Receive current / power consumption and continuous scanning.

[Tim]

Hi,

I have some questions about how BLE scanning works that haven't been answered clearly anywhere I can find, (except no-doubt the Bluetooth Spec/Tome):

1. What is the receive current for the nRF51822 when it isn't actually receiving any data. The data sheet says about 12 mA for 250 kb/s, 1 Mb/s and 2 Mb/s, but what about 0 b/s? Is it roughly the same? I would assume so but it would be good to know for certain.

2. My understanding is that BLE minimises the power draw on the central and peripheral when connected by accurately coordinating the timing of communication - i.e. communication only happens once per advertising interval. In-between both the central and peripheral sleep. Is this correct?

3. Presumably this means that to have a low latency link you need a small advertising interval. In order to maintain low power consumption on the peripheral, the terribly-named "latency" parameter means that it can skip lots of advertising events unless it has a notification/indication to send, correct?

4. For something like iBeacon to work, presumably the central (iPhone) must be scanning continuously. How does that work if it draws 12 mA? That seems like an unacceptable power draw for a phone, or does Apple have a more efficient bluetooth chip?

Cheers,

Tim

[Håkon Alseth]

Hi Tim,

1. The on-air data rate for BLE is 1MBit, and the typical current draw when in RX is then 13 mA. Even if you do not receive anything, you're still actively listening for a particular pattern of data.

2. You have the option to go to sleep, unless you have some processing to do (polling a sensor, processing data, or similar).

3. Yes, you need a low connection interval in order to maintain a low latency link. In order to keep current consumption down, you can then enable slave latency allowing you to skip a number of connection intervals if you do not have any new data to send.

4. Most phones uses a combo-IC, which has BT+Wifi etc. The current draw is usually not constant, but duty cycled between scanning for bluetooth classic and low energy devices. I am not sure how it's done on iOS devices, but I assume it's duty cycled to keep power consumption lower.

Best regards Håkon