nRF52833 increasing att mtu data length

RE: nRF52833 increasing att mtu data length

jacob jacob — Wed, 18 Dec 2024 14:57:00 GMT

I figured out what I was doing wrong, and I wanted to share incase it is useful to anyone. my issue was that when I called characteristic_add I passed in a .max_len of 20.

both peripheral and central agreed that max length would be 244 based on the messaging I saw.

but this is not the only thing sd_ble_gatts_value_set checks before returning [NRF_ERROR_DATA_SIZE]. it also checks .max_len value set by characteristic_add.

in summary to get packets with data of more than 20bytes:

set these define:
NRF_SDH_BLE_GAP_DATA_LENGTH = 244
NRF_SDH_BLE_GATT_MAX_MTU_SIZE = 244
NRF_SDH_BLE_GAP_EVENT_LENGTH = 400

AND
pass characteristic_add .max_len of desired data value.

doing all this will let you send packets of more then 20bytes.

thanks einar for the help.

RE: nRF52833 increasing att mtu data length

Einar Thorsrud — Thu, 12 Dec 2024 09:44:59 GMT

Hi,

I see. The only other thing I can think of is that the maximum length for the characteristic is lower? Which max_len was used when you added the characteristic with sd_ble_gatts_characteristic_add() or indirectly with characteristic_add()?

RE: nRF52833 increasing att mtu data length

jacob jacob — Mon, 09 Dec 2024 19:31:15 GMT

thanks for the response.
this is my settup:
NRF_SDH_BLE_GAP_DATA_LENGTH = 244
NRF_SDH_BLE_GATT_MAX_MTU_SIZE = 244
NRF_SDH_BLE_GAP_EVENT_LENGTH = 400

I get the message in the debug terminal:
<debug> nrf_ble_gatt: ATT MTU updated to 244 bytes on connection 0x3 (response).

then when I call sd_ble_gatts_value_set passing a .len of 40 I still get the following error:
<error> app: ERROR 12 [NRF_ERROR_DATA_SIZE] at C:\Users\...

RE: nRF52833 increasing att mtu data length

Einar Thorsrud — Mon, 09 Dec 2024 15:28:42 GMT

Hi,

I assume you use the SoftDevice Handler (SDH) library which is used in the examples? If os, you should adjust a few parameters in sdk_config.h:

NRF_SDH_BLE_GAP_DATA_LENGTH
NRF_SDH_BLE_GATT_MAX_MTU_SIZE
NRF_SDH_BLE_GAP_EVENT_LENGTH

You can refer to examples/ble_peripheral/ble_app_uart/pca10056/s140/config/sdk_config.h for an example.