[SOLVED] Question about Target Power

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  • [SOLVED] Question about Target Power

    I'm just designing a board and was going to put a connection on the debug header for Target Power (which my JLink supports and I've tested).

    Is it ok to have Target Power connected in the case the board is self-powered or otherwise externally powered? I was intending on connecting Target Power direct to VCC (5V) on the board so I can power the entire thing through the debug connector when testing, my concern is that when in production, where the board will have an external 5V supply supplying VCC, if I then plug in the JLink I'm supplying 5V to pin 19 (Target Power) instead of sourcing it. Do the JLinks have internal protection against this?

  • Hi rols,

    this depends on the hardware revision of your J-Link.
    As long as you do not turn on target power, a recent J-Link (HW V10.1) tolerates up to 8V externally on pin 19.

    Beware of turning on target power with an externally connected power source!
    This could cause damage to not only the J-Link but also to the PC or USB hub the J-Link is connected to.

    I would recommend you to place a protection diode for pin 19 on your board or simply to use a special connector/cable with cut pin 19 in production.

    Best regards,
    Arne
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  • ok good thing I asked then - sometimes it is good to be paranoid. My HW JLink hardware revision is 9.3, so that's quite a bit older. I can guarantee the board power will never be more than 5V, so what I need to guard against is having target and board power on at the same time. I did put a 0 ohm resistor on the circuit with the idea of not installing it on a production board, and removing it after testing a test board so that pin 19 is just not connected to VCC. I think perhaps I'll change the design and switch that 0 ohm resistor for a diode to protect the JLink in the event I do have both power supplies connected, again with the idea of not even installing it at all on a production board. The drop from 5V across the diode will still be enough to power the board so that should work.

    Thanks for the reply.