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Re: Cortex-Mx MCUs with SWD access locked

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On 27/06/2022 08:54, pozz wrote:
> Il 24/06/2022 18:26, David Brown ha scritto:
>> On 24/06/2022 16:42, dalai lamah wrote:
>>> Un bel giorno pozz digitò:
>>>
>>>>> Or maybe some ground loops, for example if you have connected the 
>>>>> J-Link
>>>>> probe to a desktop PC (or in general a PC not isolated from mains) 
>>>>> and your
>>>>> boards are somewhat connected to mains or earth ground too.
>>>>
>>>> Yes, I use a desktop PC connected to mains and the boards are supplied
>>>> from an AC/DC connected to mains too.
>>>>
>>>> Is this a problematic scenario?
>>>
>>> Normally it shouldn't be, but it also depends on what the board does. 
>>> If it
>>> contains some power stages running at high frequency, it is quite common
>>> (especially if the EMI filtering is subpar) that switching currents can
>>> pass through the AC/DC power supply and create problems.
>>>
>>> If this keeps happening, try using a laptop isolated from mains and 
>>> see if
>>> it helps.
>>>
>>
>> Another trick is to make sure you have a separate ground connection 
>> between the board and the PC before attaching the JTAG.  A wire from 
>> the board ground to the PC's case is usually fine.
>>
> 
> Sorry I don't follow very well these arguments.
> 
> You say to have a separate ground connection between the board and the 
> PC, but you suggest to connect a wire between them. It seems 
> contradictory to me.

No, the wire is the connection.

The point is to stop any brief surges passing through the debugger and 
USB system.  When boards get connected or unconnected to supplies and 
other equipment, there can sometimes be slight differences in the ground 
potential.  This can lead to a current pulse, which is sometimes large 
due to capacitances, and this in turn can lead to voltage pulses.  If 
there is a nice, clean path for the current to follow along a direct 
connecting wire, that's where the current will go, and the debugger and 
USB is spared the pulses.

> 
> What do you mean with board ground? GND, the reference voltage for 3.3V 
> of the MCU?

Yes, generally.

> Usually I work on boards with only one low-voltage power 
> supply input (12Vdc, plus and minus) that is down-scaled to 3.3V by a 
> DC/DC switching regulator. There isn't an *earth* connection on the 
> board. Mounting holes are connected to GND, but most of the time, the 
> board is not mounted on my bench.
> 

These mounting holes are often a good point to connect such a ground 
wire - just as the mounting screws on the case of the PC are typically a 
good point on the PC side.

Ideally, the power supply enters a board at one point, and you have a 
star connection from there to any zero volt references (so that noise 
from ground for high power switching does not disturb the 
microcontroller, or high frequency noise on the microcontroller ground 
does not disturb sensitive analogue parts, etc.).  And ideally, this 
ground connection to the PC also comes from that star.  In practice, for 
simpler boards a single ground reference plane is usually all you need, 
and all you need here is for your ground connection wire to have a good 
low impedance connection.  A croc clip wire clipped to the mounting 
holes or screws is fine.

(I am referring to local ground, or zero volt reference - not "true 
ground" or "earth".  You only need an earth connection if you have high 
voltages and safety concerns.)

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Thread

Cortex-Mx MCUs with SWD access locked pozz <pozzugno@gmail.com> - 2022-06-23 23:38 +0200
  Re: Cortex-Mx MCUs with SWD access locked pozz <pozzugno@gmail.com> - 2022-06-24 11:25 +0200
    Re: Cortex-Mx MCUs with SWD access locked dalai lamah <antonio12358@hotmail.com> - 2022-06-24 12:22 +0200
      Re: Cortex-Mx MCUs with SWD access locked pozz <pozzugno@gmail.com> - 2022-06-24 16:27 +0200
        Re: Cortex-Mx MCUs with SWD access locked dalai lamah <antonio12358@hotmail.com> - 2022-06-24 16:42 +0200
          Re: Cortex-Mx MCUs with SWD access locked pozz <pozzugno@gmail.com> - 2022-06-24 16:56 +0200
            Re: Cortex-Mx MCUs with SWD access locked dalai lamah <antonio12358@hotmail.com> - 2022-06-24 17:16 +0200
          Re: Cortex-Mx MCUs with SWD access locked David Brown <david.brown@hesbynett.no> - 2022-06-24 18:26 +0200
            Re: Cortex-Mx MCUs with SWD access locked Dimiter_Popoff <dp@tgi-sci.com> - 2022-06-24 20:00 +0300
            Re: Cortex-Mx MCUs with SWD access locked pozz <pozzugno@gmail.com> - 2022-06-27 08:54 +0200
              Re: Cortex-Mx MCUs with SWD access locked David Brown <david.brown@hesbynett.no> - 2022-06-27 09:23 +0200
    Re: Cortex-Mx MCUs with SWD access locked David Brown <david.brown@hesbynett.no> - 2022-06-24 14:15 +0200
      Re: Cortex-Mx MCUs with SWD access locked pozz <pozzugno@gmail.com> - 2022-06-24 16:25 +0200
        Re: Cortex-Mx MCUs with SWD access locked Stef <me@this.is.invalid> - 2022-06-24 20:29 +0200
          Re: Cortex-Mx MCUs with SWD access locked David Brown <david.brown@hesbynett.no> - 2022-06-24 20:34 +0200
            Re: Cortex-Mx MCUs with SWD access locked Stef <me@this.is.invalid> - 2022-06-24 22:12 +0200
              Re: Cortex-Mx MCUs with SWD access locked David Brown <david.brown@hesbynett.no> - 2022-06-25 10:19 +0200
          Re: Cortex-Mx MCUs with SWD access locked pozz <pozzugno@gmail.com> - 2022-06-27 09:06 +0200
            Re: Cortex-Mx MCUs with SWD access locked David Brown <david.brown@hesbynett.no> - 2022-06-27 09:25 +0200
        Re: Cortex-Mx MCUs with SWD access locked Reinhardt Behm <rbehm@hushmail.com> - 2022-06-26 14:25 +0000
          Re: Cortex-Mx MCUs with SWD access locked pozz <pozzugno@gmail.com> - 2022-06-27 10:07 +0200
            Re: Cortex-Mx MCUs with SWD access locked Reinhardt Behm <rbehm@hushmail.com> - 2022-06-27 15:11 +0000
              Re: Cortex-Mx MCUs with SWD access locked pozz <pozzugno@gmail.com> - 2022-06-27 23:50 +0200
                Re: Cortex-Mx MCUs with SWD access locked Stef <me@this.is.invalid> - 2022-06-28 11:23 +0200
                Re: Cortex-Mx MCUs with SWD access locked pozz <pozzugno@gmail.com> - 2022-06-28 12:42 +0200
                Re: Cortex-Mx MCUs with SWD access locked Stef <me@this.is.invalid> - 2022-06-28 17:06 +0200

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