NRF51 Low energy Wake up

[Simon Ampleman]

Hello,

Is there any reference designs / documentation or ideas on how we can wake up NRF51 when there is radio activity on a specific frequency ?

In our design, we need the device to operate on coin-cell battery for as long as possible. Two devices must automatically find each other when they are in a close range. Currently, we are running in Bluetooth Low Energy mode, and each device must stay most of its time in RX mode (>12mA).

We would like the device to transmit a low energy wake up signal that the other device would receive and then trigger the full bluetooth capabilities. This way, most of the device time would be in sleep mode.

Any kind of wake up signal module would be fine if it can spread 2 to 5 meters, be low cost and small.

Thank you!

Simon

[Stefan Birnir Sverrisson]

Hi

The way it works is that BLE peripheral device transmits an advertising packet which a BLE central device will scan for. When a central device has detected an advertising packet from a peripheral device, it can establish a connection with the peripheral.

So the device needs to be awake with the receiver enabled in order to detect any other transmitting device. This is what happens when a peripheral device advertises in connectable mode. The peripheral will transmit an advertising packet, then enable the receiver for a short time period in order to receive packet from the central device (scan request packet or connection request packet).

The advertising packets are transmitted periodically and in between transmissions the peripheral device sleeps in order to save power. The user can adjust the advertising interval in order to save current or to connect with the central faster. You can find more information about this on this thread, see the "Tuning Advertising parameters" section. The getting started guide could also be of help.

Update 15.1.2014 In order to listen for certain on-air frequency, you need to have a receiver enabled, which consumes power, there is no way around that. The trick is to do that for a very short time period. When advertising, the sleep current is ~3uA and the receiver current is ~13mA. When the advertising interval is 10 seconds, the average current consumption is ~6uA. When the advertising interval is 1 second, the average current consumption is ~33uA with 0dBm transmit power and 27uA with -20dBm transmit power.

Update 2 15.1.2014 Perhaps I understand your question a little better now. You can of course have an external chip monitor a frequency band of choice, then when activity is detected it would set a signal via GPIO and wake up the nRF51 device. The nRF51 can stay in system off low power mode where current consumption is ~0.6uA, then wake up when it receives the signal and start to advertise in order to connect to a BLE central device. There are some sleep/wakeup examples on Nordic's Github page. There are also some BLE examples in the nRF51 SDK that use System Off low power mode, e.g. the power profiling application

[Simon Ampleman]

Thanks for your answer.

However, what I am looking for is different than the standard Bluetooth mechanism. I want the Bluetooth system and CPU to Sleep, not to be in receive mode at ~10-15mA. I want a separate system that consumes less energy that listens for any activity on a specific band, it would wake the whole system for a limited time when it see something might wants to communicate with it (very rare condition in our system, so a lot of battery life gain). There is a lot on documentation and research on Low-Frequency Wake-Up Trigger, but I can't find any manufacturer that have an actual chip built for this purpose. Hope that I am more clear than in my previous post.

Thank you.

Simon

[Simon Ampleman]

Thank you for the update.

I agree with your first sentence, "I need a receiver enabled, which consumes power". This receiver consumes ~13mA when it's enabled. Let's say, I use a LF wake-up receiver such as AMS AS3933, it consumes 2.3µA when it's enabled, that would be a huge gain on the battery, right? When a LF wake-up signal is received, this would wake up the NRF microcontroller and the bluetooth transceiver would start to communicate with the external device, then go to sleep again.