diff --git a/cores/arduino/stm32/usb/usbd_if.c b/cores/arduino/stm32/usb/usbd_if.c
index 752be7fa04..f0584cd5f5 100644
--- a/cores/arduino/stm32/usb/usbd_if.c
+++ b/cores/arduino/stm32/usb/usbd_if.c
@@ -10,36 +10,148 @@
 #include "usbd_if.h"
 #include "usbd_cdc_if.h"
 
+#if !defined(USBD_REENUM_DISABLED)
+
+/*
+ * Below, support for re-attaching to USB is handled. USB-attachment is
+ * detected by the presence of a pullup on one of the USB datalines.
+ *
+ * To force the USB host to re-enumerate, we must detach (disable
+ * pullup), wait a bit, and re-attach (enable pullup). This can be done
+ * while running, but must also happen at startup (including the detach
+ * and delay, since USB might have been active just before reset, so we
+ * must force a re-enumeration). On a cold boot (e.g. USB just plugged
+ * in), the forced detach might not be needed, but will not be harmful
+ * either.
+ *
+ * For pullup control, a number of cases are supported:
+ *  1. An external pullup controllable through a GPIO (e.g. using an
+ *     external transistor). The default output state (e.g. when the
+ *     GPIO is floating) can be enabled or disabled, and the output
+ *     polarity can be configured.
+ *  2. A pullup built into the chip's USB core. This is automatically
+ *     enabled by hardware or USB HAL code, and can be manually
+ *     controlled using HAL functions.
+ *  3. A fixed (always enabled) external pullup. To still allow detaching,
+ *     a bit of a hack is used: By setting the D+ to OUTPUT LOW, it
+ *     *looks* like the pullup is removed (the host has own, bigger,
+ *     pulldown, to detect detaching, so writing LOW looks the same).
+ *     This probably violates USB specifications, but does seem to work
+ *     well in practice. It probably also should only be used briefly,
+ *     keeping the pin OUTPUT LOW for longer might be problematic.
+ *
+ * These scenarios are considered in order: If the variant defines an
+ * external pullup control pin, it is used, else if an internal pullup
+ * is present, that is used, else the D+ trick is used.
+ *
+ * To configure #1, the variant should either define USBD_ATTACH_PIN and
+ * USBD_ATTACH_LEVEL when the pullup is disabled by default and the pin
+ * can be used to *attach*, or USBD_DETACH_PIN and USBD_DETACH_LEVEL
+ * when the pullup is enabled by default and the pin can be used to
+ * *detach*.
+ */
+
+/* Compatibility with the old way to specify this */
+#if defined(USB_DISC_PIN) && !defined(USBD_ATTACH_PIN)
+#define USBD_ATTACH_PIN USB_DISC_PIN
+#define USBD_ATTACH_LEVEL LOW
+#warning "USB_DISC_PIN is deprecated, use USBD_ATTACH_PIN instead"
+#endif /* defined(USB_DISC_PIN) && !defined(USBD_ATTACH_PIN) */
+
+/* Some sanity checks */
+#if defined(USBD_ATTACH_PIN) && defined(USBD_DETACH_PIN)
+#error "Cannot define both USBD_ATTACH_PIN and USBD_DETACH_PIN"
+#endif /* defined(USBD_ATTACH_PIN) && defined(USBD_DETACH_PIN) */
+#if defined(USBD_ATTACH_PIN) && !defined(USBD_ATTACH_LEVEL)
+#error "USBD_ATTACH_PIN also needs USBD_ATTACH_LEVEL defined"
+#endif /* defined(USBD_ATTACH_PIN) && !defined(USBD_ATTACH_LEVEL) */
+#if defined(USBD_DETACH_PIN) && !defined(USBD_DETACH_LEVEL)
+#error "USBD_DETACH_PIN also needs USBD_DETACH_LEVEL defined"
+#endif /* defined(USBD_DETACH_PIN) && !defined(USBD_DETACH_LEVEL) */
+
+/* Either of these bits indicate that there are internal pullups */
+#if defined(USB_BCDR_DPPU) || defined(USB_OTG_DCTL_SDIS)
+#define USBD_HAVE_INTERNAL_PULLUPS
