How to send a byte array by battery level notification

RE: How to send a byte array by battery level notification

bjorn-spockeli — Mon, 11 Jun 2018 05:58:53 GMT

jacky: Have you set the size of the characteristic to 20 bytes? Please post the code where you add the battery_level characteristic.

RE: How to send a byte array by battery level notification

RK — Sat, 09 Jun 2018 23:41:50 GMT

"simply failed to work" - not very much information - failed to work how, what error message, what happened?

"So I guess the Nordic layerstack might have certain internal restrict .. " - wrong guess, the stack doesn't care one jot about the data you're sending out and has no knowledge of well-known services.

Bjorn gave you code, just make a 20 byte buffer, put data in it, pass it to the sending routine along with the length and it will be sent. If that doesn't happen then post clearly what doesn't work and what you're seeing.

RE: How to send a byte array by battery level notification

jacky — Sat, 09 Jun 2018 22:32:54 GMT

Hi Bjorn,

I have tried a lot times, and the code simply failed to work when I tried to send 20-byte packet through the notification of battery-level characteristic. So I guess the Nordic layerstack might have certain internal restrict to this Well-known service and characteristics. Could you verify it? And If possible, could you send me a sample code that successfully sending out 20 byte packets through Battery_Level characteristic?

Thanks,

Jacky

RE: How to send a byte array by battery level notification

bjorn-spockeli — Wed, 23 May 2018 11:53:50 GMT

Hi Jacky,

Yes, you will need to split it into 20-byte chuncks.

BLE characteristic only deals with octets or arrays of octets, i.e. byte-streams. If you're sending something bigger than a byte, then you need to format the data so that it can be placed in a byte array.

So if you for instance want to sent a 32-bit unsigned integer, then you will have to set the size of the characteristic to 4*sizeof(uint8_t), i.e. attr_char_value.init_len/max_len = 4*sizeof(uint8_t) in the function that adds the characteristic and then divide the 32-bit integer into four 8-bit usigned integers, e.g.

uint32_t large_number = 64000;
uint8_t ble_data[4] = {0};
uint16_t ble_data_len = sizeof(ble_data);

for(int i = 0; i < sizeof(ble_data);i++) { ble_data[i] = large_number >> (i*8); }

Then in your function that calls sd_ble_gatts_hvx you simply point to the byte array and specify the length of the array.

        ble_gatts_hvx_params_t hvx_params;

        memset(&hvx_params, 0, sizeof(hvx_params));

        hvx_params.handle = p_cus->custom_value_handles.value_handle;
        hvx_params.type   = BLE_GATT_HVX_NOTIFICATION;
        hvx_params.offset = 0;
        hvx_params.p_len  = &ble_data_len;
        hvx_params.p_data = ble_data

        err_code = sd_ble_gatts_hvx(p_cus->conn_handle, &hvx_params);