USBFAN — STM32F103 USB CDC 2-Channel Fan Controller

A tiny, open USB fan controller on a STM32F103C8T6 Blue Pill. It enumerates as a virtual COM port and drives 2 PWM fans with 2 tachometer (RPM) inputs, controlled by FanControl through a bundled plugin — or by anything that can speak a few ASCII lines over a serial port.

• MCU: STM32F103C8T6 (Blue Pill, LQFP48, 72 MHz, 64 KB flash / 20 KB RAM)
• USB: CDC (Virtual COM Port), VID 0x0483 / PID 0x5740 — no vendor driver needed
• 2× PWM fan channels @ 25 kHz, 2× tachometer inputs, 1 status LED
• ~60% flash / 35% RAM usage, plenty of headroom

中文说明见文末 / 中文説明見文末  (Chinese README at the bottom)

Features

• Dual-channel PWM + tach: ch0/ch1 each get an independent 0–100 % duty PWM output and a real-time RPM reading from the fan's tach line.
• Safe boot default: fans start at 50 % duty until a host sends a command.
• USB re-enumeration on reset: the Blue Pill's D+ pull-up is hard-wired, so the firmware drives a short SE0 pulse at boot to make Windows re-enumerate after a flash/reset without unplugging the cable.
• Crosstalk-immune tach: a 2 ms debounce plus whole-burst suppression rejects 25 kHz PWM crosstalk that would otherwise read as a phantom ~15000 RPM.
• FanControl plugin with presence detection: when the device is unplugged, the plugin shows no fans; plugging it back in makes them reappear automatically.

Hardware / wiring

Function	ch0	ch1
PWM output (25 kHz, push-pull, 0–3.3 V)	PB7 (TIM4_CH2)	PB8 (TIM4_CH3)
Tach input (falling edge, internal pull-up)	PA1 (EXTI1)	PB14 (EXTI14)
Status LED (active-low)	PC13 (onboard Blue Pill LED)

Tach wiring gotcha: keep the tach signal ground short and local to the board GND. Routing it through the mains PE (e.g. a PSU earth) creates a ground loop that couples the 25 kHz PWM into the tach line.

Serial protocol

Line-based ASCII, \n terminated (CR ignored):

Direction	Format	Example
Host → device	D<ch>:<val>\n (ch = 0..1, val = 0..100)	D0:75\n
Device → host (≈1 Hz)	R<ch>:<rpm>\n	R0:1250\n

Build

Toolchain: CMake + Ninja + arm-none-eabi-gcc (from STM32CubeCLT, all on PATH).

cmake --preset Debug
cmake --build --preset Debug      # -> build/Debug/USBFAN.elf

Flash

ST-Link via SWD:

STM32_Programmer_CLI.exe -c port=SWD -w build\Debug\USBFAN.elf -v -rst

The board re-enumerates on its own after flashing — no USB replug needed.

FanControl plugin

tools/FanControl.Stm32FanCdc/ — a net10.0 plugin (matches FanControl's runtime) implementing IPlugin3 (presence detection via RefreshRequested). It finds the COM port by VID/PID (0x0483/0x5740) through the registry and requires no NuGet packages (references FanControl's own FanControl.Plugins.dll and System.IO.Ports.dll).

dotnet build tools\FanControl.Stm32FanCdc\FanControl.Stm32FanCdc.csproj -c Release
# copy to the FanControl plugins folder (needs admin):
#   bin\Release\net10.0\FanControl.Stm32FanCdc.dll  ->  C:\Program Files (x86)\FanControl\Plugins\

Project structure

Core/            firmware (main.c, fan_hw.c, HAL init)
USB_DEVICE/      CubeMX USB CDC glue (usbd_cdc_if.c holds the D/R parser)
Drivers/         STM32F1 HAL + CMSIS
Middlewares/     ST USB Device Library (CDC)
tools/FanControl.Stm32FanCdc/   FanControl plugin source
USBFAN.ioc       STM32CubeMX project (regenerate-safe; SYS = Serial Wire keeps SWD)
cmake/           CMake presets + CubeMX CMake

---

中文说明 (Chinese)

USBFAN — STM32F103 USB CDC 双通道风扇控制器

基于 STM32F103C8T6 Blue Pill 的开源 USB 风扇控制器：以虚拟串口枚举， 驱动 2 路 PWM 风扇 + 2 路测速（RPM）输入，通过附带的插件由 FanControl 控制，或任何能用串口发几行 ASCII 的程序直接控制。

• USB CDC 虚拟串口，VID 0x0483 / PID 0x5740，免驱动
• 2 路 25 kHz PWM + 2 路 tach + 1 个状态 LED
• Flash 占用 ~60% / RAM ~35%

特性

• 双通道 PWM + tach：每路独立 0–100% 占空比输出 + 实时转速读数
• 安全默认 50%：开机即 50% 占空，直到主机发指令
• 复位自动重枚举：Blue Pill 的 D+ 上拉硬接死，固件开机拉低 DP/DM 50ms 制造断开→重连，烧录/复位后不用拔 USB
• tach 抗串扰：2ms 消抖 + 突发整体抑制，滤掉 25 kHz PWM 串扰（否则会误读成 ~15000 RPM）
• 插件在位检测：拔掉设备风扇即消失，插回自动出现

引脚

功能	ch0	ch1
PWM 输出（25 kHz，推挽，0–3.3V）	PB7（TIM4_CH2）	PB8（TIM4_CH3）
Tach 输入（下降沿，内部上拉）	PA1（EXTI1）	PB14（EXTI14）
状态 LED（低电平点亮）	PC13（板载）

地线注意：tach 信号地要就近接板子 GND。走电网 PE 会形成地环路，把 25 kHz PWM 耦合进 tach。

串口协议

ASCII 行，\n 结尾（忽略 CR）：

方向	格式	示例
主机 → 设备	D<ch>:<val>\n（ch=0..1，val=0..100）	D0:75\n
设备 → 主机（约 1Hz）	R<ch>:<rpm>\n	R0:1250\n

构建 / 烧录

cmake --preset Debug
cmake --build --preset Debug        # -> build/Debug/USBFAN.elf
STM32_Programmer_CLI.exe -c port=SWD -w build\Debug\USBFAN.elf -v -rst

FanControl 插件

tools/FanControl.Stm32FanCdc/，net10.0，实现 IPlugin3（在位检测），按 VID/PID 从注册表找 COM 口，无 NuGet 依赖。

dotnet build tools\FanControl.Stm32FanCdc\FanControl.Stm32FanCdc.csproj -c Release
# 把 bin\Release\net10.0\FanControl.Stm32FanCdc.dll 拷到
#   C:\Program Files (x86)\FanControl\Plugins\   （需管理员）