milk-v duo 系列开发板采用的 CV18xx 系列 USB 下载，通过一系列模式后，官方有提供相关文档，但是都比较散。在群友指点下，通过一系列摸索和实践，终于可以通过 USB 下载 milk-v duo了。
相关文档：
1、进入下载模式
在 milk-v duo datasheet https://github.com/milkv-duo/duo-files/blob/main/duo/hardware/duo-datasheet-v1.2.pdf 中有介绍进入 USB 下载模式的方法：
The MILKV-DUO can operate in USB Device mode for firmware burning and downloading when USB_VBUS_DET (PIN45/ADC2) is detected through a resistor-divided
5V_USB_IN and powered high. However, if the USB port is not being used for burning,
USB_VBUS_DET (PIN45/ADC2) can be directly connected to ground, and the MILKV-DUO
will operate in Host mode, enabling it to connect to external USB devices such as USB
HUBs or USB 2.0 devices.
By default, PIN45 on the MILKV-DUO mainboard is used as an input for ADC2, with a
maximum 3.3V level divided to 1.65V using a voltage divider consisting of two resistors.
If the user wants to use PIN45 as a USB burning detection pin, it is recommended to add
a button in the baseboard design. When the button is pressed, the system will power on,
and USB_VBUS_DET will detect a high level, causing the USB interface to enter Device
mode for firmware downloading. On the other hand, when the button is not pressed,
USB_VBUS_DET detects a low level, and the USB interface enters Host mode, allowing
external connections to USB HUBs or USB 2.0 devices.
可以看到拉高芯片的 PIN45 脚（USB_VBUS_DET），即可进入 USB 烧录模式。
查看 milk-v duo 的原理图 https://github.com/milkv-duo/duo-files/blob/main/duo/hardware/duo-schematic-v1.2.pdf。
可以看到这个管脚通过分压电路之后，连接到了开发板的 32 脚。不过 ADC2 是 1.8V 电压，所以外接电压不能超过 3.6V，否则有可能会烧坏管脚。
所以我们将 开发板的 32 脚上拉到 3.3V 后，即可进入 USB 下载模式。
2、u-boot 代码修改
上面说到拉高 USB_VBUS_DET 管脚可以进入 USB 烧录模式。不过 milk-v duo 的 SDK 在 u-boot 中将 USB_VBUS_DET 管脚定义为 ADC5 了，所以需要将这部分代码做修改。
修改 build/boards/cv180x/cv1800b_milkv_duo_sd/u-boot/cvi_board_init.c 文件，将以下语句注释。
// PINMUX_CONFIG(USB_VBUS_DET, XGPIOB_6); // ADC2
如使用的是 spi-norflash、spi-nandflash 可修改对应目录下的 cvi_board_init.c 文件。
修改完成后重新编译。
3、USB 驱动安装
-
连接 milk-v duo 开发板上的 USB 口
-
正常上电后进入 u-boot，在 u-boot 命令行中输入：
cv180x_c906# cvi_utask vid 0x3346 pid 0x1001
注意，milk-v duo 这里用的是 0x1001，否则后面连接下载软件会失败。其他板子如 duo-256 和duo-s 待验证。
如连接的是 windows 操作系统，此时会弹出 USB Com Port 设备。
- 下载 zadig 官网 https://zadig.akeo.ie 下载 zadig 最新版软件 https://github.com/pbatard/libwdi/releases/download/v1.5.0/zadig-2.8.exe。
下载完成后运行 zadig，选择 list all devices。
然后选择 USB Com Port 设备，并将 Driver 切换至 libusb-win32
点击 Reinstall Driver 开始安装。
4、USB 下载
当前 USB 下载测试在 windows 下进行，Linux 操作系统下暂未测试。
- duo-buildroot-sdk 编译后的已经生成了用于 USB 下载的文件，位于 install 目录下，复制 install 下对应的 soc_cv1800b_milkv_duo_spinand 文件夹至 windows 下。
本人使用的是 nandflash。
-
安装 python3，并添加 python3 至环境变量。
-
安装 python 库
pip install pyusb pyserial
-
断开 milk-v duo 上的 usb 连接和电源，连接串口线至 PC。
-
切换到
soc_cv1800b_milkv_duo_spinand\tools\usb_dl目录，输入
py cv181x_dl.py --libusb --image_dir C:\Users\cys\Desktop\soc_cv1800b_milkv_duo_spinor
INFO: Using libusb
INFO: CV181X USB download start
fip_path: C:\Users\cys\Desktop\soc_cv1800b_milkv_duo_spinand\fip.bin
CV181X USB download start
Connecting to ROM
Waiting for USB port: |
其中 C:\Users\cys\Desktop\soc_cv1800b_milkv_duo_spinor 为 fip.bin 所在目录。
此时下载软件会提示 Waiting for USB port
- 上拉 USB_VBUS_DET 即开发板 32 管脚，连接 USB，并上电，下载软件会自动开始下载。
INFO: Using libusb
INFO: CV181X USB download start
fip_path: C:\Users\cys\Desktop\soc_cv1800b_milkv_duo_spinand\fip.bin
CV181X USB download start
Connecting to ROM
COM6ing for USB port: /
USB VID:PID=3346:1000 SER=123456789ABC LOCATION=1-3.1
done
Send cv_dl_magic.bin...
