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TM4C1294NCPDT: USB0VBUS pin: two questions

michael nudelman
Intellectual 620 points
Part Number: TM4C1294NCPDT

Hi,

I had to deal with one problem and have just been informed on the other.

1. Upon power-up (autonomos, not via VBUS) in maybe two out of many devices (USB0VBUS pin is configured as USB, not PB1 - I did not write the code) the pin were consistently driven low, loading VBUS (successfully - 5V was going down to under 4V). After unplugging / plugging back the USB cable the USB would start functioning. Eventually the pin was damaged. I remedied this by inserting 1K resistor in series with the pin, and in the spun version also put a 3.6V Zener after it to counter ESD and 5V in case it does something to the pin. So far seems to work.

2. Today we have been made aware that when the device is powered down but the USB Master (our device is used as a slave) is still plugged in, the CPU stays powered. I have not found anything mentioning the CPU could be powered via the USB0VBUS pin (the only connection to any power at that point.)
What gives?

Best, Mike

  • 0
    TI__Guru 72500 points

    All of the I/O pins have some form of ESD protection that eventually routes voltages higher than VDD to the VDD supply. In this case you are powering the device through the 1K resistor and the ESD circuit.  Looking at page 1852 of the spec, you need to limit the current to 2mA. 5v/2mA = 2.5K Ohm.

  • 0 in reply to Bob Crosby
    Intellectual 620 points
    Bob

    thanks for replying.

    Isn't USB0VBUS the 5V pin (the rest is indeed 3.3V ones). Also 1K resistor is, as I said, feeds into a 3.6V zener before going to the USB0VBUS pin, which should limit it both current and voltage-wise. And, I would need 2.5K only if I had a short on teh other end; if powering the CPU I obviously will have some voltage developing on it?
  • 0 in reply to michael nudelman
    TI__Guru 72500 points
    The zener protects against over voltage, but does not keep the device from being powered through the PB1 pin. You can calculate the current through PB1 if you measure the voltage drop across the 1K resistor (assuming the voltage at PB1 is less than 3.6V). Are there any other pins that might still be powering the device?
  • 0 in reply to Bob Crosby
    Intellectual 620 points

    No, no other pins.

    Also we could not duplicate that effect today, usng the board with no protection whatsoever and a computer USB port.

    So at the minimum, it is not consistent.