Does the observer or the central role consume less power?

[tyler]

I am working on a ble application using an nRF51822 that requires low power consumption. I'm curious if on average the observer or the central role will consume less power for a system that only transmits data from the peripheral device once every couple months.

[endnode]

Edit: sorry I might have misunderstood the question. If ask specifically between GAP Observer and Central then indeed Central is the way to go for lower power consumption (as well as data throughput!). When the link is connected the stack turns Tx and RX radio only for fraction of millisecond to serve the PDUs and all the rest it's in sleep mode (except time when your app computes something on MCU). To run reasonable Observer you would need at least 10-20% duty cycle and that's lot of time running Rx on full current draw.

Original elaboration about GAP Observer/Central and Broadcaster/Peripheral roles:

No, it's the opposite. Firstly Observer is using Rx which has constant power consumption in opposite to Tx which might be higher in the most powerful setting but normally you run the chip at -4/-8dBm and that's lower then nominal Rx current draw. Secondly to transmit you need to turn the radio just for fraction of millisecond. But when you listen to asynchronous broadcasts you need to listen most of the time to get some reasonable "detection" rate. That's also inherent in BLE technology since the beginning: device with lower power budget should be GAP Peripheral (or Broadcaster/Advertiser if you want). The only situation when the power consumption is basically equal between these two roles is when you run "permanent" connection link (both sides of the link need more or less the same Tx and Rx time). This is also the reason why initial rich OS APIs like Android/Windows/iOS didn't support GAP Peripheral role (because BLE was meant for "personal area" low power devices like sensors and wearables where smart phone/tablet/PC is device with much larger resources like bigger battery, every day charging or power directly from the grid).

[tyler]

Ok thanks. I was asking specifically about GAP observer and central.