| | | | | When closely examining the power consumption |
| How to design ultra low power for ZigBee RF4CE and | | | | behavior of electronic circuits, it becomes apparent |
| 802.15.4 Wireless sensor networks | | | | that what initially looks like a flat current curve actually |
| The new Communication Controller Centric | | | | bears more resemblance to a mountain range with |
| Transceiver Chip Design Architecture | | | | peaks and valleys. When certain functional blocks |
| | | | | become active, they draw peak current. When two |
| By Cees Links, CEO of GreenPeak | | | | functional blocks switch on simultaneously, the peak |
| | | | | amplitude doubles. |
| Wireless Technology is evolving from communications | | | | The secret to reducing the peak power lies in carefully |
| to between people and computers to communications | | | | managing the turn-on and turn-off time for key |
| between machines. There is a third wave of wireless | | | | functions so that double peaks can be avoided. |
| that is following the almost ubiquitous integration of cell | | | | |
| phones and wireless Internet (Wi-Fi) into our lives. | | | | Synchronized Wake Up and Sleeping enables |
| | | | | reduction of power consumption for low power mesh |
| This third wireless wave consists of wireless sense | | | | networks |
| and control networks that can connect and control all | | | | One of the most dramatic differences between |
| kinds of equipment in our homes and businesses – | | | | wireless sensor communications technology and other |
| from freezers to light switches, from consumer | | | | well known wireless technologies is the ability of |
| electronics (TV, DVD-player) and remote controls to | | | | sensor nodes to forward messages from other nodes |
| sensors, for detection or protection, and to central door | | | | located further down the communications chain. This |
| locking and window locking in our homes (as we are | | | | technique, known as mesh routing or multi-hop |
| used to in our cars). | | | | networking, provides an effective and reliable means |
| | | | | of spanning large infrastructures, beyond the range of |
| Unfortunately, using today's wireless technologies, most | | | | what a single wireless link can do. |
| of those wireless sensors and controls require the use | | | | For a node to forward a message received from |
| of a significant quantity of batteries creating | | | | another node, it needs to be in an awake and receiving |
| environmental concerns (think toxic chemicals and | | | | mode when the original wireless message arrives. |
| heavy metals) as well as a serious maintenance | | | | Unfortunately, the reception mode requires so much |
| problem (continuously exchanging batteries). Therefore | | | | power that it can drain batteries in a matter of a few |
| ultra low power wireless networks that require very | | | | days. As this power lifespan is too short for most |
| little power are of great interest. | | | | real-life applications, the most straightforward solution, |
| | | | | as specified by most industry standards, is to limit the |
| This includes systems that can run off of a single cell | | | | multi-hop capability to the nodes that are permanently |
| battery for the life of a device as well as wireless | | | | connected to the main power. In such a framework, |
| networks and sensors that can be powered by | | | | low-power devices, which are assumed to be in a |
| energy harvesting (sometimes called energy | | | | power-down mode most of the time, are not capable |
| scavenging). Creating ultra low power wireless | | | | of retransmitting messages from other devices. These |
| networks and systems that can run off the energy | | | | low-power devices, known as end-devices, are |
| that is available in the environment instead of batteries | | | | located at the end or beginning of the communications |
| is a very exciting emerging technology. | | | | chain. |
| | | | | This framework, which combines mains-powered |
| Last year, the ZigBee organization partnered with | | | | mesh routing devices and low-power end-devices, |
| several of the largest consumer electronics companies | | | | works for some applications. Take, for example, an |
| in the world (Panasonic, Philips, Sony and Samsung) to | | | | office lighting application utilizing interconnected wireless |
| form what is known as ZigBee RF4CE (Radio | | | | lamps and light switches. The lamps, which are |
| Frequency for Consumer Electronics). This industry | | | | connected to the main power source, house the mesh |
| partnership signals the development of an entire new | | | | routing communication nodes. The switches, which are |
| generation of remote control devices – for TVs, for | | | | not mains powered, are a natural place for the |
| home and office automation, for many other types of | | | | end-devices. |
| remote control products that communicate via low | | | | Many other applications do not fit well in such a |
| power RF instead of the decades old IR (infrared). By | | | | framework. In applications like gas detection, fire |
| using these new communication technologies, we soon | | | | detection, access control, precision farming, battlefield |
| shall be seeing a wide range of remote devices that | | | | monitoring, perimeter surveillance, warehouse |
| are not only interoperable among brands and models, | | | | temperature monitoring, etc., mains power is not readily |
| but require so little power that their batteries will never | | | | available or even present. Running a power cable in |
| have be changed or recharged. It is even possible to | | | | these applications would be cost prohibitive, offsetting |
| design and build remotes that will not require any | | | | the benefit of wireless communication. |
| batteries at all and will get their power from energy | | | | To address this class of applications requires |
| harvesting. | | | | low-power multi-hop networking, or low-power routing, |
| | | | | in which all of the nodes, including the mesh routing |
| Challenges of wireless sensor networks | | | | nodes, operate in low-power mode. |
| | | | | By using a "synchronized wake-up" scheme, it is |
| The biggest technical challenge for developing these | | | | possible to coordinate receiving activity in a way that |
| ultra low power sensor networks is managing the | | | | eliminates the need for the mesh routing nodes to |
| energy consumption without reducing range or | | | | continually operate in receive mode, thereby |
| functionality, like speed and standards compliance. The | | | | significantly reducing power consumption. The picture |
| resulting elimination of battery replacement will then | | | | below depicts how low-power-routing works when |
| simplify maintenance and provide a higher level of | | | | Node A wants to send a message to Node C, through |
| ease of use and safety. | | | | Node B. All nodes in the pictures are low-power nodes, |
