Support boards with a voltage divider on VBUS

This can be useful to allow measuring the actual USB voltage, rather
than just detect presence.

Previously the USB code would always enable an internal pulldown on the
VBUS, which would skew the reading (and, if the divider is fairly high
impedance, can even prevent detecting VBUS presence using a digital
read).

With this commit, boards can indicate they use an external divider using
the STM32L0_CONFIG_PIN_VBUS_HAS_DIVIDER macro in their variant.h. When
this is defined, the internal pullup is not enabled to prevent it from
causing issues and because it is no longer needed (the external divider
functions as a pulldown).

Author: matthijskooijman

cores/arduino/USBCore.cpp

@@ -54,8 +54,17 @@
 bool USBDeviceClass::begin()
 {
 #if defined(USB_CLASS)
+    #if defined(STM32L0_CONFIG_PIN_VBUS_HAS_DIVIDER)
+    // On boards with an external divider (to allow measuring
+    // voltage rather than just on/off), do not enable the pulldown
+    // since that influences the divider (and the divider works as
+    // an external pulldown too).
+    bool needs_pulldown = false;
+    #else
+    bool needs_pulldown = true;
+    #endif
     return USBD_Initialize(USB_VID, USB_PID, (const uint8_t*)USB_MANUFACTURER, (const uint8_t*)USB_PRODUCT, USB_CLASS,
-                           STM32L0_CONFIG_PIN_VBUS, STM32L0_USB_IRQ_PRIORITY,
+                           STM32L0_CONFIG_PIN_VBUS, needs_pulldown, STM32L0_USB_IRQ_PRIORITY,
                            &USBDeviceClass::connectCallback, &USBDeviceClass::disconnectCallback, &USBDeviceClass::suspendCallback, &USBDeviceClass::resumeCallback);
 #endif
     return false;

cores/arduino/wiring_private.h

@@ -59,7 +59,7 @@ extern void USBD_CDC_Initialize(void *);
 extern void USBD_CDC_MSC_Initialize(void *);
 
 extern bool USBD_Initialize(uint16_t vid, uint16_t pid, const uint8_t *manufacturer, const uint8_t *product, void(*initialize)(void *),
-                            unsigned int pin_vbus, unsigned int priority,
+                            unsigned int pin_vbus, bool pin_vbus_needs_pulldown, unsigned int priority,
                             void(*connect_callback)(void), void(*disconnect_callback)(void), void(*suspend_callback)(void), void(*resume_callback)(void));
 extern void USBD_Teardown(void);
 extern void USBD_Attach(void);

system/STM32L0xx/Source/USB/usbd_conf.c

@@ -182,7 +182,7 @@ static void USBD_VBUSChangedIrq(void)
 }
 
 bool USBD_Initialize(uint16_t vid, uint16_t pid, const uint8_t *manufacturer, const uint8_t *product, void(*initialize)(struct _USBD_HandleTypeDef *),
-                     unsigned int pin_vbus, unsigned int priority,
+                     unsigned int pin_vbus, bool pin_vbus_needs_pulldown, unsigned int priority,
                      void(*connect_callback)(void), void(*disconnect_callback)(void), void(*suspend_callback)(void), void(*resume_callback)(void))
 {
     if (stm32l0_system_pclk1() < 10000000)
@@ -213,7 +213,10 @@ bool USBD_Initialize(uint16_t vid, uint16_t pid, const uint8_t *manufacturer, co
 
         stm32l0_lptim_timeout_create(&USBD_VBUSTimeout);
 
-        stm32l0_gpio_pin_configure(usbd_pin_vbus, (STM32L0_GPIO_PARK_HIZ | STM32L0_GPIO_PUPD_PULLDOWN | STM32L0_GPIO_OSPEED_LOW | STM32L0_GPIO_OTYPE_PUSHPULL | STM32L0_GPIO_MODE_INPUT));
+        uint32_t mode = (STM32L0_GPIO_PARK_HIZ | STM32L0_GPIO_OSPEED_LOW | STM32L0_GPIO_OTYPE_PUSHPULL | STM32L0_GPIO_MODE_INPUT);
+        if (pin_vbus_needs_pulldown)
+            mode |= STM32L0_GPIO_PUPD_PULLDOWN;
+        stm32l0_gpio_pin_configure(usbd_pin_vbus, mode);
 
         if (stm32l0_gpio_pin_read(usbd_pin_vbus))
         {