BLE Characteristics write data Issue

[chandan]

Hi, I want to write 510 bytes of data to my BLE Characteristics, so I set parameters ble_gatts_attr_t max_len as 510 and its variable length attribute. Also I set  NRF_SDH_BLE_GATT_MAX_MTU_SIZE as 512. I observed below things:

- When I send data length of 510 or more, I am receiving only 507.

- When I send data length of 509, I am receiving correct data length 509.

I am not not able to find issue. Any suggestions?

Thanks

[Kenneth]

Hi,

You can find that BLE_GATTS_VAR_ATTR_LEN_MAX is 512 bytes from:
https://infocenter.nordicsemi.com/topic/com.nordic.infocenter.s112.api.v7.2.0/group___b_l_e___g_a_t_t_s___a_t_t_r___l_e_n_s___m_a_x.html

However this max length include a 3 byte overhead, so the actual useable data that can be notified is ATT_MTU - 3 bytes (=509 in your case) as written here:
https://devzone.nordicsemi.com/support-private/support/214779#permalink=399234

I am little unsure why it is truncated to 507 bytes when you are using 510 or more, but it may be that the packet is actually split into several packets (queued writes).

Best regards,
Kenneth