Support multiple connections (5+) on central

Question

Hi, 
 I am working on a project where we need to connect to multiple peripherals which also include mobile devices. (up-to 10) on BLE central. 
 We need to measure the distance with individual devices in regular intervals. 
 We are referring to nrf_dm example to do so and we are currently observing connection issue on the central. 
 When we try to connect with more than 2/3 devices central is not scanning anymore, we confirm other peripherals are advertising and yet the central did not scanned the device. 
 Below is our prj.conf file 
 CONFIG_BT=y
CONFIG_BT_CENTRAL=y
CONFIG_BT_CTLR_PHY_CODED=y
# CONFIG_BT_PHY_UPDATE=y
CONFIG_BT_USER_PHY_UPDATE=y
CONFIG_BT_EXT_ADV=y
CONFIG_BT_CTLR_ADV_EXT=y

CONFIG_BT_SCAN=y
CONFIG_BT_SCAN_FILTER_ENABLE=n
CONFIG_BT_MAX_CONN=6

# Allow to read RSSI for the connected peripheral

CONFIG_BT_CTLR_CONN_RSSI=y
CONFIG_NEWLIB_LIBC=y
CONFIG_NEWLIB_LIBC_MIN_REQUIRED_HEAP_SIZE=1024

# Use internal clock source
CONFIG_CLOCK_CONTROL_NRF=y
CONFIG_CLOCK_CONTROL=y
CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y
CONFIG_CLOCK_CONTROL_NRF_K32SRC_500PPM=y

# PERIPHERAL
CONFIG_BT_EXT_ADV=y
CONFIG_BT_PERIPHERAL=y
CONFIG_BT_PER_ADV=y
CONFIG_SETTINGS=y
CONFIG_BT_SETTINGS=y

# Distance Measurement
CONFIG_DM_MODULE=y
CONFIG_MPSL=y
CONFIG_MPSL_TIMESLOT_SESSION_COUNT=3
CONFIG_MPSL_WORK_STACK_SIZE=8192

CONFIG_DM_GPIO_DEBUG=y
CONFIG_NCS_SAMPLES_DEFAULTS=y

CONFIG_BT_DEVICE_NAME="Notify"
CONFIG_DM_INITIATOR_DELAY_US=2000
CONFIG_DM_MIN_TIME_BETWEEN_TIMESLOTS_US=10000
CONFIG_DM_TIMESLOT_QUEUE_COUNT_SAME_PEER=3

CONFIG_BT_GATT_CACHING=n
CONFIG_SETTINGS_NONE=y

CONFIG_BT_GATT_CLIENT=y
CONFIG_BT_DEBUG_LOG=y
CONFIG_LOG_MAX_LEVEL=4
CONFIG_PRINTK=n
CONFIG_DM_HIGH_PRECISION_CALC=y

# Enable PWM buzzer module
CONFIG_PWM=y
CONFIG_NRFX_GPIOTE0=y
CONFIG_NRFX_PDM=y
CONFIG_NRFX_PWM0=y
CONFIG_NRFX_PWM1=y
CONFIG_NRFX_PWM2=y
CONFIG_DEBUG=y
CONFIG_DEBUG_OPTIMIZATIONS=y
CONFIG_DEBUG_THREAD_INFO=y

# Enable ADC
CONFIG_ADC=y

# Enable NVS
CONFIG_NVS=y
CONFIG_FLASH=y
CONFIG_FLASH_PAGE_LAYOUT=y
CONFIG_MPU_ALLOW_FLASH_WRITE=y
CONFIG_DM_RANGING_OFFSET_US=100000
CONFIG_BT_PERIPHERAL_PREF_TIMEOUT=50
CONFIG_BT_PERIPHERAL_PREF_MIN_INT=120
CONFIG_BT_PERIPHERAL_PREF_MAX_INT=160
CONFIG_BT_CTLR_TX_PWR_PLUS_8=y

CONFIG_REBOOT=y

# Below configurations to enable DFU
# Enable mcumgr.

CONFIG_NCS_SAMPLE_MCUMGR_BT_OTA_DFU=y
CONFIG_NCS_SAMPLE_MCUMGR_BT_OTA_DFU_SPEEDUP=y
CONFIG_BOOTLOADER_MCUBOOT=y
CONFIG_SIGN_IMAGES=y
CONFIG_SECURE_BOOT=y
CONFIG_SB_SIGNING_KEY_FILE="priv_key.pem"
CONFIG_BT_L2CAP_TX_MTU=252
CONFIG_BT_BUF_ACL_RX_SIZE=256
CONFIG_PPI_TRACE=y

# Some command handlers require a large stack.
CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE=4096

# power management
CONFIG_UART_ASYNC_API=y
CONFIG_BOARD_ENABLE_DCDC=y
CONFIG_PM_DEVICE=y

We also tried restart scanning on dm intervals and no use. 
 We are using nrf52840 device and SDK v2.6.0 
 Please guide us on any config we are missing or if our method is wrong. 
 Thank you in advance.

