BLE receive mode current consumption calculation

[shwetank vishnu]

Hello community,

I have been working on nRF BLE module. I want to make a project that has a small relay control for an appliance. The device will be connected to a mobile application over BLE. I want to power the whole from two AA batteries. The battery capacity is 2600mAh. I have figured out the overall current consumption over 24Hrs which is 365uA average.

I want to know what will be the current consumption of the BLE in receiving mode. 

1. Do i need to be always listening i.e. the bluetooth connection will always be on taking more current?

2. If I put the module to sleep mode, will my radio be still on?

3. Lets say I connect the nRF BLE initially and keep it on, how much current will it draw?

Thanks 

Regards

[Stian Røed Hafskjold]

In the Bluetooth LE world you have several roles depending on the application. Before any connection is made, you have a scanner and an advertiser. The scanner will be drawing most current and will be the one initiating the connection. The other device needs to be advertising in order to be found by the scanner. Usually the mobile phone is scanning and the battery device is advertising. If you want the battery device to be found by the mobile phone you need to always be advertising (you can advertise at a 1 second interval or more, which would consume very little current), or you can push a button or similar on the battery device to start advertising.

After the connection is requested by the central (scanner) the peripheral (advertiser) can choose to skip listening for some connection packets in order to save current, this is called slave latency.

Please check out the Online Power Profiler to see how much the total average current will be in both advertising and connected modes.

 

1. Do i need to be always listening i.e. the bluetooth connection will always be on taking more current?

 The device needs to be transmitting advertising packets in order to be discovered by the scanner.

 

2. If I put the module to sleep mode, will my radio be still on?

Depends on how you define "sleep". The CPU will always be sleeping between the radio packets.

 

3. Lets say I connect the nRF BLE initially and keep it on, how much current will it draw?

 It depends on the connection interval. If you just maintain a 1 second connection interval without any data being transmitted, the average current will be in the < 10 uA range.

[shwetank vishnu]

Ok, got your overview.

Now lets say i get past the scanning part and my HOST is connected with the battery device. Now I want to send commands to the battery device from the host. The commands can be random throughout the day but the packet size is ofcourse same. Now I know how can I make nRF work as a beacon by setting interval time to advertise the data, but if I want my device to receive some data (generally on/off for a relay) how would that be?

So as far as I understand:-

1. I connect the battery nRF to the HOST (takes initial burst current for few seconds or ms)

2. I keep my battery device in listening mode and wait for the HOST to give any command.

3. Do whatever I need to do with the command.

My questions:-

1. when I put the nRF in sleep mode for 1s and start listening and again put it to sleep, will my connection be lost and I need to again connect to the HOST?

What I am looking to do is to keep my BLE connection alive so it can receive whatever data comes and sleep for rest of the time.

[Stian Røed Hafskjold]

shwetank vishnu said:

1. I connect the battery nRF to the HOST (takes initial burst current for few seconds or ms)

2. I keep my battery device in listening mode and wait for the HOST to give any command.

3. Do whatever I need to do with the command.

That is correct.

shwetank vishnu said:

1. when I put the nRF in sleep mode for 1s and start listening and again put it to sleep, will my connection be lost and I need to again connect to the HOST?

What I am looking to do is to keep my BLE connection alive so it can receive whatever data comes and sleep for rest of the time.

Yes, you can keep the connection alive with a connection interval as long as 4 seconds. The device will be able to receive the command immediately on the scheduled connection event (as long as there is no on air packet loss)

The device will be sleeping between the connection events. But again, it depends on the definition of sleep. The nRF5 has two sleep modes. System ON idle, which will consume about 2 uA, and system OFF mode, which is consuming about 0.3 uA. In system OFF mode the connection is lost and the device wakes up in a reset state, and will have to re-initialize the stack and connection. Also, it can only wake up from an external event (button or similar) in system OFF mode. So I guess system ON idle is what you want. The chip goes to system ON idle between each advertising and connection event automatically.

 

 

[shwetank vishnu]

Great, thanks a lot for the information. I will test this and post any issues.

Stian Røed Hafskjold said:

So I guess system ON idle is what you want. The chip goes to system ON idle between each advertising and connection event automatically.

Perfect, so as soon as the nRF powers on I will assign a HOST to it, a connection once established will not require any more scanning. 

Then I will put it into system ON mode where I can let the chip sleep for some time (1-4s) and check for any received message from the host. During this whole process since the device radio is always active my connection will be alive and I would not require to do the scanning again. Sounds perfect.

Thanks again