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MSP-EXP430FR5994: An important note for users of the FR5994 Launchpad if you power it not using the FET

David Roberts
Intellectual 350 points

Part Number: MSP-EXP430FR5994

I'm building a circuit based on the FR5994 Launchpad, but wanted it to be capable of battery backup. My circuit contains:

- hardware related to my application

- hardware that seemlessly switches between 5V from the USB/FET and a backup battery

- a 3.3V low dropout voltage regulator which powers the FR5994 and my logic

I disconnected the J101 3.3V connector, but kept the 5V jumper in place to provide 5V power to my regulator when the USB is supplying power. This allows me to power the FR5994 but not the FET circuit (which contains its own 3.3V low dropout regulator)

Although the whole circuit worked perfectly when power originated with the USB, it worked only occasionally when the circuit was powered by a battery or external power supply. It took me a long time to figure it out, with many missteps along the way, like enhanced decoupling the 5V and 3.3V to the nth degree. I haven't noticed any documentation which helped me resolve the issue, but it might be plainly available... if so, my apologies.

The problem originates with the power-on circuit S3/R2/C2 on the schematic. and J101  jumper RSTn/SBWTDIO.

Because I did not disconnect RSTn/SBWTDIO, the RSTn signal from the FR5994 portion of the Launchpad was being shorted to ground (actually, not allowed to rise above 1.5V) by the pin clamping diodes in pin 43 of the FET microprocessor MSP101, which was unpowered. If this problem is not addressed elsewhere, perhaps the Launchpad documentation should be adjusted.

  • 0
    TI__Genius 14930 points
    David,

    Thank you for documenting what you encountered! This is great info for someone trying to modify the Launchpad for purposes outside the realm of it's recommended use-case, but since it is not a problem that would arise within the normal use-case of the Launchpad, it doesn't merit a change in our Launchpad documentation. The User's Guide does note to remove the jumpers to the debug circuit of the Launchpad to allow for DIYers to power the device externally and implement any project they want.

    Again thank you for documenting this on E2E so that anyone searching on the internet with this problem may come across this as their answer.

    Best regards,

    Matt Calvo
  • 0 in reply to Matt Calvo
    Intellectual 350 points
    Yes, I accept your conclusion. For my (I guess) unusual application, the issue is complicated by:
    1. needing the RSTn jumper in place when reprogramming the Launchpad
    2. needing the RSTn jumper removed for normal operation of my circuit.
    I expect that sooner or later, TI will produce a version of Launchpad (not just FR5994) that provides connectors for battery backup and perhaps my experience will help in the design. Since the two J101 RSTn pins protrude in the "up" direction, whereas the Launchpad connects to my circuit in the reverse "down" direction, I will have to have "flying" connectors that attach to the J101 RSTn pins, detect when USB 5V is present then connect the two pins together electrically, but disconnect them when USB power is absent.
    Thanks for your comments.

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