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TM4C129ENCPDT: The behavior of the pins(PA2-PA5) has changed for some reason

g.f.
Guru 15520 points
Part Number: TM4C129ENCPDT


Hi,

My customer are using TM4C129ENCPDTI3(latest revision 3) in their product
and having some issue.

On their customer board, LED are connected to TM4C129ENCPDTI3 and
the LED are controlled via PA2-PA5 pin of this MCU.

The issue is that LED light get ON after Power on Reset get released
even the user progam isn't flashed to the internal flash memory yet.
They manufactured this unit over 8,000 so far without this issue, but
it has been confirmed that it occurs in 5 units with recently manufactured products.
But after flashing the user program, the MCU will operate appropriately.

We checked the errata sheet of TM4C129ENCPDTI3, but we couldn't find any errata that match this issue.

They have following question:
Q1.Has the TM4C129ENCPDTI3 specification changed recently which are not updated in datasheet?

Q2.Before flashing the program to TM4C129ENCPDTI3 (factory default),
I think PA2-PA5 pin is in the INPUT state. Is this wrong?

Q3.Is it possible to change the status of PA2-PA5 pins externally by setting the pins of TM4C129ENCPDTI3?

Could you please answer to these questions?

best regards,
g.f.

  • 0
    TI__Guru 72500 points

    g.f. said:
    Q1.Has the TM4C129ENCPDTI3 specification changed recently which are not updated in datasheet?

    No

    g.f. said:
    Q2.Before flashing the program to TM4C129ENCPDTI3 (factory default), I think PA2-PA5 pin is in the INPUT state. Is this wrong?

    That is correct.

    g.f. said:
    Q3.Is it possible to change the status of PA2-PA5 pins externally by setting the pins of TM4C129ENCPDTI3?

    If voltages are applied to the GPIO pins before the device power is supplied, the device may be improperly back-powered through the IO pins which may cause improper operation. 

    Are the LEDs driven directly from the GPIO pins, or is there a driver chip or transistor? If driven directly, is the GPIO pin attached to the anode or cathode of the LED (is the GPIO sourcing or sinking the current)?

  • 0 in reply to Bob Crosby
    Guru 15520 points

    Hi Bob,

    Thank you for the reply.

    >If driven directly, is the GPIO pin attached to the anode or
    >cathode of the LED (is the GPIO sourcing or sinking the current)?

    The LED are connected to PA2-PA5 pins directly and the pins are connected to cathode of the LED.

    best regards,
    g.f.

  • 0 in reply to g.f.
    TI__Guru 72500 points

    I assume you have a current limiting resistor attached to the anode and the resistor is attached to a voltage source. If that voltage source turns on before the 3.3V to the TM4C, it might trigger the ESD protection SCR. However, this would happen at power-up. It would not wait for power-on reset to be released. Also, it would occur independent of the code in the device. However, if the code in the device turns on the LED, the current will be diverted from the SCR to the output buffer's N-channel transistor. This would allow the SCR to turn off. The circuit would then operate properly.

    I have no explanation of the behavior the way you have described it.