Data rate vs power consumption vs latency

[kwas]

Hi. We're looking at nordic modules for a range of new miniaturized health sensors.

Does anyone know how to compute the current consumption of a BLE module given the data rate and latency? It seems there are too many parameters to this question and we can't risk months of development time trying to figure it out experimentally. There seems to be much confusion out there, some platforms (iphone) limit ble data rate to less than theoretically possible, and some modules limit you to using RFCOMM which has drawbacks and performance penalties. We know that it's possible to connect and send multiple packets (up to 6 at a time), but there are reports that when this is done, the theoretical increase in bandwidth is not achieved.

[kwas]

What does the advertising interval affect? I am ok with having a BLE connection take 10 seconds to establish. I am ok with this being possible only at a certain point in time (ex. a button is pressed on the transmitting device). Other than that I need a throughput of 4096 bytes per second, max latency 250 ms, not sure about the crystal, I can have it of any sort needed to accomplish the goal. thanks

[Ole Morten]

The advertising interval affects the time it takes to create a connection. Setting it low will increase average current consumption and shorten connection times, while setting it high will reduce average current consumption and increase connection times.

As you can see in the question I linked to, 4 kB/s is the maximum of what you can expect, under optimal conditions, with an iPhone and an nRF51822. In practice, you'll probably experience sustained throughput lower than this due to some packets being lost on-air, and due to the phone may not always be able to keep up. You don't say anything about what Central devices you target with your application, but if it's a phone, others will most often have similar restrictions as the iOS devices.

Latency in the data throughput will be given by the connection interval, which due to your high throughput requirements will have to be one of the lowest ones, for instance 30 ms.

When doing high-speed data transfers (6 packets of 20 bytes, every 30 ms), you can expect average current consumption of about 2 mA. Just keeping a link with 30 ms interval will give average consumption of about 340 µA.