VBUS sensing detection

Based on the USB specification, a USB device must use VBUS sensing detection. When the device detects the host presence, it connects its pull-up resistor to either a D+ or D- data signal. This allows the host to detect the device presence on the bus.

Pin PA9, a five volt-tolerant pin, which includes an additional function natively dedicated to VBUS sensing. The absolute maximum ratings table of the datasheet indicates that the five volt-tolerant pin voltage cannot exceed VDD + 4 V. The user must avoid the situation when the MCU is not powered, and a 5 V VBUS is connected to PA9: this violates the condition on absolute maximum ratings, and can result in permanent damage to the device. For this purpose, it is mandatory to reduce the voltage on PA9 below 4 V.

For a reliable and safe VBUS detection, the use of a voltage divider is recommended to guarantee absolute maximum ratings.

To minimize the DC current, the total resistance between VBUS and GND must be high. The basic GPIO input levels (VIL, VIH) can now be used with the resistor divider. As this method uses the regular GPIO functionality, the VBUS sensing function (OTG_GCCFG.VBDEN), embedded in PA9, should not be enabled.

The GPIOs available in STM32 families exhibit varying maximum operating conditions at their inputs. These conditions are typically restricted to a value that depends on certain VDDs. For instance, the expression min(VDD, VDDA, VDD3V3_USB, VBAT) +3.9 represents one such example. To detect VBUS through GPIOs, it is imperative to adhere to these tolerances even in the absence of VDDs. In the worst-case scenario (that is, absence of all VDDs), the aforementioned expression would yield a value of 3.9 V. This higher tolerance enables easier detection. However, if the guaranteed input tolerance is less than 3.6 V, a different approach is required.

Resistor values have been assessed assuming a ± 1% tolerance. They are checked against VIL/VIH across the different STM32 families, to guarantee switching when V_BUS is between 0.8 and 3.67 V.

It is also possible to use only three resistors with a solder bridge as shown in Figure 5 Resistor divider supporting both 1.8 and 3.3V ranges.

When the GPIO input tolerance is lower, a simple resistor divider is no longer electrically compatible. A simple and cheap way to detect VBUS presence in this case is to resort to an N-channel MOSFET, the recommended device is a 2N7002. Note that this scheme inverts the detection compared with a resistor divider (low level when VBUS is present). See Figure 6 VBUS detection when GPIO max below.

This section describes the hardware requirements for correct operation of the USB peripheral.

In peripheral mode, the V_BUS power is always provided through the cable. The USB FS impedance driver is always managed internally to avoid the need to add external serial resistors on the data line path.

To optimize the power consumption on self-powered platforms, only a USB PHY and a controller must be started on the VBUS detection.

It is recommended to implement a resistor bridge (refer to VBUS sensing detection for more details). It is also recommended to use an ESD protection device and to place it as close as possible to the USB connector.

To be compliant with the USB 2.0 full-speed electrical specification, the USB_DP (D+) pin must be pulled up with a 1.5 kΩ resistor to a voltage in the 3.0 to 3.6 V range.

In several STM32 MCUs, the pull-up resistor is already implemented. The user must add this resistor in other STM32 MCUs (refer to the 'Embedded pull-up resistor on USB_DP line' column in AN4879_USB-on-STM32-products.html#id_3644__table_irq_4n2_5fb).

Note:

Since OTG_HS PHY hardware internally behaves similarly for VBUS sensing, the same external resistor approach applies. The difference between FS and HS PHY is basically in data signaling speed, not in VBUS detection logic.

A DP pull-up must be connected only when VBUS is plugged. A GPIO from the MCU is used to drive it after the VBUS detection.

A bus-powered application is an application where the supply comes exclusively from VBUS. In order to keep the host alive and the VBUS available, both the PHY and the controller must always be active.

It is recommended to use an external low-dropout regulator (LDO) to lower the input supply of the MCU (LDO39050PU33R or an equivalent component can be used), and to place the ESD protection chip (if used) as close as possible to the USB connector.

This section describes the implementation for the USB FS downstream port, available on all the STM32 microcontrollers supporting host connection.

As required by the USB specification, if a VBUS overload occurs, it must be indicated to the user. The information regarding a VBUS overload is provided to the STM32 device via a switch with overcurrent protection (STMPS2151STR or equivalent)., as shown in the figure below.

The ESD protection chip, if used, has to be placed as close as possible to the USB connector.