New Chip Architecture Slashes Power Requirements
It is possible to further reduce ZigBee's RF4CE's overall power consumption by 65% or more by using a communication controller centric chip design instead of a microcontroller centric design (MCU), along with what is known as "synchronized wake-ups."
Figure 3 – GreenPeak’s communication controller-centric architecture versus traditional microcontroller-centric approach. Most transceiver solutions require that the MCU be switched on the whole time during the transmission of a package. By using a communication controller, the MCU is only required to process the data to be transmitted or received.
Because most low power processor centric radio designs require a microcontroller to handle all the intelligence for the transceiver, the microcontroller needs to be awake the entire time, which requires additional power. Instead, by using a more energy efficient communication controller approach, the transceiver can transmit and receive the data independently from the microprocessor. Thus the microprocessor is only awakened and used when it is needed to further process the data.
Synchronizing the wake ups means that the communications controller decides when to wake up and check for messages. The device can be off most of the entire time – thereby greatly reducing overall energy consumption. This is especially effective for the home's various environmental, security and location sensors. Because of the scheduler and synchronizer inside the communication controller, the system only wakes up for a brief moment to check to see if there are any messages and then goes back to sleep.
By using a hardware based scheduler and synchronizer within the chip itself, the radio only wakes up as needed to see if there is any data that needs to be sent. If not, it returns to sleep. If there is data to be sent, the controller then wakes up the microcontroller. The chip then communicates the information and then goes back to sleep until the next time it is scheduled to wake. 9999 times out of 10,000 – there is no message to be sent and the controller does not need to energize the microprocessor. Every time that data is sent, the chips also transmit a synchronization message to ensure that they all wake up together on the next duty cycle.
Figure 4 – By letting the microprocessor sleep until it is needed, it is possible to save over 65% of energy usage as compared to a the typical always on traditional transceiver
Figure 4 illustrates that by letting the communications controller decide when to wake up and check for messages, it is possible to greatly reduce overall energy consumption. Because of the scheduler and synchronizer inside the communication controller, the system only wakes up for a brief moment to check to see if there are any messages and goes back to sleep.
Peak current savings – Managing Turn On and Turn Off
Figure 5 depicts the current consumption in three typical wireless sensor node states for a commonly used wireless sensor platform. In state one, the microprocessor and transceiver are in sleep mode (10µA). In state two, the microprocessor is switched on while the transceiver is asleep (10 mA). In state three, both the transceiver and the microprocessor are awake (27 mA).
Figure 5 – the three wireless node states and typical power consumption
When closely examining the power consumption behavior of electronic circuits, it becomes apparent that what initially looks like a flat current curve actually bears more resemblance to a mountain range with peaks and valleys. When certain functional blocks become active, they draw peak current. When two functional blocks switch on simultaneously, the peak amplitude doubles.
The secret to reducing the peak power lies in carefully managing the turn-on and turn-off time for key functions so that double peaks can be avoided.
Advantages of pre-integrated chips for designing ZRC solutions
Using pre-integrated ZigBee RF4CE chips targeting specific application models enables the fast and easy development of robust and low cost ZigBee RF4CE remote control applications. As the industry is still in the transition cycle between IR and RF, it makes sense to include the full IR functionality for use in legacy product designs as well as an embedded keyboard scanner for use in the remote control.
The ZigBee RF4CE chip for the remote control needs to have the best possible power optimization, while the ZigBee chip for the set-top box or TV set does not need to optimized for power, but it does need to offer the proper interfaces for easy integration. For TV sets, a UART interface is preferred while for a set-top box, an SPI/TWI interface is preferred.
Bringing the complete RF4CE functionality for each application into a single device makes low cost and reliable RF remote controls a reality. Pre-integration brings the cost of the total solution down and makes the choice for RF remote controls even easier.
About the Author
Cees Links is a pioneer of the wireless LAN industry, a visionary leader bringing the world of mobile computing and continuous networking together. With his leadership, the first wireless LANs were developed which ultimately became house-hold technology integrated into the PC’s and notebooks we all use today. His group also pioneered the development of access points, home networking routers and hotspot base stations, all widely used today.