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cleonb322
Associate III
April 22, 2017
Question

Do I need a 1.5K pullup on USB D+ line?

  • April 22, 2017
  • 2 replies
  • 1228 views
Posted on April 22, 2017 at 08:12

Do i need a pullup 1.5K resistor on D+ for USB Full Speed when the USB is configured as a Device (peripheral)? I recently build a custom circuit board without the 1.5K pullup and the PC didn't detect it?  unfortunately I damaged the circuit board so i cannot retest it with the 1.5K resistor.

My connections:

D-  = PA11 (with 22 ohm resistor in series),

D+ = PA12 (with 22 ohm resistor in series), 

V_USB is not configured.

I use the same code on my STM32F407 discovery board and it  works fine therefor it cannot be the code. I am using the HAL lib with STMCube).

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    2 replies

    S.Ma
    S.Ma
    Principal
    April 22, 2017
    Posted on April 22, 2017 at 13:20

    Don't have the anser as there are few generations of USB peripheral, and newer ones avoid the need to put external resistor.

    Unless someone has the answer right away, dig by finding on-line the discovery board schematic for the same STM32 Family, open the schematic and check out the USB FS lines.

    re.wolff9
    re.wolff9
    Senior
    April 23, 2017
    Posted on April 23, 2017 at 21:14

    You custom board also has a '407? Or a '405? 

    No, you don't need a 1.5k pullup. The discovery board hooks up VBUS to PA9. If you don't do that, it can be a bit tricky to figure out how to disable the check for vbus. The symptoms are exactly as you describe: PC doesnot see any device. 

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