Chinese J-Link VTref/3.3V weirdness

[mondalaci]

Hi guys,

I purchased a J-Link V9 clone, and to my surprise, I measure 3.3V between GND and VTref (pin 1) regardless of whether I run the `power on` vs `power off` command in J-Link Commander. With an original Segger J-Link, I always measure zero volts between VTref and GND, as should be the case.

I found a jumper on the PCB of the clone, but it doesn't affect this behavior. I'm wondering what its purpose is.

Do any of you know what's going on with the VTref of the clone?

[abyrvalg]

VTref shouldn’t be affected by power on/off cmd at all, it’s an input supplying those two LVC buffers. Perhaps the 3v3 you are observing is just some weak leakage from MCU’s outputs connected to the other side of LVCs. And the power out is pin 2.

Edit: the sw-controlled power output is pin 19 indeed (and that can be seen clearly on the photo - a thick track connects it to a transistor), while pin 2 is a fixed output selectable by jumper.

[GromBeestje]

There are various chinese clones out there. They usually output 3.3 volts on the VTref as a "feature" to power the circuit from the programmer.
Usually they come with a jumper places, and removing the jumper should disable this. However, I have seen some versions with a leakage voltage on the VTref pin when the jumper was removed. With the jumper removed, try putting some pull down resistor at VTref and see if the voltage goes away.

[barshatriplee]

Have you got any schematic? I think it will help more if you share that (in case you got it from the seller).

[mondalaci]

I don't have a schematic to share, but I've pulled down VTref (pin 1) by connecting it to GND (pin 4) via a 5.1K resistor. The jumper was in the right position, according to the photo I attached previously. J-Link Commander still measured 3.3V.

When the jumper is in the left position or removed, and the pull-down resistor is connected, J-Link Commander measures 1.12V, and the status LED switches to red instead of its usual green color.