AN1040 STMICROELECTRONICS | Alldatasheet
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MONITORING THE V BUS SIGNAL FOR USB SELF-POWERED DEVICES by Microcontroller Division INTRODUCTION One of the USB Compliance Checklist for Peripherals items asks the following question: “Is the device’s pull-up active only when VBUS is high?”. This item refers to chapter 7.1.5 “Device Speed Identification” of the USB Specification. It is mentioned that “the voltage source on the pull-up resistor must be derived from or controlled by the power supplied on the USB cable such that when V BUS is removed, the pull-up resistor does not supply current on the data line to which it is attached” This applies only to self-powered devices where power does not come from VBUS .
1 VBUS MONITORING
3.3V voltage regulator that pulls up the appropriate data line with a 1.5 kΩ ±5% resistor. This regulator can be switched on or off by software via the PDWN (Power Dow n) bit. This can be implemented using an I/O pin configured as input CMOS.
- Monitoring by polling the I/O pin in the main loop of the microcontroller program.
- Monitoring by interrupt if the I/O pin connected to the V
VBUS is removed, an interrupt will be generated on the falling edge. Figure 1. VBUS Configuration
2 REFERENCE DOCUMENTS
- Universal Serial Bus Specification, Compaq Intel Microsoft NEC, Revision 1.1, September 23, 1998 2. USB Compliance Checklist, “Peripherals (excluding Hubs) for the Universal Serial Bus Specification (version 1.1)”, USB Implementors Forum, Version 1.06, August 16, 1999
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