Problem with sd_ble_gatts_hvx without notifications enabled

RE: Problem with sd_ble_gatts_hvx without notifications enabled

Eugene0469 — Fri, 18 Jan 2019 09:31:20 GMT

Hello. I'd like to read the temperature values in my nordic thingy without enabling notifications. Is there a way one can use the function sd_ble_gatts_value_set() to do this?

RE: Problem with sd_ble_gatts_hvx without notifications enabled

Anders Strand — Fri, 10 Feb 2017 10:07:47 GMT

Hey. Would you mind sharing a code snippet that works for you? it would be nice too know how and where you calculated the CRC as well.

RE: Problem with sd_ble_gatts_hvx without notifications enabled

Gauti — Fri, 13 Jan 2017 10:50:21 GMT

Manually enable notification and calculate the CRC (CRC-CCIT - 0xFFFF). Update the new attributes along with CRC using the function sd_ble_gatts_sys_attr_set(). With the updated system attributes, use sd_ble_gatts_hvx() to write the values. This seems to be working fine. Of course we have to explicitly read the value from the mobile. Each CCCD is 3 byte long (SDK12.1). This is followed by 2 byte CRC at the end. So if the CCCD count is 3, length would be: 3x6 + 2 = 14

RE: Problem with sd_ble_gatts_hvx without notifications enabled

Gauti — Fri, 13 Jan 2017 10:41:24 GMT

Yes true, I used sd_ble_gatts_value_set() to set the value directly as you mentioned. I did not get any error code, meaning return from this function was 0. I did try to explicitly read from the phone to fetch the value, but this was not successful. However I used another method. I manually enabled notification by using the function sd_ble_gatts_sys_attr_set(). I calculated the CRC (CRC-CCIT - 0xFFFF) and updated the same using this function. With the updated system attributes, I used sd_ble_gatts_hvx() to write the values. This seems to be working fine. Of course I have to explicitly read the value from the mobile.

RE: Problem with sd_ble_gatts_hvx without notifications enabled

Jørgen Holmefjord — Thu, 12 Jan 2017 12:33:01 GMT

That link only say that you cannot write the cccd of the characteristics using sd_ble_gatts_value_set(). You should still be able to set the value of the characteristics using this function. I tested this on the BLE UART example, and that works fine. Are you getting any error codes from sd_ble_gatts_value_set() when you use that code? Note that you explicitly have to read the characteristics on the phone to get the value, you will not get any hints that a new value is available.

RE: Problem with sd_ble_gatts_hvx without notifications enabled

Gauti — Wed, 11 Jan 2017 14:39:01 GMT

Hi, Thanks for your reply! I did add the below piece of code in on_connect() with no luck:

uint32_t          err_code;
uint8_t           default_protocol_mode;
ble_gatts_value_t gatts_value;

lsp_nus->conn_handle = lsp_ble_evt->evt.gap_evt.conn_handle;

if (lsp_nus->rx_handles.value_handle)
{
    // Set Protocol Mode characteristic value to default value
    default_protocol_mode = 0;

    // Initialize value struct.
    memset(&gatts_value, 0, sizeof(gatts_value));

    gatts_value.len     = sizeof(uint8_t);
    gatts_value.offset  = 0;
    gatts_value.p_value = &default_protocol_mode;

    err_code = sd_ble_gatts_value_set(lsp_nus->conn_handle,
    		lsp_nus->rx_handles.value_handle, &gatts_value);

The following link: devzone.nordicsemi.com/.../

claims that sd_ble_gatts_value_set() might not serve the purpose. Any thing else that I can try to make this work?

RE: Problem with sd_ble_gatts_hvx without notifications enabled

Jørgen Holmefjord — Wed, 11 Jan 2017 07:48:26 GMT

Hi,

If notifications is not enabled, you should use sd_ble_gatts_value_set() to set the value of the characteristics. The other side then needs to read the characteristics to get the new value.

Best regards,

Jørgen