| | | | | sleeping most of the time. |
| Ultra low power consumption | | | | By synchronizing the sleep/wake-up cycles of the |
| It is obvious that current consumption – milli-amps | | | | nodes to each other, nodes wake up when they |
| – and duty cycling are important in wireless sensor | | | | expect a message from a neighboring node. This |
| networks. However, minimizing current consumption is | | | | enables the routing nodes to operate in a nearly |
| only part of the solution. There are several essential | | | | powerless sleeping state most of the time, thereby |
| issues key to developing low power wireless sensor | | | | achieving ultra-low-power operation. Clearly, more |
| applications, but it all starts with the development of an | | | | wake-ups will occur than strictly required to carry the |
| ultra low power transceiver radio chips. | | | | data, as neighboring nodes will not always have data |
| By using a communication controller centric chip design | | | | to transmit. However, the additional power required for |
| instead of a microcontroller centric design, along with | | | | periodic wake-ups and synchronization is more than |
| synchronized wake-ups, it is possible to reduce overall | | | | offset by the power saved by eliminating the need for |
| power consumption by 65% or more. | | | | continuous receive mode operation. |
| Most transceiver solutions require that the MCU be | | | | Since its inception, wireless sensor technology has |
| switched on the whole time during the transmission of | | | | been linked with low-power electronics. Most |
| a package. By using GreenPeak Technology's | | | | low-power wireless sensor networks have been |
| GP500 communication controller, the MCU is only | | | | designed for low power, meaning that they consume |
| required to process the data to be transmitted or | | | | little power when switched on. That is not enough. By |
| received. | | | | using communication centric transceiver chips, wireless |
| Most low power radio networks rely on a processor | | | | mesh networks, and synchronized wake up and sleep |
| centric approach that requires a microcontroller to | | | | cycles, developers can now create systems that don't |
| handle all the intelligence for the transceiver. This | | | | even need batteries and instead, can utilize energy |
| requires the microcontroller to be awake the entire | | | | harvesting to power the sensor network from |
| time that in turn requires additional power. By using a | | | | environmental power sources. |
| more energy efficient communication controller | | | | |
| approach, the transceiver can transmit and receive the | | | | The wireless sensor network standard – IEEE |
| data independently from the microprocessor and the | | | | 802.15.4 |
| microprocessor is only awakened and used when it is | | | | |
| needed to further process the data. | | | | For wireless sensor transceivers the dominant and |
| By using a hardware based scheduler and | | | | probably only real standard is the IEEE 802.15.4 |
| synchronizer within the chip itself, the radio only wakes | | | | specification. However, there have been efforts to use |
| up as needed to see if there is any data that needs to | | | | Bluetooth and Wi-Fi for low power sensor applications. |
| be sent. If not, it returns to sleep. If there is data to be | | | | In most of the cases reported, Bluetooth and W-Fi |
| sent, the controller then wakes up the microcontroller. | | | | were used in a non-standard way, in fact weaving the |
| The chip then communicates the information and then | | | | principles of IEEE 802.15.4 in their native implementation. |
| goes back to sleep until the next time it is scheduled to | | | | It is nowadays widely accepted that the IEEE 802.15.4 |
| wake. 9999 times out of 10,000 – there is no | | | | offers the best basis for wireless sensor network |
| message to be sent and the controller does not need | | | | applications. |
| to energize the microprocessor. Every time that data | | | | |
| is sent, the chips also transmit a synchronization | | | | Besides the IEEE 802.15.4 standard, a number of |
| message to ensure that they all wake up together on | | | | technology suppliers have chosen to build proprietary |
| the next duty cycle. | | | | transceivers. The main motivation seems to be a |
| By letting the communications controller decide when | | | | reduction of the complexity and thus a potential lower |
| to wake up and check for messages, it is possible to | | | | cost point. However, it remains to be seen if a |
| greatly reduce overall energy consumption. Because | | | | proprietary solution will ever reach sufficient volumes |
| of the scheduler and synchronizer inside the | | | | to actually reach that theoretically lower cost point. |
| communication controller, the system only wakes up | | | | Additionally, reducing the complexity automatically goes |
| for a brief moment to check to see if there are any | | | | hand in hand with sacrificing performance and thus |
| messages and goes back to sleep. By letting the | | | | limiting the applicability. |
| microprocessor sleep until it is needed, it is possible to | | | | |
| save over 65% of energy usage as compared to a | | | | Proprietary technologies are vulnerable, for two |
| the typical always on traditional transceiver | | | | reasons: (1) the owner of the technology controls the |
| | | | | specification and thus also the price, and (2) the |
| If you multiply this individual node power saving by a | | | | customer depends on the technology owner for |
| wireless network of over 100 nodes, it is obvious that | | | | upgrades and uninterrupted sourcing. |
| the entire network will be able to operate using vastly | | | | |
| less power than a traditional microprocessor based | | | | Even within the boundaries of standards, technology |
| network. | | | | providers can discover and leverage differentiation |
| | | | | opportunities. |
| Peak current savings | | | | |
| There are three typical wireless sensor node states | | | | As an example GreenPeak has developed transceiver |
| for a commonly used wireless sensor platform. Each | | | | and network stack technology that is compliant to the |
| has its own level of current consumption. In state one, | | | | IEEE 802.15.4/2.4 GHz standard but includes additional |
| the microprocessor and transceiver are in sleep mode | | | | functionalities that enable its use for ultra low power |
| (10µA). In state two, the microprocessor is switched | | | | applications. An ultra-low-power application is defined |
| on while the transceiver is asleep (10 mA). In state | | | | as an application that is able to live off a coin cell |
| three, both the transceiver and the microprocessor are | | | | battery or off energy harvested from the environment |
| awake (27 mA). | | | | through a solar cell, a vibration energy harvester or |
| | | | | any other environment energy converter. |