+#endif /* defined(USB_BCDR_DPPU) || defined(USB_OTG_DCTL_SDIS) */
+
+/* Figure out which USB instance is used. This mirrors the decision made
+ * in USBD_LL_Init in usbd_conf.c. */
+#if defined(USE_USB_HS)
+#define USBD_USB_INSTANCE USB_OTG_HS
+#elif defined(USB_OTG_FS)
+#define USBD_USB_INSTANCE USB_OTG_FS
+#elif defined(USB)
+#define USBD_USB_INSTANCE USB
+#endif
+
+/*
+ * Translate pin number to a pin name (using the same define for both
+ * attach and detach pins) and also define the complementary level. This
+ * allows using the same code for the default-enabled and
+ * default-disabled case as well as for the D+ trick (which does not
+ * know the pin number, only the pin name).
+ */
+#if defined(USBD_ATTACH_PIN)
+#define USBD_PULLUP_CONTROL_PINNAME digitalPinToPinName(USBD_ATTACH_PIN)
+#define USBD_DETACH_LEVEL !(USBD_ATTACH_LEVEL)
+#elif defined(USBD_DETACH_PIN)
+#define USBD_PULLUP_CONTROL_PINNAME digitalPinToPinName(USBD_DETACH_PIN)
+#define USBD_ATTACH_LEVEL !(USBD_DETACH_LEVEL)
+#elif !defined(USBD_HAVE_INTERNAL_PULLUPS)
+/* When no USB attach and detach pins were defined, and there are also
+ * no internal pullups, assume there is a fixed external pullup and apply
+ * the D+ trick. This should happen only for the USB peripheral, since
+ * USB_OTG_HS and USB_OTG_FS always have internal pullups. */
+#if !defined(USB)
+#error "Unexpected USB configuration"
+#endif
+#define USBD_PULLUP_CONTROL_PINNAME USB_DP
+#define USBD_DETACH_LEVEL LOW
+// USBD_ATTACH_LEVEL not needed.
+#define USBD_DP_TRICK
+#endif
+
 /**
-  * @brief  Force to re-enumerate USB
+  * @brief  Force to re-enumerate USB.
+  *
+  * This is intended to be called at startup by core code. It could be
+  * used at runtime, while USB is connected, to force re-enumeration
+  * too, but that does not work in all cases (when USB is enabled on an
+  * F103C8, setting output mode on the DP pin no longer has any effect).
+  *
   * @param  None
   * @retval None
   */
-
 WEAK void USBD_reenumerate(void)
 {
-#ifndef USBD_REENUM_DISABLED
-  /* Re-enumerate the USB */
-#ifdef USB_DISC_PIN
-  pinMode(USB_DISC_PIN, OUTPUT);
-  digitalWrite(USB_DISC_PIN, LOW);
+#if defined(USBD_PULLUP_CONTROL_PINNAME)
+  /* Detach */
+  pin_function(USBD_PULLUP_CONTROL_PINNAME, STM_PIN_DATA(STM_MODE_OUTPUT_PP, GPIO_NOPULL, 0));
+  digitalWriteFast(USBD_PULLUP_CONTROL_PINNAME, USBD_DETACH_LEVEL);
+
+  /* Wait */
+  delay(USBD_ENUM_DELAY);
+
+  /* Attach */
+#if defined(USBD_DP_TRICK)
+  /* Revert back to input (floating), needed for the D+ trick */
+  pin_function(USBD_PULLUP_CONTROL_PINNAME, STM_PIN_DATA(STM_MODE_INPUT, GPIO_NOPULL, 0));
 #else
-#ifdef USE_USB_HS_IN_FS
-  PinName pinDP = USB_OTG_HS_DP;
-#elif defined(USB_OTG_FS)
-  PinName pinDP = USB_OTG_FS_DP;
-#else /* USB */
-  PinName pinDP =  USB_DP;
-#endif
-  pin_function(pinDP, STM_PIN_DATA(STM_MODE_OUTPUT_PP, GPIO_NOPULL, 0));
-  digitalWriteFast(pinDP, LOW);
+  digitalWriteFast(USBD_PULLUP_CONTROL_PINNAME, USBD_ATTACH_LEVEL);
+#endif /* defined(USBD_PULLUP_CONTROL_FLOATING) */
+#elif defined(USBD_HAVE_INTERNAL_PULLUPS)
+  USB_DevDisconnect(USBD_USB_INSTANCE);
   delay(USBD_ENUM_DELAY);
-  pin_function(pinDP, STM_PIN_DATA(STM_MODE_INPUT, GPIO_NOPULL, 0));
-  /*delay(USBD_ENUM_DELAY);*/
-#endif /* USB_DISC_PIN */
-#endif /* USBD_REENUM_DISABLED */
+  USB_DevConnect(USBD_USB_INSTANCE);
+#else
+#warning "No USB attach/detach method, USB might not be reliable through system resets"
+#endif
 }
 
