This is a demonstration of how to create a GATT service and characteristic.

Dependencies:   BLE_API mbed nRF51822

Fork of BLE_EvothingsExample_GATT by Austin Blackstone


This code demonstrates how to use the BLE_API to create a GATT service and characteristic to toggle a LED on / off.


This code is a demonstration of how to create a custom service (UUID=0xA0000) with two characteristics, a read only characteristic (UUID=0xA001) and a write characteristic (UUID=0xA002). What is written to the write characteristic will be copied across to the read characteristic and broadcast out. If a single byte is written it will be used to toggle the on board LED, if more than 1 byte is written the data will be written out to the terminal. The default max size is 10bytes.


This is a video of how to use this example. Please note that in this video Android is used to read / write the characteristics. This could just as easily been done with the Evothings App listed below or an equivalent iOS app. Any app that can read/write characteristics could have been used.


characteristic and service UUID's are initialized here


uint16_t customServiceUUID  = 0xA000;
uint16_t readCharUUID       = 0xA001;
uint16_t writeCharUUID      = 0xA002;

We set up the list of available UUID's that will be advertised as part of the GAP advertising packet here. (it is good form to make this match the services you are actually advertising, so in this case we should set it to be 0xA000, but we are choosing to set it to 0xFFFF so it is easier to distinguish on the scanning app)


static const uint16_t uuid16_list[]        = {0xFFFF};
ble.accumulateAdvertisingPayload(GapAdvertisingData::COMPLETE_LIST_16BIT_SERVICE_IDS, (uint8_t *)uuid16_list, sizeof(uuid16_list)); // UUID's broadcast in advertising packet

Next we set up the characteristics and then put them together into a service. Notice that each characteristic has a maximum of 10Bytes of Value space, this can be expanded up to 512B.


// Set Up custom Characteristics
static uint8_t readValue[10] = {0};
ReadOnlyArrayGattCharacteristic<uint8_t, sizeof(readValue)> readChar(readCharUUID, readValue);
static uint8_t writeValue[10] = {0};
WriteOnlyArrayGattCharacteristic<uint8_t, sizeof(writeValue)> writeChar(writeCharUUID, writeValue);
// Set up custom service
GattCharacteristic *characteristics[] = {&readChar, &writeChar};
GattService        customService(customServiceUUID, characteristics, sizeof(characteristics) / sizeof(GattCharacteristic *));

Next we setup the writeCharCallback() function that will be called when a BLE onDataWritten event is triggered (when someone writes to a characteristic on the device). This function will check to see what characteristic is being written to and then handle it appropriately. In this case there is only one writable characteristic so its redundant, but good form for more complex service/characteristic combinations.


void writeCharCallback(const GattCharacteristicWriteCBParams *params)
    // check to see what characteristic was written, by handle
    if(params->charHandle == writeChar.getValueHandle()) {
        // toggle LED if only 1 byte is written
        if(params->len == 1) {
            led = params->data[0];
        // print the data if more than 1 byte is written
        else {
            printf("\n\r Data received: length = %d, data = 0x",params->len); 
        // update the readChar with the value of writeChar
// register callback function

The last thing to do is register the service with the BLE object so it is available.


    // add our custom service

Viewing Data

You can use either the LightBlue app on iOS or the nRF Master Control Panel application on Android to view the advertising data. Alternatively you can use a custom GATT Evothings App to view the data.


Evothings is a rapid prototyping environment that uses cordova to enable you to rapidly develop smartphone applications in Javascript. Please download the Evothings workbench to your computer and the Evothings client to your smartphone. The Evothings workbench will come with a program called "mbed Evothings GATT", this Evothings Smartphone program is meant to be used with the embedded mbed code. For instructions on how to use evothings please see the reference section below.