AN1040 datasheet(2/4 Pages) STMICROELECTRONICS

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1 VBUS MONITORING

ST7 and ST9 microcontrollers with an embedded USB interface are equipped with an on-chip

3.3V voltage regulator that pulls up the appropriate data line with a 1.5 k

Ω ±5% resistor.

This regulator can be switched on or off by software via the PDWN (

Power Down) bit.

When the USB microcontroller is used in a self-powered application, it has to monitor the VBUS

signal to power off the 3.3 V voltage regulator if the USB cable is removed or if the VBUS is

switched off.

This can be implemented using an I/O pin configured as input CMOS.

Two monitoring schemes are possible:

1. Monitoring by polling the I/O pin in the main loop of the microcontroller program.

2. Monitoring by interrupt if the I/O pin connected to the VBUS signal has interrupt capability. If

VBUS is removed, an interrupt will be generated on the falling edge.

Figure 1. VBUS Configuration

When the VBUS signal is removed, the program must set the PDWN bit. This will power off the

3.3V on-chip voltage regulator that supplies the external pull-up resistor and the transceiver

and as a result, current will not be supplied on the data line attached to the VBUS signal.

Main Loop

Vbus

D+

D-

GND

1.5KOhm

USBVCC

USBDP

USBDM

USBGND

USB

Microcontroller

USBconnector B

I/Opin

I/OPin High

Set PDWNbit of

USBCTLRregister

to power off 3.3V

voltage regulator

Main loop process

YES

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