Power management method for use in electronic devices

Description of equivalent WO 02065262 A2

POWER MANAGEMENT METHOD FOR USE IN ELECTRONIC DEVICES

BACKGROUND OF THE INVENTION

00031. Field of the invention

0004This invention relates to a power management method for use in electronic devices. It is particularly applicable to combination devices powered by batteries comprising one portion which acts as a master and another portion which acts as a slave. Devices combining handheld computers (such as personal digital assistants) together with cellular telephones, pagers and other portable radio devices are examples of such combination devices.

00052 Description of the prior art

0006Power management for handheld devices such as personal digital assistants (PDAs), cellular telephones and handheld computers present distinct problems to be resolved.

0007Two problems in particular must be addressed: battery life and the protection of data stored in the device. Battery life is addressed by a number of different methods of reducing a device's power consumption, including causing the device to shut down (i.e. enter a low power consumption state) when not in use, referred to as going into a 'sleep' mode, and by causing devices such as cellular telephones to camp, i.e., cycle on and off many times a minute, switching on only very briefly to determine if a call or message is being transmitted to the cellular telephone.

0008Certain types of electronic data storage, for example random access memories or RAM, typically require that the device retain some electrical charge in its batteries to preserve stored data — if the battery is completely discharged, the data will be lost. To avoid loss of data, it is therefore common for such electronic devices to shut down completely before a certain minimum safe battery charge is reached. Alternatively, some devices, such as the Psion® 5^mx have a secondary battery to protect memory - though this device also shuts down at a minimum battery charge. While such shut-down arrangements are an effective way of protecting memory, they are extremely irritating to users, since they necessarily inhibit access to the functions of the device; premature shut down, i.e., a shut down before a critical battery energy level is reached, is yet more irritating and therefore undesirable.

0009Many such portable devices also use rechargeable batteries. However, rechargeable batteries' characteristics (including the amount of energy they can store and provide) for a given charge vary (i) across their lifespan; (ii) because users often have second or replacement batteries; and (iii) because manufacturing of such batteries often does not produce batteries with consistent characteristics.

0010Combining two different types of consumer electronic device, such as a handheld computer and a cellular telephone (or a wireless LAN emitter), into a single device (a so- called "one-box" solution) presents special difficulties. First, the power conservation strategies followed by the devices are different - the cellular telephone camps, i.e., constantly cycles on and off — while the handheld computer will typically sleep or power down to preserve power. Nonetheless, both devices will typically be powered by a single battery. Secondly, one device must usually be the dominant or controlling device, i.e., the 'master', and the other device the subordinate or controlled device, usually referred to as the 'slave'. Usually it is the master which will control system memory and determine when power levels have reached a critically low level, requiring a complete shut down of the device so as to protect system memory. However, if the master is the handheld computer component of the device, it will not normally be able to monitor power levels while it is in a sleep state, although the cellular telephone radio component, which is camping, will continue to consume electrical power.

0011As a practical matter, the handheld computer component must normally be the master and will supply most of the principal telephone functions and features, while the functions of the cellular radio component must be limited. Thus it is the handhled computer component that would usually dial calls, decide to accept calls, display data, maintain memory, configure the telephone and control power. The slave cellular radio component would usually provide signal transmission/reception as well as camping. It may have limited power management to ensure minimal transmission quality, i.e., inhibiting the cellular radio from operation if the instantaneously available power levels are too low to transmit a signal of acceptable quality. In addition, some cellular standards such as the GSM standard allow the cellular radio to vary its transmission power depending on conditions in a given cell (e.g., distance from cellular mast), a useful facility which has the drawback of also varying power consumption by the slave device, leading to uncertain energy consumption rates.

0012While we have described the power management problems associated with a combined handheld computer and cellular telephone, numerous master/slave arrangements raise the same or similar problems, including devices combining handheld computers with wireless LAN radios such as radios operating on the IEEE 802.11 standard or the -Buetooth® standard.

SUMMARY OF THE INVENTION

0014In a first aspect of the invention, there is a power management method for use in an electronic device, in which the device comprises a first section which stores volatile data and is capable of entering a sleep state and a second section which can automatically and regularly power itself up and down, and a power source powering both the first and the second sections; wherein the method comprises the steps of the first section powering itself up from a sleep state automatically and a power management algorithm then operating to assess whether both the first and the second sections should be taken to a low power consumption state.

