A beaconing device announces its presence to the world by broadcasting advertisements. The Eddystone protocol is built on top of the standard BLE advertisement specification. Eddystone supports multiple data packet types:
This page describes the Eddystone open beacon format developed by Google.
Apache Mynewt currently supports Eddystone-UID and Eddystone-URL formats only. This tutorial will explain how to get an Eddystone-URL beacon going on a peripheral device.
This tutorial picks up where the BLE bare bones application tutorial concludes. The first step in creating a beaconing device is to create an empty BLE app, as explained in that tutorial. Before proceeding, you should have:
Here is a brief specification of how we want our beaconing app to behave:
Let's take these one at a time.
The first step, waiting for host-controller-sync, is mandatory in all BLE applications. The NimBLE stack is inoperable while the two components are out of sync. In a combined host-controller app, the sync happens immediately at startup. When the host and controller are separate, sync typically occurs in less than a second.
We achieve this by configuring the NimBLE host with a callback function that gets called when sync takes place:
static void ble_app_set_addr() { } static void ble_app_advertise(); { } static void ble_app_on_sync(void) { /* Generate a non-resolvable private address. */ ble_app_set_addr(); /* Advertise indefinitely. */ ble_app_advertise(); } int main(int argc, char **argv) { sysinit(); ble_hs_cfg.sync_cb = ble_app_on_sync; /* As the last thing, process events from default event queue. */ while (1) { os_eventq_run(os_eventq_dflt_get()); } }
ble_hs_cfg.sync_cb points to the function that should be called when sync occurs. Our callback function, ble_app_on_sync(), kicks off the control flow that we specified above. Now we need to fill in the two stub functions.
A BLE device needs an address to do just about anything. Some devices have a public Bluetooth address burned into them, but this is not always the case. Furthermore, the NimBLE controller might not know how to read an address out of your particular hardware. For a beaconing device, we generally don't care what address gets used since nothing will be connecting to us.
A reliable solution is to generate a non-resolvable private address (nRPA) each time the application runs. Such an address contains no identifying information, and they are expected to change frequently.
static void ble_app_set_addr(void) { ble_addr_t addr; int rc; rc = ble_hs_id_gen_rnd(1, &addr); assert(rc == 0); rc = ble_hs_id_set_rnd(addr.val); assert(rc == 0); } static void ble_app_advertise(); { } static void ble_app_on_sync(void) { /* Generate a non-resolvable private address. */ ble_app_set_addr(); /* Advertise indefinitely. */ ble_app_advertise(); }
Our new function, ble_app_set_addr(), makes two calls into the stack:
ble_hs_id_gen_rnd: Generate an nRPA.ble_hs_id_set_rnd: Configure NimBLE to use the newly-generated address.You can click either of the function names for more detailed documentation.
The first step in advertising is to configure the host with advertising data. This operation tells the host what data to use for the contents of its advertisements. The NimBLE host provides special helper functions for configuring eddystone advertisement data:
Our application will advertise eddystone URL beacons, so we are interested in the second function. We reproduce the function prototype here:
int ble_eddystone_set_adv_data_url( struct ble_hs_adv_fields *adv_fields, uint8_t url_scheme, char *url_body, uint8_t url_body_len, uint8_t url_suffix )
We'll advertise the Mynewt URL: https://mynewt.apache.org. Eddystone beacons use a form of URL compression to accommodate the limited space available in Bluetooth advertisements. The url_scheme and url_suffix fields implement this compression; they are single byte fields which correspond to strings commonly found in URLs. The following arguments translate to the https://mynewt.apache.org URL:
