boards: st: stm32n6570_dk: clarify boot pins information

Clarify the BOOT0/BOOT1 configuration needed to program the flash memory or to load the image straight in RAM (serial boot variant 'sb'). Explicit what the 2 USB connectors are used for in 'sb' variant configuration. Signed-off-by: Etienne Carriere <etienne.carriere@st.com>

Author: etienne-lms

boards/st/stm32n6570_dk/doc/index.rst

@@ -227,7 +227,7 @@ To program the board, there are two options:
  and executed from there.
 - Optionally, it can also be taken advantage from the serial boot interface provided
  by the boot ROM. In that case, firmware is directly loaded in RAM and executed from
- there. It is not retained.
+ there. It is not retained in persistent memory.
 
 Programming an application to STM32N6570_DK
 -------------------------------------------
@@ -258,6 +258,19 @@ First, connect the STM32N6570_DK to your host computer using the ST-Link USB por
  :west-args: --sysbuild -- -DCONFIG_XIP=n -DSB_CONFIG_MCUBOOT_MODE_RAM_LOAD=y
  :goals: build flash
 
+ .. note::
+ For flashing, before powering the board, set the boot pins in the following configuration:
+
+ * BOOT0: 0 (switch SW2 in position L)
+ * BOOT1: 1 (switch SW1 in position H)
+
+ After flashing, to run the application, set the boot pins in the following configuration:
+
+ * BOOT0: 0 (switch SW2 in position L)
+ * BOOT1: 0 (switch SW1 in position L)
+
+ Power off and on the board again.
+
  .. group-tab:: FSBL - ST-Link
 
  Build and flash an application using ``stm32n6570_dk/stm32n657xx/fsbl`` target.
@@ -270,26 +283,31 @@ First, connect the STM32N6570_DK to your host computer using the ST-Link USB por
  .. note::
  For flashing, before powering the board, set the boot pins in the following configuration:
 
- * BOOT0: 0
- * BOOT1: 1
+ * BOOT0: 0 (switch SW2 in position L)
+ * BOOT1: 1 (switch SW1 in position H)
 
  After flashing, to run the application, set the boot pins in the following configuration:
 
- * BOOT1: 0
+ * BOOT0: 0 (switch SW2 in position L)
+ * BOOT1: 0 (switch SW1 in position L)
 
- Power off and on the board again.
+  Power off and on the board again.
 
  .. group-tab:: FSBL - Serial Boot Loader (USB)
 
- Additionally, connect the STM32N6570_DK to your host computer using the USB port.
- In this configuration, ST-Link is used to power the board and for serial communication
- over the Virtual COM Port.
+ Additionally to the USB/ST-Link, connect the STM32N6570_DK to your
+ host computer using USB1 port (CN18).
+
+ In this configuration, ST-Link (USB/CN6) is used to power the board
+ and for serial communication over the Virtual COM Port, while
+ USB1/CN18 is used to send the Zephyr image to Boot ROM for loading it
+ in RAM and executing it.
 
  .. note::
  Before powering the board, set the boot pins in the following configuration:
 
- * BOOT0: 1
- * BOOT1: 0
+ * BOOT0: 1 (switch SW2 in position H)
+ * BOOT1: 0 (switch SW1 in position L)
 
  Build and load an application using ``stm32n6570_dk/stm32n657xx/sb`` target (you
  can also use the shortened form: ``stm32n6570_dk//sb``)