Max BLE Characteristic write size with long write API

[chandan]

Hello, We are looking to send around 1000 Bytes on characteristic. On android app we used Nordic apis to send long write for sending 1000 byes but I am receiving only 509 bytes only once. Not sure if I am doing something wrong on Nordic firmware side or its not possible.

Below is my callback function :

static void on_disp_write_authorize(ble_disp_t *p_cus, ble_evt_t const *p_ble_evt) {

 ble_gatts_evt_write_t const *p_evt_write = &p_ble_evt->evt.gatts_evt.params.authorize_request.request.write;

 dataLength =  p_evt_write->len;

}

Below is the Api:

writeCharacteristic(secondCharacteristic, "Very, very long data that will no fit into MTU")

.split()
.enqueue()

https://github.com/NordicSemiconductor/Android-BLE-Library

[Vidar Berg]

Hello,

The maxium length of an attribute value is 512 bytes according to the Bluetooth spec., so 1000 bytes is not possible, unfortunately.

[chandan]

Max length include a 3 byte overhead, so the actual useable data that can be notified is ATT_MTU - 3 bytes so max possible is 509, correct ?

Also please me know what is these 3 bytes overhead?

[Vidar Berg]

The attribute value length can be set independent of the ATT_MTU size. E.g. an attribute can still hold 512 bytes even if only the default 23 byte  MTU is supported.You are however limited to "ATT_MTU - 3" bytes when sending notifications.

The 3 byte ATT packet overhead consists of one byte for the attribute opcode and 2 bytes for the attribute handle.