| Parameter | Value |
|---|---|
| url_scheme | BLE_EDDYSTONE_URL_SCHEME_HTTPS |
| url_body | “mynewt.apache” |
| url_suffix | BLE_EDDYSTONE_URL_SUFFIX_ORG |
static void ble_app_advertise(void) { struct ble_hs_adv_fields fields; int rc; /* Configure an eddystone URL beacon to be advertised; * URL: https://apache.mynewt.org */ fields = (struct ble_hs_adv_fields){ 0 }; rc = ble_eddystone_set_adv_data_url(&fields, BLE_EDDYSTONE_URL_SCHEME_HTTPS, "mynewt.apache", 13, BLE_EDDYSTONE_URL_SUFFIX_ORG); assert(rc == 0); /* TODO: Begin advertising. */ }
Now that the host knows what to advertise, the next step is to actually begin advertising. The function to initiate advertising is: ble_gap_adv_start. This function takes several parameters. For simplicity, we reproduce the function prototype here:
int ble_gap_adv_start( uint8_t own_addr_type, const ble_addr_t *direct_addr, int32_t duration_ms, const struct ble_gap_adv_params *adv_params, ble_gap_event_fn *cb, void *cb_arg )
This function gives an application quite a bit of freedom in how advertising is to be done. The default values are mostly fine for our simple beaconing application. We will pass the following values to this function:
| Parameter | Value | Notes |
|---|---|---|
| own_addr_type | BLE_OWN_ADDR_RANDOM | Use the nRPA we generated earlier. |
| direct_addr | NULL | We are broadcasting, not targeting a peer. |
| duration_ms | BLE_HS_FOREVER | Advertise indefinitely. |
| adv_params | defaults | Can be used to specify low level advertising parameters. |
| cb | NULL | We are non-connectable, so no need for an event callback. |
| cb_arg | NULL | No callback implies no callback argument. |
These arguments are mostly self-explanatory. The exception is adv_params, which can be used to specify a number of low-level parameters. For a beaconing application, the default settings are appropriate. We specify default settings by providing a zero-filled instance of the ble_gap_adv_params struct as our argument.
static void ble_app_advertise(void) { struct ble_gap_adv_params adv_params; struct ble_hs_adv_fields fields; int rc; /* Configure an eddystone URL beacon to be advertised; * URL: https://apache.mynewt.org */ fields = (struct ble_hs_adv_fields){ 0 }; rc = ble_eddystone_set_adv_data_url(&fields, BLE_EDDYSTONE_URL_SCHEME_HTTPS, "mynewt.apache", 13, BLE_EDDYSTONE_URL_SUFFIX_ORG); assert(rc == 0); /* Begin advertising. */ adv_params = (struct ble_gap_adv_params){ 0 }; rc = ble_gap_adv_start(BLE_OWN_ADDR_RANDOM, NULL, BLE_HS_FOREVER, &adv_params, NULL, NULL); assert(rc == 0); }
That's it! Now when you run this app on your board, you should be able to see it with all your eddystone-aware devices. You can test it out with the newt run command.
For reference, here is the complete application source:
#include "sysinit/sysinit.h" #include "os/os.h" #include "console/console.h" #include "host/ble_hs.h" static void ble_app_set_addr(void) { ble_addr_t addr; int rc; rc = ble_hs_id_gen_rnd(1, &addr); assert(rc == 0); rc = ble_hs_id_set_rnd(addr.val); assert(rc == 0); } static void ble_app_advertise(void) { struct ble_gap_adv_params adv_params; struct ble_hs_adv_fields fields; int rc; /* Configure an eddystone URL beacon to be advertised; * URL: https://apache.mynewt.org */ fields = (struct ble_hs_adv_fields){ 0 }; rc = ble_eddystone_set_adv_data_url(&fields, BLE_EDDYSTONE_URL_SCHEME_HTTPS, "mynewt.apache", 13, BLE_EDDYSTONE_URL_SUFFIX_ORG); assert(rc == 0); /* Begin advertising. */ adv_params = (struct ble_gap_adv_params){ 0 }; rc = ble_gap_adv_start(BLE_OWN_ADDR_RANDOM, NULL, BLE_HS_FOREVER, &adv_params, NULL, NULL); assert(rc == 0); } static void ble_app_on_sync(void) { /* Generate a non-resolvable private address. */ ble_app_set_addr(); /* Advertise indefinitely. */ ble_app_advertise(); } int main(int argc, char **argv) { sysinit(); ble_hs_cfg.sync_cb = ble_app_on_sync; /* As the last thing, process events from default event queue. */ while (1) { os_eventq_run(os_eventq_dflt_get()); } }