Read characteristic exceeds MTU

[Malcolm]

How do I determine if a read characteristic has exceeded the maximum length of the MTU and I need to use BLOB to read all the data. How do I know I have read all data using BLOB?

Apologise will using SDK16

[Kenneth]

Hello,

Check out the documentation for sd_ble_gattc_read (uint16_t conn_handle, uint16_t handle, uint16_t offset) which will trigger the BLE_GATTC_EVT_READ_RSP event, see ble_gattc_evt_read_rsp_t .: 
https://docs.nordicsemi.com/r/6ufiCB5WB2ID45~3kILwRQ/H1j5QdVUKTWf~~mwy4FZUw?section=ga813daa5810a1d2ed31d2d6fe49d3ef11 
https://docs.nordicsemi.com/r/6ufiCB5WB2ID45~3kILwRQ/v6HRLI3gmtp0K1VFDh6mcQ 

"This function initiates or resumes a GATT Read (Long) Characteristic or Descriptor procedure. If the Characteristic or Descriptor to be read is longer than ATT_MTU - 1, this function must be called multiple times with appropriate offset to read the complete value."

So something along the lines of:

Client Server
|--- Read(handle, offset=0) ---->|
|<-- ReadRsp(len=max_chunk) -----| ← full chunk, more data
|--- Read(handle, offset+=len) ->|
|<-- ReadRsp(len=max_chunk) -----| ← full chunk, more data
|--- Read(handle, offset+=len) ->|
|<-- ReadRsp(len < max_chunk) ---| ← partial chunk, DONE

Kenneth