Data transfer to a characteristic is limited to 20 bytes

Question

Hallo, 
 we are using an nrf5340 SOC together with the SDK v1.9.0. 
 When running the sample "ncs_ble_tutorial_part1" from here 
 https://devzone.nordicsemi.com/guides/nrf-connect-sdk-guides/b/getting-started/posts/ncs-ble-tutorial-part-1-custom-service-in-peripheral-role 
 we observed that maximum 20 bytes can be written to the "MY_SERVICE_RX" characteristic. 
 My Question: 
 Which modifications have to done to increase the number of byte to be transferred from 20 to e.g. 500 ? 
 
 Best regards 
 
 Georg

Best regards Georg

Vidar Berg · Accepted Answer

Hallo,

With the default kconfig settings the Bluetooth zephyr host and Softdevice controller will give you an ATT_MTU size of 23 bytes which limits your effective payload to 20 bytes after you account for the GATT header.

With the following configuration you should be able to write up to 244 byte of payload in one write command:

<project_dir>/child_image/hci_rpmsg.conf

CONFIG_BT_BUF_ACL_TX_SIZE=251
CONFIG_BT_CTLR_DATA_LENGTH_MAX=251
CONFIG_BT_BUF_ACL_RX_SIZE=251

<project_dir>/prj.conf

CONFIG_BT_L2CAP_TX_MTU=247
CONFIG_BT_BUF_ACL_TX_SIZE=251
CONFIG_BT_BUF_ACL_RX_SIZE=251

Project from tutorial with the updated configuration settings above:

0116.ncs_ble_tutorial_part1_long_mtu_1.9.1.zip

Best regards,

Vidar

Edit: here's another thread with a similar question : https://devzone.nordicsemi.com/f/nordic-q-a/87829/updating-mtu-size-for-tx-and-rx-with-android-ble-client