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How to optimise power for battery-less BLE beacons

Posted: 12 May 2016     Print Version  Bookmark and Share

Keywords:smartphones  Bluetooth Low Energy  BLE  Wireless Sensor Node  beacon 

The limited power budget available with an EHS implies that every aspect of the embedded system should be optimised energy-wise so that it works seamlessly while powered by the EHS. There are many sub-systems in such a system that may be power hungry and need to be optimised to ensure they do not pull down the output of the EHS. Some of the key areas to target when optimising power include the following:

CPU clock frequency
The system clock frequency determines how fast a particular routine is going to be processed and how much energy it is going to consume during that time. A faster clock means faster processing but higher current consumption. Also, each device has a certain minimum and maximum clock frequency requirement, which should not be violated.

For EHS-based designs, an optimised clock frequency has to be chosen with regards to following two factors: Average current consumption and Peak current consumption.

The EHS capacity must consider both of these factors. The average current is the time average of current required during an active state. However, the peak current is the instantaneous maximum current requirement of an active state, and is often much higher than the average current. It may be possible that average current required is well within the capacity of the EHS, but the peak current will cause the sudden energy depletion of the EHS, causing the voltage to fall below the cut-off voltage. Note that the processing time is part of the calculation of average current consumption.

Figure 9 shows the power vs time plot of a particular routine processed at two different system frequencies, first at 48MHz and a second at 12MHz.

Figure 9: Current consumption while processing a routine at 48MHz (Source: Cypress).

Figure 10: Current consumption while processing a routine at 12MHz (Source: Cypress).

In this example, the 48MHz processed routine takes ~300µs to complete and consumes about 10 mA peak during this period. The 12MHz processed routine takes 1.1 ms to complete but consumes only 4 mA peak current. The average current consumed during the process is greater at 12MHz but has a lower peak current requirement. Depending on the EHS capacity, one can go ahead with a short 48MHz clock setup, a longer 12MHz clock setup, or a mix of both, where the clock frequencies are switched from one process to another. Such current profiling should be taken into account while selecting the optimised system frequency.

Low power device boot-up
Once an embedded device is powered, it goes through a boot-up procedure before it can execute application code. A typical boot-up sequence includes:
1. Initializing memory
2. Setting interrupt vectors
3. Configuring peripheral and common registers
4. Initializing external clocks, if any.

Each of these steps takes CPU processing time to complete, which in turns consumes energy. The amount of energy consumed depends on the type of device used, the system clock frequency, how large the memory/register set that is being initialized is, and the time it takes to setup external clocks. Thus, the boot-up process is a power-intensive activity and has to be optimised so that it does not consume an excessive amount of energy from the energy harvester output. Factors that should be kept in mind while writing boot up code are:
1. Initialise only those sections of memory and registers which will be used. Leave others set to default values.
2. Most wireless systems need high accuracy external clocks. These clocks, such as an external clock oscillator or watch crystal oscillator, have a large stabilisation time after start-up. Rather than waiting in active mode for the clocks to stabilise, the system should be put in low power mode (sleep/deep sleep) and awakened only when they are ready for use. Use an internal timer for this purpose.

Low power system start-up
Once the device begins executing application code, there is usually a need to start individual peripherals in the system. These peripherals could be internal to the device, such as an ADC, or may be external to the device, such as a sensor. The starting time may not be large individually for peripherals, but in combination, the overall set up could require enough processing time to drain the stored energy in the EHS.

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