Jithin A · Accepted Answer

Hi Amanda, 
 Sorry for late reply. 
 We made a lot of changes on prj conf which also include your suggestion and also to stable the DM module and it is working. 
 Sorry I cannot pinpoint the fix to the issue as we had lot of changes on the project. Let me share the final prj file which is working for us so if anyone looking for suggestion can refer to the file and compare. 
 # CENTRAL
CONFIG_BT=y
CONFIG_BT_CENTRAL=y
CONFIG_BT_SCAN=y
CONFIG_BT_SCAN_FILTER_ENABLE=n
CONFIG_BT_MAX_CONN=5
# Allow to read RSSI for the connected peripheral
CONFIG_BT_CTLR=y
CONFIG_BT_CTLR_CONN_RSSI=y

# PERIPHERAL
CONFIG_BT_PERIPHERAL=y
CONFIG_BT_PER_ADV=y
CONFIG_BT_SETTINGS=y
CONFIG_BT_GATT_CLIENT=y
CONFIG_BT_GATT_CACHING=n
CONFIG_BT_L2CAP_TX_MTU=252
CONFIG_BT_BUF_ACL_RX_SIZE=256
CONFIG_BT_PERIPHERAL_PREF_TIMEOUT=600
CONFIG_BT_PERIPHERAL_PREF_MIN_INT=120
CONFIG_BT_PERIPHERAL_PREF_MAX_INT=160
CONFIG_BT_PERIPHERAL_PREF_LATENCY=2
CONFIG_BT_CTLR_TX_PWR_PLUS_8=y
CONFIG_BT_CTLR_ADVANCED_FEATURES=y
CONFIG_BT_CTLR_TX_PWR_DYNAMIC_CONTROL=y

# PHY Coded enable for long range
# CONFIG_BT_PHY_UPDATE=y
CONFIG_BT_CTLR_PHY_CODED=y
CONFIG_BT_USER_PHY_UPDATE=y
CONFIG_BT_CTLR_ADV_EXT=y
CONFIG_BT_EXT_ADV=y

# Distance Measurement
CONFIG_DM_MODULE=y
CONFIG_DM_GPIO_DEBUG=y
CONFIG_DM_HIGH_PRECISION_CALC=y
CONFIG_DM_TIMESLOT_RESCHEDULE=y
CONFIG_DM_INITIATOR_DELAY_US=2000
CONFIG_DM_RANGING_OFFSET_US=100000
CONFIG_DM_MIN_TIME_BETWEEN_TIMESLOTS_US=100000
CONFIG_DM_TIMESLOT_QUEUE_COUNT_SAME_PEER=3
CONFIG_MPSL=y
CONFIG_MPSL_TIMESLOT_SESSION_COUNT=3
CONFIG_MPSL_WORK_STACK_SIZE=8192

CONFIG_SETTINGS_NONE=y
CONFIG_SETTINGS=y

CONFIG_NEWLIB_LIBC=y
CONFIG_NEWLIB_LIBC_MIN_REQUIRED_HEAP_SIZE=1024

# Use internal clock source
CONFIG_CLOCK_CONTROL_NRF=y
CONFIG_CLOCK_CONTROL=y
CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y
CONFIG_CLOCK_CONTROL_NRF_K32SRC_500PPM=y

# Enable PWM buzzer module
CONFIG_PWM=y
CONFIG_NRFX_PWM1=y
CONFIG_NRFX_PWM2=y

# Enable ADC
CONFIG_ADC=y

# Enable NVS
CONFIG_NVS=y

CONFIG_FLASH=y
CONFIG_FLASH_PAGE_LAYOUT=y
CONFIG_MPU_ALLOW_FLASH_WRITE=y

CONFIG_REBOOT=y

# Below configurations to enable DFU
# Enable mcumgr.
CONFIG_NCS_SAMPLES_DEFAULTS=y
CONFIG_NCS_SAMPLE_MCUMGR_BT_OTA_DFU=y
CONFIG_NCS_SAMPLE_MCUMGR_BT_OTA_DFU_SPEEDUP=y
CONFIG_BOOTLOADER_MCUBOOT=y
CONFIG_SIGN_IMAGES=y
CONFIG_SECURE_BOOT=y
CONFIG_SB_SIGNING_KEY_FILE="priv_key.pem"

# Some command handlers require a large stack.
CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE=4096

# power management
CONFIG_PM_DEVICE=y

# Disable system logs
CONFIG_LOG_MAX_LEVEL=4

Support multiple connections (5+) on central

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