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FTDI Cable powers chip through UART

Discussion in 'Hobby Electronics' started by James, Apr 20, 2010.

  1. James

    James Guest

    Hi all

    I though a few here might find this interesting.

    I'm currently working on a project using an Atmel ATMEGA324P/V (picopower
    variant), the board has no RS-232, only RS-485 (MAX487). I included a header
    for TTL serial interface. Was using this header to run some diagnostics on
    my firmware via a USB - TTL (5v) serial cable (Don you know the ones). I was
    more than a little surprised when the program actually started running and
    returning values with the power supply completely disconnected.

    Only GND, RX & TX connected on the cable

    So through the hardware UART on the AVR I was having it powered and run at
    the correct speed (seemingly) using external crystal osc. to deliver a
    perfect data stream at 19200 baud.

    Measured Vcc @ 1.7v (BOD was not set)

    It took me a while to realise what was happening since it was initially
    after I'd had the thing running and disconnected power and thought there was
    some magical smoke & mirrors buffer in Windows doing some crazy stuff.

    Anyone come across this before?

    Regards
    James













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  2. JohnnyInBris

    JohnnyInBris Guest

    The TX line from the USB to TTL converter is powering the micro through
    the input diode to VCC on the micro's RXD input. Not uncommon to see
    this as the micro's supply current is quite low.
     
  3. Don McKenzie

    Don McKenzie Guest

    Seen a similar thing where a micro powered an LCD through the parallel
    data signals, without any VCC connected.

    And I am sure I have seen another similar occurrence, but my memory
    fades. I'll give myself a bit of think music, and I may come up with it.

    I'll bet there are others readers will be aware of.

    Cheers Don...



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  4. Jasen Betts

    Jasen Betts Guest

    It's fairly widely known that CMOS logic can be powered through the
    protection diodes on it's inputs if the outputs aren't heavily loaded.



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  5. I can second this.

    Originally, the idiot who designed it included circuitry to turn the
    LCD display off to save power (this was power sensitive application)
    but, forgot to buffer the data lines, and drove the LCD directly from
    the CPU bus.

    Having realised this was a lost cause, the LCD power down functionality
    was removed (the boss wasn't going to buy yet another hardware change).

    Enter poor bugger (myself) looking at a very intermittent problem,
    where everything worked as it should, but somtimes, the LCD display
    would dim a bit. Still worked, but dim.
    Not being privvy to that part of the design phase, I never knew the
    firmware was actually doing it.

    Finally put two and two together, I realised that the "unused" LCD
    power control circuitry was actually being actuated after all.
    Turns out, the internal CMOS protection diodes on the inputs were
    enough to power up the LCD display, albeit at less than 5v (around 4.3v
    if I recall). That explained that.

    I told the new guy working on the firmware about it, and it was
    promptly addressed. Turned out the LCD power control routines were
    distributed all over the firmware, and for whatever reason, there was no
    "global" control of this. Each little section made it's own mind about
    whether or not to power up the LCD. Which explained why the original
    idiot didn't do it properly the first time around.



    Moral of the story I suppose is never get an idiot to fix their own
    screwups. If they can screw it up that well to start with, they can
    screw the correction up as well.
     
  6. Heard of a similar case where a 40 pin CPU was not soldered in at all.
    There was enough flex in the legs to keep enough conduction happening
    with the plated through-holes.

    Was in service for some years before it came in for intermittent
    faults. After soldering the legs, all was good.
     
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