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TM4C1294KCPDT: What is absolute MAX volts/amps allowed on GPIO PB1 5v tolerant input pin

Genatco
Guru 55913 points
Part Number: TM4C1294KCPDT
Other Parts Discussed in Thread: EK-TM4C1294XL

Trying to locate in any TM4C129x datasheet the absolute maximum voltage allowed on PB1. Should we consult a Stellaris datasheet for that answer or guess it ain't that much above 5v tolerant.

Several EK-TM4C1294XL have at times exhibited a POR while plugging the OTG port into 8 port USB hub powered from same PC as the ICDI USB port (JP1-ICDI). At the time POR occurred simply marked as freak incident and silently thought WTF but moved along anyway. The 8 port hub also has a very nice wall switcher, have only used after 12 AM when the computer goes to sleep for the night.

 

  • 0
    Guru 117855 points

    Might the fact that your, "8 port hub - powered from that  (very nice) wall switcher" - have someway/how - caused/created an "imperfect common Ground?"

    Should the grounds not be common - it (may) be possible that the USB cable inputs voltage (outside) the MCU's 5V specification.      Quick/dirty measurement should confirm...     This diagnosis proves (especially) likely should your,  "New, custom pcb"  be powered from OTHER than the USB cable - alone!      

    To cause such unwanted  Reset - as you report - suggests that your MCU's custom board is,  "Powered and Up/Running"  (via a separate power source)  DURING  your insertion of the  "Hub Sourced" USB cable.     Normally - "POR" IS expected (and necessary) - when power is uniquely and FIRST introduced!

  • 0
    TI__Guru 72500 points
    Hi BP101,
    The maximum voltage is 5.5V. It is not clear from the datasheet, so I will provide that feedback internally. The TM4C129 is on the same process node as the TM4C123 and here is a good paper on the 5V tolerant pins of the TM4C123: www.ti.com/.../spma053.pdf
  • 0 in reply to cb1_mobile
    Guru 55913 points

    cb1_mobile said:
    Might the fact that your, "8 port hub - powered from that  (very nice) wall switcher" - have someway/how - caused/created an "imperfect common Ground?"

    As stated the hub power is sourced via VBUS from Host computer.  Wall switcher is not plugged in but only after the late night attendant is sawing zzzz and host computer power is turned off.

    cb1_mobile said:
    To cause such unwanted  Reset - as you report - suggests that your MCU's custom board is,  "Powered and Up/Running"  (via a separate power source)  DURING  your insertion of the  "Hub Sourced" USB cable.     Normally - "POR" IS expected (and necessary) - when power is uniquely and FIRST introduced!

    That later part of POR is not part of the USB2.0 specification and definitely not ever supposed to occur. VBUS is a signal (PB1) more than it is ever being used to power the MCU.

    4.3.3 USB OTG:
    TM4C129x devices that support USB OTG mode include the signals for USB Device mode, signals for
    USB Host mode and an additional signal USB0ID located on pin PB0. This USB ID signal is the 5th 4th pin
    found on a USB micro-AB connector. If a micro-A cable end is plugged into this connector, the ID pin on
    the cable is tied to ground causing the TM4C129x device to operate as a USB host. If a micro-B cable end
    is plugged into the USB connector, the ID pin is left floating. In this case, the TM4C129x device's internal
    pull-up on the USB0ID signal causes the controller to operate in device mode.

    In order to limit damage from ESD events, a 100Ω resistor should be placed in series between the ID pin
    on the USB connector and USB0ID(PB0) on the microcontroller.

    To support full USB OTG negotiation using the SRP and HNP protocols, VBUS from the USB connector
    must be directly connected to USB0VBUS(PB1) of the microcontroller without a series resistor in between.
    In this case, USB0VBUS should be connected to an ESD suppressor such as a TVS diode, or ESD
    resistant VBUS switch.

  • 0 in reply to Bob Crosby
    Guru 55913 points
    Hi Bob,

    Notice too in order for TM4C123 be 5v tolerant PVT engineers omitted fail safe ESD upper rail diode on all GPIO ports.

    That said a similar stress issue was reported this forum with GPIO port configured PWM T5/CCP0 & EdgeCounter T5/CCP1 for regulating 24v fan speed. That circuit required external Schottky diode to halt current flow PM6 in/out to PM7 (unpowered MCU). GPIO (PM4,5,6,7) are specified non HBM compliant ESD pins to other GPIO pins. That external diode stops current out of GPIO pin if 24v is first powered prior to powering MCU, depowered in reverse order. After later reading HBM issue have moved CCP0/1 to PL4/5 and still have Schottky diode installed yet 24v is now controlled via GPIO enable pin. Point is the GPIO pins are no longer being destroyed from excessive current flow when the MCU is powered or unpowered.