0015The invention in one implementation is a method by which a master device which enters a sleep mode during periods of non-use can monitor the power consumption of the slave device, which depends on the same battery or power source and remains in operation while the master device is in a sleep state. In principle, this implementation of the invention consists of arranging the sleep mode of the master device, so that the master device will briefly and periodically partially revive and measure the rema-_αing energy, and if the energy remaining in the power source or the battery has dropped below a certain critical level, shuts both master and slave devices down completely. The decision as to when to shut down can be arranged in either of three ways — (i) the device can have a preset threshold based on a calculation of maximum likely power consumption by the slave device before the next revival of the master device, or (ϋ) the master device can engage in a dynamic calculation of power consumption rates. When this calculation determines that remaining power will drop below the level necessary to protect data before the next scheduled revival of the master, the devices are shut down, or (ϋi) the master can simply reset its next revival time so as to allow it to shut down the joint devices when the power remaining in power source or battery is at a -minimum safe level to protect the data stored in the device. In addition, the master device may in certain arrangement be capable of resetting certain device parameters of the slave device to lower its power consumption, by for example elongating the off periods in the camp mode of a cellular telephone.

BRIEF DESCRIPTION OF THE DRAWINGS

0017Figure 1 is a schematic of a master/slave device containing a computer linked to a GSM modular radio.

0018Figure 2 is a flowchart of the method of operation of the power management arrangement, with a broken line delineating the power management/ sleep steps of the flowchart.

0019Figure 3 is a flowchart illustrating the area delineated by the broken line in Figure 2 when a dynamic power management method is in use.

DETAILED DESCRIPTION

0021The invention is described with respect to a device at present under development by Psion PLC of Great Britain which comprises a GSM or UMTS cellular radio combined in a single device with a handheld computer, collectively powered by a single rechargeable battery, which is potentially replaceable by the user. The handheld computer's principle function is to store and display e-mails and other forms of data received over the GSM/UMTS network. It should be apparent that variations on this arrangement are possible, including but not limited to replacing the GSM/UMTS cellular radio with another type of cellular telephone radio or a wireless local area network radio (wireless LAN) or other slave device which is either consistently on or camps while an associated master device sleeps.

0022In one implementation as illustrated in Figure 1, the GSM radio is combined with a handheld computer which operates using a microprocessor developed by Psion and known as Halla which is based on the ARM920T™ processor and which is based on a 16/32 bit embedded reduced instruction set (RISC) cached processor macro-cell core developed by ARM Holdings PLC of the United Kingdom as part of a family of processors referred to as the ARM9™ Thumb® series. Development kits, including development boards for this ARM920T™ are available from ARM and further detailed information is available from the ARM PLC website http://www.arm.com. Halla incorporates a power measurement function which measures the energy gain of rechargeable batteries during charging and can measure subsequent power consumption providing an accurate measure of remaining battery charge. Alternately, microprocessors such as the bq2945 Gas Gauge IC manufactured by Benchmarq Microelectronics Inc., of the United States are available which measure the available charge in NiCd, NiMH, and Li-Ion batteries, and which can be integrated into a battery pack, or into the device. These devices typically reset after the battery is fully recharged, to a predetermined presumed energy capacity for the battery and measure power consumption from that point.

0023In the implementation as illustrated in Figure 1, the device has four principle components, a battery: a GSM radio module, a computer, and a real time clock ("RTC") which is always ON. The operating system of the computer is provided with a standard sleep function, which initiates a sleep routine, if after a predetermined time, no user inputs have been detected by the computer. As illustrated by the flowchart in Figure 2, the basic sleep routine consists of addressing the system gas gauge to determine the remaining battery energy ("E_R") at the time the sleep routine is initiated, and writing a record to the system's RAM of that measured energy level. If the energy level is below a preset critical energy ("E_c") the entire device is shut down (or placed in a safety mode) including both the master and the slave devices. If the remaining battery energy is more than the preset critical energy, the device writes an end time for the sleep period to the RTC, and places the master component, i.e., the computer in a sleep mode. At the end of the sleep period set by the clock, it revives the computer by causing an interrupt to be sent to the processor, invoking an interrupt handler to run a script carrying out a short routine, which does not include powering user interfaces or displays. In this first aspect of the invention the routine consists of measuring the remaining battery energy, and if it is below the preset critical energy, shutting down the device, and if it is greater than the preset critical energy writing an end time for a new sleep period to the RTC and placing the master component, i.e., the computer, into a new sleep period.

0024The system will perform a measurement of the remaining power in the battery and only place the device in a sleep mode if that remaining energy is higher than a preset rninimum for allowing the master component to sleep.

0025The sleep periods ("T_s") are calculated by dividing the available energy ("E_A"), which is equal to the remaining energy minus the critical energy (E_R- E_c) by a pre-set value for energy consumption by the cellular component per unit time while camping ("P "). ^aGd dividing the time value thus obtained by a preset number ("N") which is greater than 1, i.e., <img file="WO02065262A2_D0001.tif" />

0026N is described as a heuristic factor and will usually have a value of 1.5 to 2. N may have a variable value greater than 1, which is calculated by comparing the energy consumption rate during the previous sleep cycle and comparing it with an average calculated for either all previous sleep cycles of a finite number of previous sleep cycles, and if the energy consumption during the last sleep cycle exceeds one standard deviation, increasing the preset initial value of N by 1 (or another selected value).

0027P_c is calculated by determining the energy consumption rate by the slave during the previous sleep cycle.