+#else /* !defined(USBD_REENUM_DISABLED) */
+WEAK void USBD_reenumerate(void) { }
+#endif
+
 #ifdef USBD_USE_CDC
 void USBD_CDC_init(void)
 {
diff --git a/variants/Generic_F401Rx/variant.h b/variants/Generic_F401Rx/variant.h
index d9d8e56d96..661ea405cf 100644
--- a/variants/Generic_F401Rx/variant.h
+++ b/variants/Generic_F401Rx/variant.h
@@ -126,7 +126,9 @@ extern "C" {
 #define PIN_SERIAL_TX           PA9
 
 #ifdef ARDUINO_CoreBoard_F401RC
-#define USB_DISC_PIN            PD2
+// USB, pull this pin low to enable the USB attach pullup
+#define USBD_ATTACH_PIN         PD2
+#define USBD_ATTACH_LEVEL       LOW
 #endif
 
 #ifdef __cplusplus
diff --git a/variants/MALYANM200_F103CB/variant.cpp b/variants/MALYANM200_F103CB/variant.cpp
index d2e0259e81..3b1a499396 100644
--- a/variants/MALYANM200_F103CB/variant.cpp
+++ b/variants/MALYANM200_F103CB/variant.cpp
@@ -86,25 +86,6 @@ const PinName digitalPin[] = {
 extern "C" {
 #endif
 
-/**
-  * Initialize the variant. Turn on PB9 for USB.
-  */
-void initVariant()
-{
-  pinMode(PB9, OUTPUT);
-  digitalWrite(PB9, 1);
-}
-
-void USBD_reenumerate(void)
-{
-  pinMode(PB9, OUTPUT);
-  digitalWrite(PB9, HIGH);
-  delay(10);
-  digitalWrite(PB9, LOW);
-  delay(10);
-  digitalWrite(PB9, HIGH);
-}
-
 /**
   * @brief  System Clock Configuration
   *         The system Clock is configured as follow :
diff --git a/variants/MALYANM200_F103CB/variant.h b/variants/MALYANM200_F103CB/variant.h
index f59785c988..8e2875e69f 100644
--- a/variants/MALYANM200_F103CB/variant.h
+++ b/variants/MALYANM200_F103CB/variant.h
@@ -105,6 +105,10 @@ extern "C" {
 #define PIN_SERIAL_RX           PA10
 #define PIN_SERIAL_TX           PA9
 
+// USB, pull this pin low to *disable* the USB attach pullup
+#define USBD_DETACH_PIN         PB9
+#define USBD_DETACH_LEVEL       LOW
+
 /* Dedicated definitions */
 #ifndef MAX_PRIORITY
 #define MAX_PRIORITY            15
diff --git a/variants/MAPLEMINI_F103CB/variant.h b/variants/MAPLEMINI_F103CB/variant.h
index dbdad39ad1..6b6e26f899 100644
--- a/variants/MAPLEMINI_F103CB/variant.h
+++ b/variants/MAPLEMINI_F103CB/variant.h
@@ -108,8 +108,9 @@ extern "C" {
 #define PIN_SERIAL_RX           PA10
 #define PIN_SERIAL_TX           PA9
 
-// USB
-#define USB_DISC_PIN            PB9
+// USB, pull this pin low to enable the USB attach pullup
+#define USBD_ATTACH_PIN         PB9
+#define USBD_ATTACH_LEVEL       LOW
 
 #ifdef __cplusplus
 } // extern "C"
diff --git a/variants/board_template/variant.h b/variants/board_template/variant.h
index 3f3b205649..a636ede352 100644
--- a/variants/board_template/variant.h
+++ b/variants/board_template/variant.h
@@ -162,6 +162,19 @@ extern "C" {
 // See AN4879 https://www.st.com/content/st_com/en/search.html#q=AN4879-t=resources-page=1
 //#define USBD_VBUS_DETECTION_ENABLE
 
+// If the board has external USB pullup (on DP/DM depending on speed)
+// that can be controlled using a GPIO pin, define these:
+//  - If the the pullup is disabled (USB detached) by default, define
+//    USBD_ATTACH_PIN to the pin that, when written to
+//    USBD_ATTACH_LEVEL, attaches the pullup.
+//  - If the the pullup is enabled (attached) by default, define
+//    USBD_DETACH_PIN to the pin that, when written to
+//    USBD_DETACH_LEVEL, detaches the pullup.
+//#define USBD_ATTACH_PIN x
+//#define USBD_ATTACH_LEVEL LOW
+//#define USBD_DETACH_PIN x
+//#define USBD_DETACH_LEVEL LOW
+
 #ifdef __cplusplus
 } // extern "C"
 #endif