--- 0.0 Seconds ---
done
COM6ing for USB port: |
USB VID:PID=3346:1000 SER=123456789ABC LOCATION=1-3.1
Send 4096B fip.bin...
--- 0.02 Seconds ---
set flag
break
Connecting to ROM 2nd stage...
COM6ing for USB port: |
USB VID:PID=3346:1000 SER=123456789ABC LOCATION=1-3.1
Send cv_dl_magic.bin...
--- 0.0 Seconds ---
fip_tx_offset 4096
fip_tx_size 47616
--- 0.3 Seconds ---
set MGN1 flag
break
Connecting to ROM 2nd stage...
COM6ing for USB port: |
USB VID:PID=3346:1000 SER=123456789ABC LOCATION=1-3.1
Send cv_dl_magic.bin...
--- 0.0 Seconds ---
fip_tx_offset 51712
fip_tx_size 4096
--- 0.01 Seconds ---
set MGN1 flag
break
Connecting to ROM 2nd stage...
COM6ing for USB port: |
USB VID:PID=3346:1000 SER=123456789ABC LOCATION=1-3.1
Send cv_dl_magic.bin...
--- 0.0 Seconds ---
fip_tx_offset 55808
fip_tx_size 8192
--- 0.02 Seconds ---
set MGN1 flag
break
Connecting to ROM 2nd stage...
COM6ing for USB port: |
USB VID:PID=3346:1000 SER=123456789ABC LOCATION=1-3.1
Send cv_dl_magic.bin...
--- 0.0 Seconds ---
fip_tx_offset 64000
fip_tx_size 14336
--- 0.03 Seconds ---
set MGN1 flag
break
Connecting to ROM 2nd stage...
COM6ing for USB port: |
USB VID:PID=3346:1000 SER=123456789ABC LOCATION=1-3.1
Send cv_dl_magic.bin...
--- 0.0 Seconds ---
fip_tx_offset 78336
fip_tx_size 110592
--- 0.29 Seconds ---
set MGN1 flag
break
Connecting to ROM 2nd stage...
COM6ing for USB port: |
USB VID:PID=3346:1000 SER=123456789ABC LOCATION=1-3.1
Send cv_dl_magic.bin...
--- 0.0 Seconds ---
fip_tx_offset 188928
fip_tx_size 512
--- 0.0 Seconds ---
set MGN1 flag
break
Connecting to ROM 2nd stage...
COM6ing for USB port: |
USB VID:PID=3346:1000 SER=123456789ABC LOCATION=1-3.1
Send cv_dl_magic.bin...
--- 0.0 Seconds ---
fip_tx_offset 188928
fip_tx_size 263680
--- 1.21 Seconds ---
set MGN1 flag
break
Connecting to ROM 2nd stage...
Query pyserial device timeout!
Connected to u-boot cvi_utask by pyserial
INFO: Using libusb
LIBUSB on Windows
INFO: SEND FILE C:\Users\cys\Desktop\soc_cv1800b_milkv_duo_spinand\fip.bin
C:\Users\cys\Desktop\soc_cv1800b_milkv_duo_spinand\fip.bin is 452608 bytes
Send to address 0x82473000
--- 2.22 Seconds ---
LIBUSB on Windows
INFO: SEND FILE C:\Users\cys\Desktop\soc_cv1800b_milkv_duo_spinand\boot.spinand
--- size 64 0.00893 Seconds ---
--- size 3372100 0.39134 Seconds ---
INFO: CVI_USB_PROGRAM
INFO: SEND FILE C:\Users\cys\Desktop\soc_cv1800b_milkv_duo_spinand\rootfs.spinand
--- size 64 0.00248 Seconds ---
--- size 16777280 1.49693 Seconds ---
INFO: CVI_USB_PROGRAM
--- size 2359360 0.06944 Seconds ---
INFO: CVI_USB_PROGRAM
INFO: SEND FILE C:\Users\cys\Desktop\soc_cv1800b_milkv_duo_spinand\system.spinand
--- size 64 0.00248 Seconds ---
--- size 16777280 1.99293 Seconds ---
INFO: CVI_USB_PROGRAM
--- size 4718656 0.14087 Seconds ---
INFO: CVI_USB_PROGRAM
LIBUSB on Windows
INFO: reboot device done
INFO: USB download complete
INFO: CV181X USB download end
串口连接的日志窗口会显示 u-boot 相关日志：
Hit any key to stop autoboot: 0
## Resetting to default environment
Start USB downloading...
start usb task!
NOTICE: cvi_usb_hw_init
INFO: waiting for connection ...
INFO: detect vbus ...
