USB power supply filtering

Discussion in 'Embedded Systems and Microcontrollers' started by picstudent, Feb 25, 2009.

  1. picstudent

    Thread Starter Active Member

    Feb 3, 2009
    76
    0
    My dataaqusition system using USB interface to computer. I am using 18F4550to do that.

    My device has separate power ,I need a LED to glow in hardware and the software needs to detect System when plugged.
    My hardware needs to be clean from the SMPS noise or ripple or whatever from computer power supply.

    What is the best and effective filer mechanism to use on USB connectivity?

    Thanks
     
  2. ahwaheed

    New Member

    Feb 25, 2009
    4
    0
    did you need the LED to glow when you plugin your device into the computer Or glows when pulggin into the power supply ???
     
  3. ahwaheed

    New Member

    Feb 25, 2009
    4
    0
    if you need the LED glows when putting the device to the USB ,you simply connect a limitted resistor to the LED and connect it to the VCC,GND of the USB connector in the device board.
     
  4. picstudent

    Thread Starter Active Member

    Feb 3, 2009
    76
    0
    Do we need to connect the ground of USB connector to my board ground. Also I need to connect the Vcc of the USB connector to MCU So that firmware can detect host plugged.
    In this case Gnd needs to be conected.
    Any filetr mechanism here?

    regards
     
  5. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    You need to connect the ground together, there's no avoiding that. Otherwise there would be no common reference point for voltage on both systems.

    If you have separate power supply, there's no need to connect the Vcc. USB detection does not depend on the Vcc sensing, but on pulling D+ and D- lines to a certain voltage.

    Just make sure that the PIC is separated from the data acquisition system (ground and Vcc), to avoid interference.
     
  6. picstudent

    Thread Starter Active Member

    Feb 3, 2009
    76
    0
    My system connects to PC only at the time of data uploading. If a pin is used to detect the USB connector Vcc, the USB engine needs to be started only at the time of plugin to start enumeration.
    Microchip's FUSB board has such a mechanism. They use two inductors to couple and Vcc lines to board. But I cant find out the value of inductor.

    Thanks again
     
  7. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    Right, so you are actually sensing the Vc at the micro side. There's no problem with that, and you don't need to worry about any filtering at all. Just use a resistor to an input pin.
     
  8. picstudent

    Thread Starter Active Member

    Feb 3, 2009
    76
    0
    Thanks for the help.
     
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