Intel PXA250 and PXA210 manual USB Interface, 1. Self Powered Device

Models: PXA250 and PXA210

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USB Interface

USB Interface

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4.1Self Powered Device

Figure 4-1shows the USB interface connection for a self-powered device. The 0 ohm resistors are optional, and if not used, then connect USB UDC+ directly to the device UDC+ and connect USB UDC- directly to device UDC-. The device UDC+ and UDC- pins match the impedance of a USB cable, 90 ohms, without the use of external series resistors. You may install 0 ohm resistors on your board to compensate for minor differences between the USB cable and your board trace impedance.

The 5 to 3.3 voltage divider is required since the device GPIO pins cannot exceed 3.3 V. This voltage divider can be implemented in a number of ways. The most robust and expensive solution is to use a MAX6348 Power-On-Reset device. This solution produces a very clean signal edge and minimizes signal bounce. The more inexpensive solution is to use a 3.3 V line buffer with 5 V tolerant inputs. This solution does not reduce signal bounce, so software must compensate by reading the GPIO signal after it stabilizes. A third solution is to implement a signal bounce minimization circuit that is 5 V tolerant, but produces a 3.3 V signal to the GPIO pin.

Note: If GPIOn and GPIOx are the same pin, never put the device to sleep while the USB cable is connected to the device. During sleep, the USB controller is in reset and will not respond to the host; after sleep, the device will not respond to its host-assigned address.

Figure 4-1. Self Powered Device

USB 5V

USB UDC+

USB UDC-

USB GND

5V to 3.3V

470K

GPIOn

GPIOx

1.5K

UDC+

0ohm

(optional)

UDC-

0ohm

(optional)

Board GND

4.1.1Operation if GPIOn and GPIOx are Different Pins

Any GPIO pins can be defined as GPIOn and GPIOx. GPIOn should be a GPIO which can bring the device out of sleep. Out of reset, configure GPIOx as an input that causes the UDC+ line to float. GPIOn is configured as an input that causes an interrupt whenever a rising or falling edge is

PXA250 and PXA210 Applications Processors Design Guide

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Intel PXA250 and PXA210 manual USB Interface, Self Powered Device, Operation if GPIOn and GPIOx are Different Pins