Manually committed changes from https://github.com/rogerclarkmelbourne/Arduino_STM32/pull/401, as that PR included changes to permissions on unrelated files in the tools

STM32F1/cores/maple/libmaple/usb/stm32f1/usb.c

@@ -120,6 +120,12 @@ static void usb_suspend(void) {
     USBLIB->state = USB_SUSPENDED;
 }
 
+void usb_power_off(void) {
+    USB_BASE->CNTR = USB_CNTR_FRES;
+    USB_BASE->ISTR = 0;
+    USB_BASE->CNTR = USB_CNTR_FRES + USB_CNTR_PDWN;
+}
+
 static void usb_resume_init(void) {
     uint16 cntr;
 
@@ -268,6 +274,7 @@ static void handle_out0(void);
 
 static void dispatch_ctr_lp() {
     uint16 istr;
+
     while (((istr = USB_BASE->ISTR) & USB_ISTR_CTR) != 0) {
         /* TODO WTF, figure this out: RM0008 says CTR is read-only,
          * but ST's firmware claims it's clear-only, and emphasizes
@@ -288,6 +295,7 @@ static void dispatch_ctr_lp() {
     }
 }
 
+
 /* FIXME Dataflow on endpoint 0 RX/TX status is based off of ST's
  * code, and is ugly/confusing in its use of SaveRState/SaveTState.
  * Fixing this requires filling in handle_in0(), handle_setup0(),

STM32F1/cores/maple/libmaple/usb/stm32f1/usb_cdcacm.c

@@ -80,7 +80,6 @@ static uint8* usbGetConfigDescriptor(uint16 length);
 static uint8* usbGetStringDescriptor(uint16 length);
 static void usbSetConfiguration(void);
 static void usbSetDeviceAddress(void);
-
 /*
  * Descriptors
  */
@@ -392,6 +391,8 @@ void usb_cdcacm_enable(gpio_dev *disc_dev, uint8 disc_bit) {
 	}
 	
     /* Initialize the USB peripheral. */
+    /* One of the callbacks that will automatically happen from this will be to usbInit(),
+       which will power up the USB peripheral. */
     usb_init_usblib(USBLIB, ep_int_in, ep_int_out);
 }
 
@@ -403,6 +404,9 @@ void usb_cdcacm_disable(gpio_dev *disc_dev, uint8 disc_bit) {
 	{
 		gpio_write_bit(disc_dev, disc_bit, 1);
 	}
+    /* Powerdown the USB peripheral. It gets powered up again with usbInit(), which
+       gets called when usb_cdcacm_enable() is called. */
+    usb_power_off(); 
 }
 
 void usb_cdcacm_putc(char ch) {
@@ -644,7 +648,8 @@ static uint8* vcomGetSetLineCoding(uint16 length) {
 static void usbInit(void) {
     pInformation->Current_Configuration = 0;
 
-    USB_BASE->CNTR = USB_CNTR_FRES;
+    // Reset and power up the peripheral.
+    USB_BASE->CNTR = USB_CNTR_FRES; 
 
     USBLIB->irq_mask = 0;
     USB_BASE->CNTR = USBLIB->irq_mask;

STM32F1/system/libmaple/include/libmaple/usb.h

@@ -157,6 +157,8 @@ typedef struct usblib_dev {
 
 extern usblib_dev *USBLIB;
 
+void usb_power_off(void);
+
 void usb_init_usblib(usblib_dev *dev,
                      void (**ep_int_in)(void),
                      void (**ep_int_out)(void));