INFO: bulkBufAlloc: 0000000081badd80
INFO: cmdBufAlloc: 0000000081badfc0
INFO: ep0BuffAlloc: 0000000081beae80
INFO: setup_buf: 0000000081bae200
INFO: handler: 0000000081bae280
INFO: cb0_buf: 0000000081bae690
INFO: cb1_buf: 0000000081bae720
INFO: cb2_buf: 0000000081bae7b0
INFO: rsp_buf: 0000000081bae840
INFO: acm_buf: 0000000081bae890
NOTICE: Overwrite fip_src to FIP_SRC_USB
NOTICE: fip_src 6
NOTICE: bind()
NOTICE: Patch VID 3346
NOTICE: Patch VID 3346
NOTICE: Application: disconnect
NOTICE: Patch VID 3346
NOTICE: Patch VID 3346
NOTICE: USB enumeration done
NOTICE: connection speed: 3
NOTICE: CVI_USB_PRG_CMD
NOTICE: run command: setenv filesize 0x6e800
INFO: CVI_USB_BREAK
NOTICE: Leave transfer loop
NOTICE: Application: disconnect
NOTICE: unbind()
NOTICE: USB stop
addr 0000000082473000
fip.bin
do_cvi_update_spinand with version 0x1823a001
Scan and erase first 20 blocks
Generated checknum=0xebca6b98
fip size=452608 bytes
write 1 copy of fip
write 2 copy of fip
Saving Environment to NAND... Erasing redundant NAND...
Erasing at 0x8a0000 -- 100% complete.
Writing to redundant NAND... OK
OK
start usb task!
NOTICE: cvi_usb_hw_init
INFO: waiting for connection ...
INFO: detect vbus ...
INFO: bulkBufAlloc: 0000000081baee00
INFO: cmdBufAlloc: 0000000081baf040
INFO: ep0BuffAlloc: 0000000081beae80
INFO: setup_buf: 0000000081baf280
INFO: handler: 0000000081baf300
INFO: cb0_buf: 0000000081baf710
INFO: cb1_buf: 0000000081baf7a0
INFO: cb2_buf: 0000000081baf830
INFO: rsp_buf: 0000000081baf8c0
INFO: acm_buf: 0000000081baf910
NOTICE: Overwrite fip_src to FIP_SRC_USB
NOTICE: fip_src 6
NOTICE: bind()
NOTICE: Patch VID 3346
NOTICE: Patch VID 3346
NOTICE: Application: disconnect
NOTICE: Patch VID 3346
NOTICE: Patch VID 3346
NOTICE: USB enumeration done
NOTICE: connection speed: 3
NAND erase.part: device 0 offset 0x280000, size 0x600000
Erasing at 0x860000 -- 100% complete.
OK
NAND write: device 0 offset 0x280000, size 0x337404
3372036 bytes written: OK
nand nand speed 3.494 MB/s
NOTICE: CVI_USB_PROGRAM done
NAND erase.part: device 0 offset 0x8c0000, size 0x2800000
Erasing at 0x30a0000 -- 100% complete.
OK
NAND write: device 0 offset 0x8c0000, size 0x1000000
16777216 bytes written: OK
nand nand speed 3.536 MB/s
NOTICE: CVI_USB_PROGRAM done
NAND write: device 0 offset 0x18c0000, size 0x240000
2359296 bytes written: OK
nand nand speed 3.553 MB/s
NOTICE: CVI_USB_PROGRAM done
NAND erase.part: device 0 offset 0x30c0000, size 0x4f40000
Skipping bad block at 0x07f80000
Skipping bad block at 0x07fa0000
Skipping bad block at 0x07fc0000
Skipping bad block at 0x07fe0000
OK
NAND write: device 0 offset 0x30c0000, size 0x1000000
16777216 bytes written: OK
nand nand speed 3.671 MB/s
NOTICE: CVI_USB_PROGRAM done
NAND write: device 0 offset 0x40c0000, size 0x480000
4718592 bytes written: OK
nand nand speed 3.618 MB/s
NOTICE: CVI_USB_PROGRAM done
INFO: CVI_USB_BREAK
NOTICE: Leave transfer loop
NOTICE: Application: disconnect
NOTICE: unbind()
NOTICE: USB stop
start usb task!
NOTICE: cvi_usb_hw_init
INFO: waiting for connection ...
INFO: detect vbus ...
INFO: bulkBufAlloc: 0000000081bb0180
INFO: cmdBufAlloc: 0000000081bb03c0
INFO: ep0BuffAlloc: 0000000081beae80
INFO: setup_buf: 0000000081bb0600
INFO: handler: 0000000081bb0680
INFO: cb0_buf: 0000000081bb0a90
INFO: cb1_buf: 0000000081bb0b20
INFO: cb2_buf: 0000000081bb0bb0
INFO: rsp_buf: 0000000081bb0c40
INFO: acm_buf: 0000000081bb0c90
NOTICE: Overwrite fip_src to FIP_SRC_USB
NOTICE: fip_src 6
NOTICE: bind()
NOTICE: Patch VID 3346
NOTICE: Patch VID 3346
NOTICE: Application: disconnect
NOTICE: Patch VID 3346
NOTICE: Patch VID 3346
NOTICE: USB enumeration done
NOTICE: connection speed: 3
NOTICE: CVI_USB_REBOOT
至此 USB 下载完成。
再此要特别感谢在 milk-v 微信群中给于指导的网友 xiaomin、hui，以及milk-v工程师 Carbon。