Sleep mode with LPN and no mesh

RE: Sleep mode with LPN and no mesh

tesc — Wed, 12 Jan 2022 15:47:36 GMT

Hi,

I cannot think of any (at least not straight-forward) way to count the number of nearby devices. However it would probably be better on wakeup just to try to find a friend (for a brief period of time). It should be in the same ballpark energy consumption, if not even better.

With a normal BLE connection, the connection is timed. A timer is running on the device, which triggers the radio peripheral to go in RX mode, which when receiving a packet triggers the stack (waking the CPU for handling the packet.) This means, the device is in RX mode only at the proper times for the connection event, and is turned off otherwise.

In BLE, you also have settings such as "slave latency", which lets the peripheral device skip a configurable number of connection events, i.e. radio turned off for longer time period, when it does not have any data to send. It means however that the central device (usually the phone) cannot send data to the peripheral device until the peripheral device acutally listens for a connection event.

That is during an ongoing connection. For leaving the connection, then reconnecting when needed, you can do wake on NFC, wake on movement (e.g. by use of external accelerometer), wake on button press, or wake on button.

Also, for setting up a BLE connection, the peripheral device runs advertisements, for which the central device listens. Advertising interval and to some extent type and size of advertisement will affect power consumption. Examples typically starts with rapid advertiing, then go over to longer advertising interval, and then stop completely after a couple of minutes. Then restarts (from low advertising interval) if triggered.

Regards,
Terje

RE: Sleep mode with LPN and no mesh

Alex Ross — Wed, 12 Jan 2022 15:35:20 GMT

Is it possible to get an internal count of nearby devices? I'm thinking something to the effect of:

-Wakeup every second and get a count of nearby devices (or attempt to find a friend)
-If count is 0 then go back to sleep
-Else attempt to find a friend
-No friend go back to sleep

How long it would it take to determine if a friend is nearby?

Could you also clarify to me how in normal BLE mode devices wakeup? I'm thinking something like a tag. How does it wake itself up out of sleep mode when the phone wants to get an update?

RE: Sleep mode with LPN and no mesh

tesc — Wed, 12 Jan 2022 15:25:03 GMT

Hi,

Current draw of 5 mA for normal Bluetooth mesh nodes is as expected, as they have to be in RX when not in TX. RX draw current running on DC-DC for nRF52833 is typically 4.6 mA, ref. the nRF52833 Product Specification.

LPN will save power, but as you write, when losing connection to the friend node the default action is to start looking for a new one.

It is not possible to wake on radio traffic, no. From what I understand, in order to wake e.g. on RSSI, one would need to have the radio already running in RX mode. Having the radio switched on and waking the CPU on radio traffic would not do much, as the main power draw is from the radio peripheral itself in RX mode.

Switching mesh activity off/on, deciding when to search for friends and when not to, that could be the way to go, yes. Either through periodically waking, look for friends, then go back to a sleep mode if none found, or through some smart method for deciding when to look for friends (e.g. NFC tag (for which wake on field detect is supported), movement detected (e.g. by an accelerometer), button press, etc.)

Regards,
Terje