NRF51 BLE mbed ADC

RE: NRF51 BLE mbed ADC

Tim — Tue, 21 Jun 2016 07:49:36 GMT

All bluetooth modules. There is a TCP-like layer call L2CAP but you can never get access to it. You have to use GATT which runs on top of L2CAP. BLE 4.2 did introduce "Connection Oriented Channels" which give you access to L2CAP (or something like that), but it's not available in iOS or Android. Give it 5 years or so, maybe. See www.nordicsemi.com/.../Bluetooth-technology-opens-up-to-IP-traffic

RE: NRF51 BLE mbed ADC

Sri_guy — Sat, 18 Jun 2016 15:50:10 GMT

Ok, this code is really helpful. When you say BLE does not support TCP-like connection. Does that apply to all bluetooth modules, or just mbed? Thanks.

RE: NRF51 BLE mbed ADC

Tim — Fri, 17 Jun 2016 12:27:10 GMT

I suggest you try the new mBed Simple BLE library: developer.mbed.org/.../

Unfortunately BLE doesn't yet expose a TCP-like connection where you can just send data (it's coming though). You have to use GATT which imposes a structure on your communications. Data is stored in GATT Characteristics, and you 'send' it by having the client (phone app) enable notifications on your characteristic, and then writing to the characteristic on the device.

Characteristics are grouped into Services. The easiest way to get a feel for this is to download the Master Control Panel app, connect to some BLE device and play around.

You can send floats perfectly fine. Although you may as well send as uint16 since that's what is actually read, and it is shorted. Something like this:

#include "mbed.h"
#include "SimpleBLE.h"
 
AnalogIn voltage(A0);
 
SimpleBLE ble("VoltageSensor");
 
// Create a new characteristic under service 0x8000, char 0x8001
// This characteristic will support READ and NOTIFY.
SimpleChar<float> voltageValue = ble.readOnly_float(0x8000, 0x8001);
 
void read() {
    voltageValue = voltage.read();
}
 
int main(int, char**) {
    Ticker t;
    t.attach(&read, 1.0f); // read new value every second
 
    ble.start();
    while (1) { ble.waitForEvent(); }
}

By the way you will get no-where near 1000 Hz unless you pack multiple reads into each notification. BLE data rates are typically around 1 kB/s, up to a theoretical max of around 6 kB/s (I forget the exact number). It depends on the phone, and how you set the connection parameters.

RE: NRF51 BLE mbed ADC

Stefan Birnir Sverrisson — Fri, 17 Jun 2016 12:09:43 GMT

You cant transmit float over BLE, only bytes are accepted, i.e. uint8_t. So if you have a float, you need to code that into a byte and then decode that on the BLE receiving side. You can see in the ble_nus.c file, ble_nus_string_send function how data is sent over BLE with calling sd_ble_gatts_hvx function. I.e. this ADC example uses the NUS (Nordic UART Service) service.

There is also possibility to calculate battery remaining in procentage, before sending it over BLE, like done here