or configuration may affect actual performance. Buyers should
consult other sources of information to evaluate the performance of systems or components they are considering purchasing. For more information on
performance tests and on the performance of Intel products, visit http://www.intel.com/performance/resources/benchmark_limitations.htm
This document is for informational purposes only. INTEL MAKES NO WARRANTIES, EXPRESS OR IMPLIED, IN THIS DOCUMENT.
*Other names and brands may be claimed as the property of others.
Copyright © 2008 Intel Corporation. All rights reserved.
Intel, the Intel logo, Atom, and Core are trademarks of Intel Corporation in the U.S. and other countries.
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