Power consumption issue with 2 periperhal units connected to the same central unit

[Svein]

Hi,
I have a situation(problem) I do not fully understand, and I hope you can help me to sort it out.

Background
1 Central and 2 peripheral units; all three is powered by a nRF52840.
Both peripheral units can connect to the CU in a fix order. Unit 1 first and then unit 2.
The process in CU is that it starts scanning with filtering (MAC address of unit 1) and connects if units 1 is advertising. At connect of unit 1 it starts scanning for unit 2 with filtering on name, not address.

Situation:
I measure power consumption of unit 1 using the power profiler II and observe that the power consumption is around 30 µA in system on idle and around 15 µA when unit 2 connects and scanning is stopped.
Why is it so?

Kind regards

Svein

[Einar Thorsrud]

Hi Svein,

Unit 1 here is the peripheral, and it consumes 30 µA before connecting and 15 µA after connecting. Is this just the idle current, or is this the average current consumption? If it is the idle current (in system ON sleep with nothing happening), then that tis not expected. I do not know about your firmware, but from a BLE perspective, the same resources needs to be active in idle (essentially the 32.768 kHz clock and RTC0) regardless of if the device is advertising or in a connection.

However, if these numbers are the average current consumption numbers which seems more likely to me just based on the numbers, this could make sense. In that case, you can adjust the current consumption by increasing or decreasing the advertising interval and connection interval. You can experiment with the effect of this in the online power profiler.

Einar

[Svein]

Hi Einar, thanks for the quick response.

I think I should try to clarify my problem/question further.

As mentioned, I have 2 peripheral units that I want to connect to the same central unit. (Everything works fine, but…)

I use NUS service both for peripheral unit 1 and peripheral unit 2.

 

Situation:

1)            Peripheral unit 1 is in connected state and in System ON Idle(sleep). The only things in peripheral unit 1

               that is ongoing is that it has to responds on some of the connection intervals.

                I have set connection interval to 10 ms and slave latency to 100. Scanning in CU is stopped.

                In this situation average power consumption in peripheral unit 1 is 15 µA

 

2)            Then I initiate scanning in the central unit and the power consumption in peripheral unit 1 increase to 30 µA.

 

3)            Peripheral unit 2 connects to Central unit and scanning is stopped. The power consumption in peripheral

               unit 1 is then falling to 15 µA

 

Question:

I thought that if a peripheral unit is in connected state, it will not react if the central unit it is connected to starts a scanning; so my question is: Why is the power consumption doubled for a connected device if the connected central unit starts a scanning?

Kind regards

Svein

[Einar Thorsrud]

Hi Svein,

Svein said:

Why is the power consumption doubled for a connected device if the connected central unit starts a scanning?

I can think of two potential reasons. One is if the peripheral continues to advertise and the central does active scanning, so the peripheral will have to reply with a scan response packet. It should not double the current consumption though, so this is less likely. Another perhaps more likely reason is that more connection events are dropped because of the central scanning. Perhaps you can see how the current consumption of the peripheral is if you significantly reduce the scanning duty cycle of the central (scan  less frequently and for a shorter amount of time)?