Fork of
USBDevice
by 
Tomas Cerskus
Slightly modified original USBDevice library to support F401RE.
On F401RE the data pins of your USB connector should be attached to PA12 (D+) and PA11(D-). It is also required to connect the +5V USB line to PA9.
F401RE requires 48MHz clock for USB. Therefore in order for this to work you will need to change the default clock settings:
Clock settings for USB
#include "stm32f4xx_hal.h"
RCC_OscInitTypeDef RCC_OscInitStruct;
RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
RCC_OscInitStruct.HSIState = RCC_HSI_ON;
RCC_OscInitStruct.HSICalibrationValue = 16;
RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSI;
RCC_OscInitStruct.PLL.PLLM = 16;
RCC_OscInitStruct.PLL.PLLN = 336;
RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV4;
RCC_OscInitStruct.PLL.PLLQ = 7;
if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK) {
error("RTC error: LSI clock initialization failed.");
}
NOTE: Changing the clock frequency might affect the behavior of other libraries. I only tested the Serial library.
UPDATE: Clock settings should not to be changed anymore! Looks like the newer mbed library has the required clock enabled.