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LAUNCHXL-F28379D: General board issue

Thuan Duong
Intellectual 455 points
Part Number: LAUNCHXL-F28379D

Hello TI Officers,

I am using the LAUNCHXL-F28379D, and I have some questions: 

1. I had an experiment where I connect 2 wires: GND and a voltage 3V to the board. But the strange thing is that, when I didn't even power the board, I connected my GND wire to the GND pin, and the 3V pin to any header pin except 3V3 or 5V (either GPIO pins or ADC/DAC pins), I saw the LEDs D9 lights up. Actually, I measured the TP13 and all devices connected to 3V3 line and 5V line are all having 1.8V. Why is that ? I can't believe that by supplying any GPIOs with a voltage the board would have voltage on the power supply line itself. That's not true isn't it ?  

2. For this board, do I need to add any capacitance to the ADC pins to help the ADC process ?

Regards,
An. 

  • 0
    Guru 30330 points
    Thuan,

    I am not quite sure I understand how you have connected the board . Is the LaunchPad not powered, then you connect GND and a 3 V signal to the unpowered MCU? It is not recommended to apply a voltage to a pin while the device is not powered. You can damage the device by doing so, and the behavior will be undefined. The pins are all connected internally to various supplies and it is possible that you are turning on portions of the device in potentially damaging ways. Do not do this.

    2. It depends what you are trying to accomplish. Generally sizing the ADC input circuitry properly for your desired ADC operation and performance is recommended.

    -Mark
  • 0 in reply to Mark Labbato
    Intellectual 455 points
    Hi Mark,

    1. Yes I didn't power the board, and connected GND and 3V signal to a male pin header. It was a strange phenomenon to me that by giving voltage to a pin of MCU the power line also has voltage. Anyways, I wouldn't do it again.

    2. I am trying to read analog signal from a distance sensor. It gives a value from 0 - 3V. I tried connecting directly to the Launchpad and it was reading well. But I want to know if there is any need on adding capacitance on the ADC inputs to suit the timing requirements ?

    Regards,
    An.
  • 0 in reply to Thuan Duong
    TI__Guru 60865 points
    Hi Thuan,

    For (2) What is the output impedance of your sensor? If it has an internal buffer, what is the bandwidth? How often do you need to sample the output?
  • 0 in reply to Devin Cottier
    Intellectual 455 points

    Hello Devin,

    My sensor has an output impedance of 100 Ohm, and I need to sample it every 50us, no buffer in it. 

    Regards,
    An.