updated USB_OTG in dfu.rst, usb_device.rst and secure-boot-v2.rst

docs/en/api-guides/dfu.rst

@@ -5,7 +5,7 @@ Device Firmware Upgrade via USB
 
 Typically, the firmware of the {IDF_TARGET_NAME} is flashed via the chip's serial port. However, flashing via the serial port requires a USB to serial converter chip (e.g., CP210x or FTDI) to be connected to the {IDF_TARGET_NAME} (see :doc:`Establish Serial Connection with {IDF_TARGET_NAME} <../get-started/establish-serial-connection>` for more details). The {IDF_TARGET_NAME} contains a USB OTG peripheral making it possible to connect the {IDF_TARGET_NAME} to the host directly via USB (thus not requiring a USB to serial converter chip).
 
-Device Firmware Upgrade (DFU) is a mechanism for upgrading the firmware of the {IDF_TARGET_NAME} directly via the Universal Serial Bus (USB).
+Device Firmware Upgrade (DFU) is a mechanism for upgrading the firmware of the {IDF_TARGET_NAME} directly via the Universal Serial Bus (USB). However, enabling Secure Boot or flash encryption disables the USB-OTG USB stack in the ROM, disallowing updates via the serial emulation or DFU on that port.
 
 - :ref:`get-started-get-prerequisites` of the Getting Started Guide introduces the software requirements of DFU.
 - Section :ref:`api_guide_dfu_build` describes how to build firmware for DFU with ESP-IDF.

docs/en/api-reference/peripherals/usb_device.rst

@@ -15,7 +15,7 @@ The driver allows you to use {IDF_TARGET_NAME} chips to develop USB devices on a
 
 TinyUSB stack is distributed via `IDF Component Registry <https://components.espressif.com/components/espressif/esp_tinyusb>`_.
 
-Our USB-OTG implementation is limited to {IDF_TARGET_USB_EP_NUM} USB endpoints ({IDF_TARGET_USB_EP_NUM_INOUT} IN/OUT endpoints and {IDF_TARGET_USB_EP_NUM_IN} IN endpoint) - find more information in `technical reference manual <{IDF_TARGET_TRM_EN_URL}>`_.
+Our USB-OTG implementation is limited to {IDF_TARGET_USB_EP_NUM} USB endpoints ({IDF_TARGET_USB_EP_NUM_INOUT} IN/OUT endpoints and {IDF_TARGET_USB_EP_NUM_IN} IN endpoint) . Please note that enabling Secure Boot or flash encryption disables the USB-OTG USB stack in the ROM, disallowing updates via the serial emulation or Device Firmware Update (DFU) on that port. For more details, please refer to `technical reference manual <{IDF_TARGET_TRM_EN_URL}>`_.
 
 Features
 --------

docs/en/security/secure-boot-v2.rst

@@ -322,6 +322,8 @@ Restrictions after Secure Boot is enabled
 
 - After Secure Boot is enabled, no further eFuses can be read protected. (If :doc:`/security/flash-encryption` is enabled then the bootloader will ensure that any flash encryption key generated on first boot will already be read protected.) If :ref:`CONFIG_SECURE_BOOT_INSECURE` is enabled then this behavior can be disabled, but this is not recommended.
 
+- Please note that enabling Secure Boot or flash encryption disables the USB-OTG USB stack in the ROM, disallowing updates via the serial emulation or Device Firmware Update (DFU) on that port.
+
 .. _secure-boot-v2-generate-key:
 
 Generating Secure Boot Signing Key

docs/zh_CN/api-guides/dfu.rst

@@ -5,7 +5,7 @@
 
 一般情况下，{IDF_TARGET_NAME} 的固件是通过芯片的串口烧录。但是，通过串口烧录 {IDF_TARGET_NAME} 需要连接 USB 转串口转换器（如 CP210x 或 FTDI），详细信息可参阅 :doc:`与 {IDF_TARGET_NAME} 创建串口连接<../get-started/establish-serial-connection>`。{IDF_TARGET_NAME} 包含一个 USB OTG 外设，使其可以通过 USB 将 {IDF_TARGET_NAME} 直接连接到主机，即不需要 USB 转串口转换器也可完成烧录。
 
-设备固件升级 (DFU) 是一种通过通用串行总线 (USB) 升级设备固件的机制。
+设备固件升级 (DFU) 是一种通过通用串行总线 (USB) 升级设备固件的机制。但是，启用安全启动 (Secure Boot) 或 flash 加密会禁用 ROM 中的 USB-OTG USB 堆栈，则无法通过该端口上的模拟串口或 DFU 进行更新。
 
 - 入门指南中的 :ref:`get-started-get-prerequisites` 介绍了 DFU 的软件要求。
 - :ref:`api_guide_dfu_build` 章节介绍了如何使用 ESP-IDF 构建固件。