The beginning of every GATT database (GATTDB) within the SDK begins the comment:

 

      // Service change characteristic is optional and is not present

 

What is the service change characteristic? Why would I want it? And how would I implement it?

 

Introduction:

     When a GATT client and server connect for the very first time, the client knows nothing about the server. In order to achieve a useful relationship, the client performs a GATT discovery on the server. There are a few variations in how the client can choose to go about this--search by handle, by type, etc.--we won't delve into the specifics of discovery.

     With low power in mind, these GATT discoveries are costly. Given that the client has the resources, it is far more efficient to jot down the details of the server's GATTDB and store that locally--this is called an Attribute Cache. In order to save power, upon future reconnections, the client will not perform a GATT discovery, but rather reach into its local memory to save power on both the client and server devices.

     However, a problem arises when we change the GATTDB between connections. The client will think the servers GATTDB is of the old form and begin operating on that basis. Now, this will only happen in the field under the premise that your firmware alters the GATTDB between connections.

     How do we fix this? One way is to utilize the Service Changed Characteristic. This characteristic is essentially a way for the server to indicate to the client that a change has been made to its GATTDB. The client should then update its Attribute Cache with the newest values.

 

Implementation:

 

This is how you would implement it in your GATTDB:

 

 

     const UINT8 gatt_database[]=

     {    

          // Handle 0x01: GATT service

          PRIMARY_SERVICE_UUID16 (0x0001, UUID_SERVICE_GATT),

   

          // Indicate that entire GATTDB has been altered

          // (handles: 0x0000 - 0xffff)

          CHARACTERISTIC_UUID16(0x0002, 0x0003,

               UUID_CHARACTERISTIC_SERVICE_CHANGED,

               LEGATTDB_CHAR_PROP_INDICATE,

               LEGATTDB_PERM_NONE, 4),

               0x00, 0x00, 0xff, 0xff,

   

          CHAR_DESCRIPTOR_UUID16_WRITABLE (0x0004,

               UUID_DESCRIPTOR_CLIENT_CHARACTERISTIC_CONFIGURATION,

               LEGATTDB_PERM_READABLE | LEGATTDB_PERM_WRITE_REQ, 2),

               0x00, 0x00,

          ...

     }

 

     This characteristic is implemented under the GATT Service that is mandatory to all GATTDB's. The Service Change Characteristic has a spec defined UUID. It needs only the ability to indicate and should have no required permissions. Lastly, it needs a value length of 4 bytes. These 4 bytes are used to designate the range of handles which have been changed. The first 2 bytes are the start handle, the last 2 bytes are the stop handle. As can be seen in the code above, we have specified this range to be all handles 0x0000 - 0xffff--tlling the client to completely rediscover the GATTDB.

     Since we need to use indications, this characteristic now needs a Client Configuration Descriptor--as all server initiated communications need. This descriptor is, essentially, two client-writable bytes that allow the client to give the server permission to transmit asynchronous communications--one byte is for notifications and the other is for indications. The service changed characteristic requires the use of indications (a notification with an application level ACK).

 

Use:

     The actual use of the Service Changed Characteristic requires the implementation of indications, GATT discovery by the client, and a change in the GATTDB by the server. These will not be covered here, but there exist individual resources for each:

 

     For implementation of indications see:

          Wiced-Smart-SDK/Apps/hello_client

          Wiced-Smart-SDK/Apps/hello_server

     For example firmware of a GATT change see:

          Re: How to change the GATT database after init (bleapp_set_cfg) ?

     For implementation of GATT discovery see:

          Wiced-Smart-SDK/Apps/automation_io_client

          Wiced-Smart-SDK/Apps/http_client