Tips on ULP, BLE and SoftDevice S110 with PCA10001 [nRF51822]

[Lucast85]

Hello Nordic people!

I'm developing my first firmware on nRF51822 and I need help with BLE, profiles and SoftDevice S110 stack.

Application it's a speed sensor for bicycles. I wish minimum energy consumption. Today, FW is capable to measure cadence data using RTC & LFCLK. Among the pulses due to wheel revolutions, I put the nRF CPU on sleep thus it consumes a very small amount of energy (Vdd=1.8 V, Idd=4.32 mA when CPU ON and Idd=3.05 uA when CPU sleep). I'm very happy ;)

Now it's time to send the speed information to external devices through BLE, using SoftDevice S110.

I think 8-16 bit of data are enough for speed information. I want a speed-related, variable frame-rate:

• at speed lower than 15 km/h, wheel revolution period is higher than 500 ms thus frame-rate higher than 1/500 ms will be a waste of energy. In this situation, frame-rate will match the wheel's revolution period (>500 ms) and Frame_rate<(1/500) ms.
• at speed higher than 15 km/h, wheel revolution period is lower than 500 ms. However send data via radio too often is still a waste of energy (end-user has no advantages in a non-real time applications like this) thus maximum desirable frame-rate is Frame_rate_max=1/500 ms. The end-user BLE equipment will have a maximum display-refresh rate of 1/500 ms. It's a good compromise.

Summarizing, maximum frame-rate is 2 FPS (1/500 ms). When no wheel's revolutions happened in 4 seconds, I send a 0 km/h data and switch off nRF51822 thus minimum frame-rate is 0.25 FPS (1/4 s).

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I'm starting to read BLE manuals but is still a jungle of information. I briefly saw the GATT and CSC profile but I'm not sure it's the better choice to obtain extra low power consumption.

Maybe it's smarter using a custom profile? Have you got some useful links, BLE quick start, simple nRF51822 project and profiles/services and tips about this application? Please, put me on the right way :) Thank you all!

[Stefan Birnir Sverrisson]

Hi

With your frame rate of 2Hz, you should be able to obtain very low current consumption.

Perhaps you will benefit of looking at this blog. There is also a current consumption guide here.

To create a proprietary profile, nAN-36 is a good starting point. It is perhaps a little outdated but the principles should be still valid. A more updated accompanied ble_app_lbs code is available on Nordic's Github page

Connection interval tips. Choose 500ms connection interval, and a slave latency of 7, to allow the device to respond to the central device in every 8th connection event if there is no data to send for 8*500ms. Supervision timeout of perhaps 5-10 seconds.

[Lucast85]

Thank you for fast response! I try to download last GitHub version of nrf51-ble-app-lbs into PCA10001 board but, with USB dongle and Master Control Panel, I cannot see any BLE device (see here). I think it's a "nrf51-ble-app-lbs" problem because the dongle and MCP works well with other BLE examples applications (e.g. ble_app_hrs). I use SoftDevice 7.1 and SDK v.6.0.0.43681. Are there any solution to nAN36 application or are there others app notes to quick jump to BLE world? Thank you again.

[Stefan Birnir Sverrisson]

The ble_app_lbs current version uses an older SDK and softdevice, as stated in the readme for the example on Github. So if you flash softdevice 6.0.0, and put the Github ble_app_lbs example into SDK 5.2.0, compile it and then flash it, it should work.

[Lucast85]

I think to modify the code described in nAN-36 as follow:

1. delete the LED characteristic;
2. add LPCOMP functionality and measure speed with help of RTC and LFCLK;
3. modify the BUTTON characteristic replacing button status data with speed data computed at point "2";
4. update connection parameters programmatically (related with bicycle's speed);
5. Delete all extra functionality minimizing current consumption. It could work? Or it's better to start from the "template application" (example project for PCA10028)? Thank you!

[Lucast85]

The code was updated recently and now it work also with SD 7.1 and SDK 6.0.0.43681.