Adding Baud Rate Characteristic to UART example

[aaron.mcneil]

Hello All,

I am implementing a simple UART bridge over BLE and want to add a Characteristic to update the baud rate being used. I thought I added code to support a new characteristic everywhere it needs to go, but when I scan for characteristics with the nRF connect app or another BLE scanner, it does not show up in the list.  The plan is to look for the newly defined BLE_NUS_EVT_BAUD event in the nus_data_handler and update the baud rate accordingly. Is there anything I am missing here when it comes to adding a characteristic?

I added the following lines of code to the UART example:

in ble_nus.h

struct ble_nus_s
{
uint8_t uuid_type; /**< UUID type for Nordic UART Service Base UUID. */
uint16_t service_handle; /**< Handle of Nordic UART Service (as provided by the SoftDevice). */
ble_gatts_char_handles_t tx_handles; /**< Handles related to the TX characteristic (as provided by the SoftDevice). */
ble_gatts_char_handles_t baud_handles;
ble_gatts_char_handles_t rx_handles; /**< Handles related to the RX characteristic (as provided by the SoftDevice). */
blcm_link_ctx_storage_t * const p_link_ctx_storage; /**< Pointer to link context storage with handles of all current connections and its context. */
ble_nus_data_handler_t data_handler; /**< Event handler to be called for handling received data. */
};

in ble_nus.c

#define BLE_UUID_NUS_BAUD_CHARACTERISTIC 0x0004 /**< The UUID of the BAUD Characteristic. */
#define BLE_UUID_NUS_TX_CHARACTERISTIC 0x0003 /**< The UUID of the TX Characteristic. */
#define BLE_UUID_NUS_RX_CHARACTERISTIC 0x0002 /**< The UUID of the RX Characteristic. */

#define BLE_NUS_MAX_BAUD_CHAR_LEN BLE_NUS_MAX_DATA_LEN /**< Maximum length of the BAUD Characteristic (in bytes). */
#define BLE_NUS_MAX_RX_CHAR_LEN BLE_NUS_MAX_DATA_LEN /**< Maximum length of the RX Characteristic (in bytes). */
#define BLE_NUS_MAX_TX_CHAR_LEN BLE_NUS_MAX_DATA_LEN /**< Maximum length of the TX Characteristic (in bytes). */

in on_write in ble_nus.c

else if ((p_evt_write->handle == p_nus->baud_handles.value_handle) &&
(p_nus->data_handler != NULL))
{
evt.type = BLE_NUS_EVT_BAUD;
evt.params.rx_data.p_data = p_evt_write->data;
evt.params.rx_data.length = p_evt_write->len;

p_nus->data_handler(&evt);
}

in ble_nus_init in ble_nus.c

// Add the BAUD Characteristic.
memset(&add_char_params, 0, sizeof(add_char_params));
add_char_params.uuid = BLE_UUID_NUS_BAUD_CHARACTERISTIC;
add_char_params.uuid_type = p_nus->uuid_type;
add_char_params.max_len = BLE_NUS_MAX_RX_CHAR_LEN;
add_char_params.init_len = sizeof(uint8_t);
add_char_params.is_var_len = true;
add_char_params.char_props.write = 1;
add_char_params.char_props.read = 1;

add_char_params.read_access = SEC_OPEN;
add_char_params.write_access = SEC_OPEN;

err_code = characteristic_add(p_nus->service_handle, &add_char_params, &p_nus->baud_handles);
if (err_code != NRF_SUCCESS)
{
return err_code;
}

[Susheel Nuguru]

Hi Aaron,

I can see your new BAUD characteristic when i used your code

[aaron.mcneil]

Thats really strange. Here is the screenshot of mine running the same code:

When I experiment and change the UUID of the Tx Char to a 4 instead of a 3, it still shows the Tx Char as 6E4000003 also. Really confusing. Is there another element I am not handling to the advertising portion?

[Susheel Nuguru]

can you test this using nRF Connect for desktop instead of a phone. I think something in the phone is caching the services from the old connection based on device address

[aaron.mcneil]

Thank you for your response. Once I had my PC forget the device and then ran the scanner the characteristic showed up as expected.

Building on this, are there any examples of reading a characteristic value, as well as updating the value so the phone/windows side can read it back? I can successfully subscribe to the characteristic on the phone/windows side now. When I write to that characteristic I get the correct p_evt->type in the nus_handler. But where do I go from there? I need to read what value was sent to me and update the buad rate in the uart and then update the characteristics value so it can be read back by the phone or windows app.