samples: Bluetooth: observer: Extended Scanning on BBC Micro Bit board

Add configuration overlay file to support observer sample
with Extended Scanning on BBC Micro Bit board.

Due to slow CPU, there will be assertions, and this commit
(for now) validates build-only. And the sample may run for
a duration until it asserts.

Asserts:

- ASSERTION FAIL [start_us == (aux_start_us + 1U)]
  @ WEST_TOPDIR/zephyr/subsys/bluetooth/controller/ll_sw/
  nordic/lll/lll_scan_aux.c:359
  This will happen for small aux offset value, definitely
  for the 300 us because CPU usage latency to setup such
  auxiliary PDU reception on nRF51 is high due to slow CPU.

- ASSERTION FAIL [0]
  @ WEST_TOPDIR/zephyr/subsys/bluetooth/controller/ll_sw/
  nordic/lll/lll_scan_aux.c:592
    prepare_cb: Actual EVENT_OVERHEAD_START_US = 579
  This will happen due to CPU usage latencies scheduling
  the radio events, due to slow CPU.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>

samples/bluetooth/observer/README.rst

@@ -16,6 +16,23 @@ If the used Bluetooth Low Energy Controller supports Extended Scanning, you may
 enable :kconfig:option:`CONFIG_BT_EXT_ADV` in the project configuration file. Refer to the
 project configuration file for further details.
 
+Building Extended Scanning support for BBC Micro Bit board
+**********************************************************
+
+.. code-block:: console
+
+   west build -b bbc_microbit . -- -DCONF_FILE='prj_extended.conf' -DEXTRA_CONF_FILE='overlay_bbc_microbit-bt_ll_sw_split.conf'
+
+Thread Analysis for BBC Micro Bit board
+***************************************
+
+Due to resource constraints on the BBC Micro Bit board, thread analysis can be enabled to profile
+the RAM usage and thread stack sizes be updated to successfully build and run the sample.
+
+.. code-block:: console
+
+   west build -b bbc_microbit . -- -DCONF_FILE='prj_extended.conf' -DEXTRA_CONF_FILE='debug.conf;overlay_bbc_microbit-bt_ll_sw_split.conf'
+
 Requirements
 ************
 

samples/bluetooth/observer/debug.conf

@@ -0,0 +1,5 @@
+# Enable thread analysis
+CONFIG_THREAD_ANALYZER=y
+CONFIG_THREAD_ANALYZER_AUTO=y
+CONFIG_THREAD_ANALYZER_AUTO_INTERVAL=5
+CONFIG_THREAD_NAME=y

samples/bluetooth/observer/dts/arm/nordic/override.dtsi

@@ -0,0 +1,7 @@
+/* Keep default IRQ priority low for peripherals to reduce Radio ISR latency.
+ * ARM Cortex-M4 lowest priority value of 5, i.e. considering Zephyr reserved 2
+ * levels for Exceptions and ZLI (if enabled).
+ * ARM Cortex-M0 lowest priority value of 3, i.e. we use it as Zephyr has no
+ * support for ZLI on Cortex-M0.
+ */
+#define NRF_DEFAULT_IRQ_PRIORITY 3

samples/bluetooth/observer/overlay_bbc_microbit-bt_ll_sw_split.conf

@@ -0,0 +1,53 @@
+# Adjust Stack Sizes for reduce RAM usage
+CONFIG_MAIN_STACK_SIZE=1024
+CONFIG_IDLE_STACK_SIZE=128
+CONFIG_ISR_STACK_SIZE=1024
+CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE=1024
+
+# Enable Extended Scanning
+# CONFIG_BT_EXT_ADV=y
+
+# Set maximum scan data length for Extended Scanning
+CONFIG_BT_EXT_SCAN_BUF_SIZE=192
+
+# Zephyr Bluetooth LE Controller needs 16 event buffers to generate Extended
+# Advertising Report for receiving the complete 1650 bytes of data.
+# Use 4 to be able to receive a minimal extended advertising with chains.
+CONFIG_BT_BUF_EVT_RX_COUNT=4
+
+# Use Zephyr Bluetooth Low Energy Controller implementation
+CONFIG_BT_LL_SW_SPLIT=y
+
+# Set maximum scan data length for Extended Scanning in Bluetooth LE Controller.
+CONFIG_BT_CTLR_SCAN_DATA_LEN_MAX=192
+
+# Increase Zephyr Bluetooth LE Controller Rx buffer to receive complete chain
+# of PDUs. Need 9 for maximum of 1650 bytes, but we use 3 as minimum to receive
+# at least a primary PDU, an auxiliary PDU and a chain PDU.
+CONFIG_BT_CTLR_RX_BUFFERS=3
+
+# Enable advanced features
+CONFIG_BT_CTLR_ADVANCED_FEATURES=y
+
+# Adjust execution context priorities to achieve lower Radio ISR latencies
+CONFIG_BT_CTLR_LLL_PRIO=0
+CONFIG_BT_CTLR_ULL_HIGH_PRIO=1
+CONFIG_BT_CTLR_ULL_LOW_PRIO=1
+
+# Use just-in-time collision resolution in ticker and LLL pipeline
+CONFIG_BT_CTLR_LOW_LAT=n
+CONFIG_BT_CTLR_LOW_LAT_ULL_DONE=n
+CONFIG_BT_TICKER_LOW_LAT=n
+
+# Increase the below to receive interleaved advertising chains
+CONFIG_BT_CTLR_SCAN_AUX_SET=3
+CONFIG_BT_CTLR_LOW_LAT_ULL=y
+# CONFIG_BT_CTLR_SCAN_AUX_USE_CHAINS=y
+# CONFIG_BT_CTLR_SCAN_AUX_CHAIN_COUNT=3
+
+# Use unreserved timespace scanning
+CONFIG_BT_CTLR_SCAN_UNRESERVED=y
+
+# Code size reduction
+CONFIG_ISR_TABLES_LOCAL_DECLARATION=y
+CONFIG_LTO=y

samples/bluetooth/observer/sample.yaml

@@ -3,20 +3,33 @@ sample:
 tests:
   sample.bluetooth.observer:
     harness: bluetooth
+    tags: bluetooth
     platform_allow:
       - qemu_cortex_m3
       - qemu_x86
       - nrf52840dk/nrf52840
     integration_platforms:
       - qemu_cortex_m3
-    tags: bluetooth
+      - nrf52840dk/nrf52840
   sample.bluetooth.observer.extended:
     harness: bluetooth
-    extra_args: CONF_FILE="prj_extended.conf"
+    tags: bluetooth
+    extra_args:
+      - CONF_FILE="prj_extended.conf"
     platform_allow:
       - qemu_cortex_m3
       - qemu_x86
       - nrf52840dk/nrf52840
-    tags: bluetooth
     integration_platforms:
       - qemu_cortex_m3
+      - nrf52840dk/nrf52840
+  sample.bluetooth.observer.extended.bbc_microbit.bt_ll_sw_split:
+    harness: bluetooth
+    tags: bluetooth
+    extra_args:
+      - CONF_FILE="prj_extended.conf"
+      - EXTRA_CONF_FILE="debug.conf;overlay_bbc_microbit-bt_ll_sw_split.conf"
+    platform_allow:
+      - bbc_microbit
+    integration_platforms:
+      - bbc_microbit