Long Attribute Values: writing to values larger than ATT_MTU

[Youssif]

Hi

According to BT 4.2 Spec Vol 6, Part F, Section 3.2.9:-

" To write the entire value of an attribute larger than (ATT_MTU-3) octets, the prepare write request and execute write request is used. It is possible to write the first (ATT_MTU-3) octets of a long attribute value using the write request".

Does the nordic softdevice have a function that performs the "prepare write request and execute write request" mentioned above? We have tried using the normal sd_ble_gattc_write() trying to write a value larger than ATT_MTU-3 and got the error NRF_ERROR_DATA_SIZE.

We are using S132 v3.0.0 on an nRF52.

Thanks

[Ulrich Myhre]

Hi,

If you look in the API documentation for sd_ble_gattc_write you see that it takes a ble_gattc_write_params_t. That struct has a field called write_op, where you specify the write operation you want to use.

You want to use multiple BLE_GATT_OP_PREP_WRITE_REQ, followed by a BLE_GATT_OP_EXEC_WRITE_REQ (Execute write request). The execute also needs the flags field to be set. There is an example MSC in the API documentation as well. It details which events you need to wait for.