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MSP430FR5994: Use Backchannel UART with external power

Rosh Mendis
Intellectual 730 points
Part Number: MSP430FR5994

Hi,

I want better debugging capabilities than using LED/GPIOs, when using external power.

Is there a way to use the backchannel UART when powering the MSP430FR5994 launchpad board with external power (e.g. from a power supply) ?

I also have some 3V/5V booster packs which will be mounted on the LP.

thanks

  • 0
    TI__Mastermind 32405 points
    Hi Rosh

    What's the "Backchannel UART" mean?No matter which UART channel the power source will not affect the device's function I think.

    Best regards
    Gary
  • 0
    Guru 35570 points

    As long as you have an external serial to USB converter it should be no problem.

  • 0 in reply to Keith Barkley
    Intellectual 730 points

    Hi, thanks for your replies.

    Is there a Jumper setting where I can use the UART + external power (using the 3v3 GND pins at the bottom of the board connected to a power supply / battery) on the Launchpad board ? I'm referring specifically to the USB UART, not a separate Serial-to-USB connection. thanks

  • 0 in reply to Rosh Mendis
    Guru 88460 points
    I think you can just remove the 3V3 (and 5V) jumpers on the bridge header. The USB will power the FET side and your power will feed the rest.

    Many/most Launchpads run at 3.6V (notwithstanding the silkscreen), so your RXD may be driving at 3.6V. The TXD level (at "true" 3v3) should be within bounds.
  • 0 in reply to Bruce McKenney47378
    Guru 88460 points
    I just tried this, powering an FR2311 Launchpad (minus the 3v3/5v bridge jumpers) from an FR6989 Launchpad. They were both 3.3V, so maybe the 3.6V thing was my imagination.

    No smoke. The UART worked fine, and I could even program it.
  • 0 in reply to Rosh Mendis
    Expert 1335 points
    As Bruce said, this would work fine.

    Only remember about the backfeeding your target MCU. In case your external supply is turned off, your target Vdd will be still be around ~ 2.7V, because of the 3.3V supplied through Tx (target Rx). This is plenty for MSP430 to keep it running.
  • 0 in reply to Bruce McKenney47378
    Intellectual 730 points
    Thanks, I tried this and it works, I can get UART and power externally with both 3v3 & 5v removed. of course the TX,RX jumpers should be connected.
    One thing I did notice was that you can't program the board without GND jumper being connected. CCS kept saying load 'failed'.
  • 0 in reply to Andrejs Gasilovs
    Intellectual 730 points
    Hi Andrejs, what do you mean by 'backfeeding the target MCU' ? the external supply produces a step-wise voltage waveform (oscillating) between 0.5V and 3.3V, I can see the MCU loosing power (debug LED P1.0 goes off) when below 1.8V and resets back when the voltage is higher than 1.8V. It seems like this continues to happen even when the UART (usb) is connected to the PC<->LP. if the MCU is getting power from the TX/RX pins, then shouldn't it continue to flash the debug LED and not reset <0.5v ? How can I properly test if the MCU gets power from the TX/RX after external supply is switched off ?
  • 0 in reply to Rosh Mendis
    Expert 1335 points

    Well, it is somewhat complicated topic. If you are new to MCU backfeeding thorough the protection diodes, then i recommend you to watch Dave Jones video on this topic.

    You mentioned that you have a boosterpack and stuff alike, so in this case your total current consumption might be too high. Remember that Tx pin that is sourcing 3.3V is similar to GPIO pin and it has limited current sourcing capabilities (as shown in appropriate graphs in the datasheet). As the current that is sources increases, the voltage decreases. So at some point it will fall to a level where it won't be able to power your target. But if your system total current consumption is not too high (in my tests < 20mA), then you will be able to power your circuitry with just the current coming from Tx pin. See image below where both launchpad LEDs are ON

  • 0 in reply to Andrejs Gasilovs
    Guru 88460 points

    Another option is Don't Do That -- keep the power running on both sides. Back-feeding through the UART is not good for your chip, but the damage is gradual.

    Is this a short-term hack or a circuit design? My EE has some magic that prevents it (not particularly complex I think), but I forget exactly how it works.

    [Edit: That was supposed to be directed at Rosh, but I guess I hit the wrong button.]

  • 0 in reply to Bruce McKenney47378
    Intellectual 730 points

    Thanks, both. I think what Andrejs mentioned about power backfeeding was important if I wanted to test DUT with exclusively external power, so for early-testing my design I can use both usb/uart and external power like Bruce mentioned (keep both running), and once design is verified, for isolated secondary testing, I can just remove the USB and test.

  • 0 in reply to Andrejs Gasilovs
    Intellectual 730 points
    Hi Andrejs,
    sorry to comment on this old thread, but I have a very related follow up question -
    I only want to transmit date out of the board through UART, so if I only connect the board's TX pin (i.e. 5th jumper), will I still get the backfeeding ?
    thanks
  • 0 in reply to Rosh Mendis
    Expert 1335 points
    That's a good question.

    UART is push-pull and transmitter drives the line.

    But oftentimes (and it's a good practice) the Rx line is tied to Vcc either with external or internal pull-up to prevent spurious interrupts and glitches on the line when nothing is connected. So it is possible to bring power (backfeed) even through Rx line (target Tx).

    However, in case of the MSP‑EXP430FR5994, I've checked the schematic and there's no external pull-ups, I've also measured the debuggers Rx line and it shows 0V which means no internal pull-ups. So in your case you'll be fine and there will be no backfeeding.

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