Scan power profile

[Alexandre Bard]

Hello everybody,

I'm developping an application using nRF51 Development Kit with S130 softdevice and I'm looking for the power profile of scanning event. I understand that it's not yet available in S130 specification, but I looked in S120 specification and it seems to be missing : the chapter 14 BLE Power profiles contains only the Connection event profile and the caption of Table 33 is a bit confusing, speaking of "advertising event".

So am I understanding something wrong ? Looking at the wrong place ?

[Stefan Birnir Sverrisson]

Hi Alexandre

A nRF51822 central device with S120 periodically scans for peripheral devices in order to discover advertising packets from the peripheral. While scanning, the device does so with fixed RX radio power, and that can not be configured. The current consumption while scanning is around 13mA. What you can adjust is the scan interval and scan window, which determine the duty cycle of your scanning.

While the device is not scanning it will reside in System On low power mode with current consumption of ~4uA.

If you would choose to have very low duty cycle (long scanning interval and short scanning window) , then you would consume less average power, but it will take longer for the device to discover peripherals and connect them, so it will in theory not save you any power, just make your device less responsive.

Update 25.8.2015 Continuous scanning is enabled by simply turning on the nRF51 radio receiver, which is fixed 13mA. Our new nRF52832 chip however has receiver current consumption of 6.1mA with even better sensitivity than nRF51.

The way to reduce current consumption is to simply not scan the whole time, i.e. scan periodically. That is done by adjusting the SCAN_INTERVAL and SCAN_WINDOW parameters in e.g. the ble_app_hrs_c example in the nRF51 SDK. The SCAN_WINDOW window specifies how long the device should scan inside the SCAN_INTERVAL.

If you have power constraints on both peripheral and central side, you need to balance the current consumption between the two as desired, i.e. by having the central scan with decent scan interval and the peripheral to advertise with decent advertising interval. If you let the peripheral advertise with long advertising interval and the scanner scan very seldom, then you will have to wait a very long time for the central to receive an advertising packet from the peripheral. You need to spend some current on either or both sides in this process in order to have adequate response time.

[pltv]

Hello Stefan

Could you explain more on the 13mA current consumption. I have a requirement for continuous scanning and advertising and I am observing that the program burns out the battery pretty fast. We are exploring ways of reducing current consumption. It will be useful to understand in detail how the scanning functionality affects current consumption.