nRF5340 BLE: the configuration for BLE MTU

Question

Hi Support Team, 
 I tried the BLE communication with the Nordic UART Service (NUS) and used the code in the ' Bluetooth Low Energy Fundamentals Lesson 4 - Exercise 3' course. 
 The default MTU size was too small for my application. I preferred not to change the code in the hci_rpmsg on the netCore, so I added the below configuration on the appCore side: 
 1. In the files \bt-fund-main\lesson4\ blefund_less4_exer3_solution\child_image\hci_rpmsg.conf : 
 #GATT_CLIENT needed for requesting ATT_MTU update
CONFIG_BT_GATT_CLIENT=y
#PHY update needed for updating PHY request
CONFIG_BT_USER_PHY_UPDATE=y
#For data length update
CONFIG_BT_USER_DATA_LEN_UPDATE=y
#This is the maximum data length with Nordic Softdevice controller
CONFIG_BT_CTLR_DATA_LENGTH_MAX=251
#These buffers are needed for the data length max. 
CONFIG_BT_BUF_ACL_TX_SIZE=251
CONFIG_BT_BUF_ACL_RX_SIZE=251
#This is the maximum MTU size with Nordic Softdevice controller
CONFIG_BT_L2CAP_TX_MTU=247

# Maxmum 2 connections are enough
CONFIG_BT_MAX_CONN=2 
 2. In the \bt-fund-main\lesson4\ blefund_less4_exer3_solution\prj.conf : 
 #
# Copyright (c) 2018 Nordic Semiconductor
#
# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
#

# Enable the UART driver
CONFIG_UART_ASYNC_API=y
CONFIG_NRFX_UARTE0=y
CONFIG_SERIAL=y

CONFIG_GPIO=y

CONFIG_BT_NUS_UART_BUFFER_SIZE=400

# Make sure printk is printing to the UART console
CONFIG_CONSOLE=y
CONFIG_UART_CONSOLE=y

CONFIG_HEAP_MEM_POOL_SIZE=2048
CONFIG_MAIN_STACK_SIZE=4096

CONFIG_BT=y
CONFIG_BT_PERIPHERAL=y
CONFIG_BT_DEVICE_NAME="Nordic_UART_Service"
CONFIG_BT_DEVICE_APPEARANCE=833
CONFIG_BT_MAX_CONN=1
CONFIG_BT_MAX_PAIRED=1
CONFIG_BT_RX_STACK_SIZE=2048
# Enable the NUS service
# STEP 1 - Enable the Kconfig symbol of the NUS service
CONFIG_BT_NUS=y
CONFIG_BT_NUS_AUTHEN=n
CONFIG_BT_NUS_SECURITY_ENABLED=n
# Enable bonding
CONFIG_BT_SETTINGS=n
CONFIG_FLASH=n
CONFIG_FLASH_PAGE_LAYOUT=n
CONFIG_FLASH_MAP=n
CONFIG_NVS=n
CONFIG_SETTINGS=n

# Enable DK LED and Buttons library
CONFIG_DK_LIBRARY=y

# This example requires more workqueue stack
CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE=2048

# Config logger
CONFIG_LOG=y
CONFIG_USE_SEGGER_RTT=y
CONFIG_LOG_BACKEND_RTT=y
CONFIG_LOG_BACKEND_UART=n

CONFIG_ASSERT=y

#########################Added to increase the MTU###############################
CONFIG_BT_GATT_CLIENT=y
# STEP 8 - Enable PHY updates.
CONFIG_BT_USER_PHY_UPDATE=y

# STEP 12 - Update Data Length and MTU
CONFIG_BT_USER_DATA_LEN_UPDATE=y
#CONFIG_BT_CTLR_DATA_LENGTH_MAX=251
CONFIG_BT_BUF_ACL_RX_SIZE=251
CONFIG_BT_BUF_ACL_TX_SIZE=251
CONFIG_BT_L2CAP_TX_MTU=247
 
 But it did not work. Could you help me with the below questions: 
 (1). Could you help review the above configurations? what was missed for the MTU setting? 
 (2). I'm a little confused about the configuration alignment on the host and controller sides. For Data Length, MTU, and PHY_UPDATE, which need to align the configuration on both host and controller? which only needs to change single side configuration? In Lesson 3 – Exercise 2 , it sets all in the host's prj.conf file. 
 
 Thank you very much, 
 Best regards, 
 Yanpeng Wu

Edvin · Accepted Answer

Please see the instructions on how to install the sniffer . Then you need to select the advertising device from the dropdown list near the top while sniffing. After that it should follow the peripheral into the connection. If you use encryption (other than "just works" pairing), you need to enter the passkey in the top before you pair. 
 
 You can also compare connecting to the laptop with connecting to the laptop running nRF Connect for Desktop ->Bluetooth Low Energy (which uses a DK/dongle for the BLE operations). Does it disconnect in both cases? 
 
 After capturing a sniffer trace, you can save it as a .pcapng file, and upload it here. 
 
 BR, Edvin

nRF5340 BLE: the configuration for BLE MTU

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