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ARM DS-5 ARM DSTREAM User Guide : About hot-plugging and unplugging the debug cable

About hot-plugging and unplugging the debug cable

There are times when you might want to connect or unplug a debug cable without powering off the target. You can do this provided certain conditions exist.

You can connect and disconnect the DSTREAM unit to the target hardware without affecting operation of the target if:

  • The DSTREAM unit is powered by its original, earthed-output power supply.
  • The target hardware is earthed either through its PSU or separate earth connection.


  • If the DSTREAM unit or the target hardware are not properly earthed, potential differences can occur between them, causing unwanted pulses in the debug interface during connection. This can cause unwanted resets to occur.
  • If using targets with a power supply voltage greater than 5V, ARM® recommends observing one or both of the following precautions.

    1. Disconnect the target from the DSTREAM unit before powering.
    2. Ensure that the target is plugged into its power brick before plugging the power brick into the wall socket. This step ensures that when the target powers, it is properly grounded through its own power supply.

You might want to connect the debug cable if you have a target that is operating without a DSTREAM unit connected and you want to examine the target to find out why it is behaving in a particular way. To do this, you must power-up the DSTREAM unit and configure the connection without affecting the state of the target. This requires that the DSTREAM unit is powered before it is connected to the target.

When unplugging the debug connector, you must be aware that:

  • If you are using an RTCK system, make sure that no communication is taking place between the system and the DSTREAM unit. Otherwise, if the DSTREAM unit is waiting for a return clock, it might lock up.
  • If you are not using an RTCK system, the debug software can handle this situation. However, you must arrange to do a Test Access Port (TAP) reset using the debugger when you next plug the DSTREAM unit into a target. This can be performed during the advanced configuration part of the process.
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