Vbus support #5
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Vbus support #5
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Since commit bdd33df (Refresh USB logic; callback for events; hotplug in STOP mode; STOP mode in USB SUSPEND; bunch of race conditions in the USB/CDC code; avoid SOF interrupt; 500mA max power in descriptiors), the USBD code would assume a VBUS pin is available and only initialize the USB stack when a VBUS voltage is present. This commit restores the older behavior of, when `STM32L0_CONFIG_PIN_VBUS` (so indirectly also `usbd_pin_vbus`) is `STM32L0_GPIO_PIN_NONE`, assuming VBUS is always present. USB initialization can still be controlled using `USBDevice.attach()`.
This is already done by usbd_conf.c which is also the place where it is read, so no need to do it again here.
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).
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