Reference design addresses issue of power consumption in portable products

With battery technology progressing slowly, device manufacturers have picked up the baton. The creation of ever smaller process technology nodes brings lower power consumption for the parts made on them, but designers can't rely on this alone. Looking to help designers create products that draw less power, Arrow Electronics has developed the Oryx board, described as an 'ultra low power reference design for mobile applications'. Using components from a range of leading suppliers, the board draws just 42mW in full operating mode while providing a range of functionality. While portable devices can be used in a range of different applications, they all have certain aspect in common. For instance, they will have some kind of display, sensors to capture data, a processor, a real time clock, memories and some form of input, which could be a touchscreen. The Oryx board has been designed to address a wide range of portable applications. With power efficiency in mind, the board features low power components from Sharp, NXP, Analog Devices and Linear Technology. At the heart of the board is an ARM Cortex-M0 based microcontroller from NXP. The LPC11U14 boasts a small silicon area, low power consumption and a minimal code footprint, allowing it to offer32bit performance at an 8bit price point. According to NXP, the Cortex-M0 processor promises substantial savings in system cost while retaining tool and binary compatibility with Cortex-M3 based devices. "Even at full capacity and at a clock rate of 50MHz," said Patrick Delmer, Arrow Electronics' supplier business manager, "the cpu consumes only 8mA. Different power down modes lower power consumption in different stages, depending upon the cpu's workload. If full performance isn't required, the clock rate drops to 12MHz and power consumption to 2mA." Four power profiles can be selected: default mode, the state which follows an mcu reset; performance mode, where the processor is at its maximum capacity; efficiency mode, where computing power and power consumption are balanced; and low current mode, where power consumption is reduced to a minimum. In many portable products, the display consumes the most power. However, Sharp's 1.35in LS013B4DN04 Memory LCD display consumes only 4µA. The display features 1bit storage units at each pixel, which means only the pixels which change need to refreshed. A further contribution to low power consumption comes from the fact the screen doesn't need backlighting. Sven Johannsen, business development manager for Sharp Microelectronics Europe, explained the benefit: "With conventional transmissive lcds, the microcontroller has to rewrite the entire screen frame by frame at 50 or 60Hz; even if most of the image remains the same. The combination of these benefits means the screen consumes less than 1% of the power required by a similar conventional display." The board is powered from a rechargeable lithium ion battery, which can be charged from several different sources. For instance, power can be supplied through the USB interface when connected to a pc, or through the expansion connector. A further option is to charge the battery from an external source, such as a solar panel. The board runs on 3V and all parts of the design remain under power at all times, with the exception of three areas. Power switches associated with three external peripherals allow their power consumption to be controlled. According to the developer, these parts – the spi flash memory, the memory lcd and the trimming potentiometer – have relatively high power consumption when the board is in low power mode. The board's developer's point out that monitoring low power operation requires efficient power management. The Oryx board achieves this using Linear Technology's LTC4071 shunt charger. This part allows lithium ion batteries to be charged using very low current, intermittent or continuous sources – the company suggests currents as low as 550nA can be used. It features a near zero current low battery latching disconnect function to protect low capacity batteries from deep discharge and potential damage. With its low operating current, the LTC4071 is also suited to use in energy harvesting applications and its architecture allows for a simple battery charger solution, requiring only one external resistor. In order to allow developers to begin integrating external sensor inputs into their designs, the Oryx board is equipped with the three axis ADXL345 accelerometer from Analog Devices, as well as eight PCF8885 multichannel touch sensors from NXP, which can also act as control elements. According to Arrow, all sensors consume no more than 400nA when in stand by mode. Software can be developed using the LPCXpresso integrated software development environment, which has been created by Code Red specifically for use with NXP's LPC microcontroller range. LPCXpresso, which includes all the tools necessary for cost effective software development, is based on a simplified Eclipse framework with LPC specific enhancements. Using LPCXpresso IDE, users can build an executable of any size with full code optimisation. The Oryx reference design also an LPCXpresso compatible embedded SWD/JTAG interface called LPC-LINK (see fig 1). This facility, which is said to simplify program download and debugging, can be disconnected in order to minimise power consumption when running an application on the host microcontroller. The Onyx reference design is seen to be a good starting point for the development of mobile applications ranging from data loggers and portable input terminals to intelligent remote controls and electronic toys. According to Delmer and Johannsen, the Oryx board is the 'almost perfect' development platform for data loggers. A device which acquires one reading every second requires full cpu performance only for 20ms, during which power consumption rises to 14mA. For the remaining time, the system can be in power down mode, where consumption is 6.57µA. When powered by a 45mAh lithium ion battery, the logger would operate for more than six days between charges. With a measuring frequency of one reading per minute, this rises to 167 days. An intelligent remote control could take advantage of solar charging. Because these devices are inactive for most of the time, the reference design could be used to develop a self sufficient control which is 'woken up' by the accelerometer when it is picked up. For medical applications, developers might consider a device which integrates the ability to measure temperature and pulse rate, with further data input using a keypad. Such a device could reasonably be expected to have a battery operating life of several days. In the leisure sector, devices could be developed which last for days between charges, rather than hours provided by current products. Arrow gives, as an example, a gps enabled product that could be used on an extended biking tour. The Onyx reference design is available from Arrow Electronics. Schematics and a user manual are available for download from its website, as are the development software package and an updated software library.