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# 1   2008-08-20 06:56:09 USB pull-up resistor

asadler
New member
Registered: 2008-08-18
Posts: 1

USB pull-up resistor

Is there any specific reason the USB pull-up resistor is pulled up through a transistor?
I can't see why it could not be pulled up directly by a port pin.

Upon RESET, the port would be high impedance and the resistor thus disconnected until the port pin is driven high and made an output.

Does anyone know why a transistor was used?

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# 2   2008-10-15 14:05:51 USB pull-up resistor

iotize
Administrator
Registered: 2007-07-09
Posts: 30

Re: USB pull-up resistor

Hi,

The transistor allows to re-enumarate the USB: it offers the possibility to disconnect the product to the USB and to reconnect it with a new USB description.

This schematic have been found on a ST evaluation board.

regards,

Benoit

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