How does a STLINK/V2 know when SWD or Jtag mode can be selected

[aa8pa222]

I have an eval board that requires a jump to be installed before SWD will work.  I installed the jumper and now both Jtag and SWD work as they should. My question is once I installed the jumper how does the STlink/V2 know that the jumper was installed. The jumper is on the output of an and gate that that has 2 inputs USB_Jlink_TMS and Jlink_TMS, the jumper takes the output of the and gate and goes to Jlink_TMS.  It looks like the jumper goes back to the STlink/V2 connector.  Any ideas will help.

[stmdude]

Schematics would be helpful. Since it's an eval-board, they can probably be found online.

Edit: Though, in general, I haven't seen a STLink/V2 (or V1) being able to autodetect JTAG/SWD. I've always had to tell the software which interface I wanted to use.

[aa8pa222]

Yes you do select which interface you want to use via the software.  Without the jumper you can only select Jtag. Swd is not available.  Once the jumper is in place both Jtag and swd are now available. Let me ask it a different way. What happens to a digital signal when you add it to itself,  this is what I see the jumper is doing. The output of a 2 input and gate has one of its inputs jlink_tms jumpered to the and gates output. Then this signal is sent to the mcu. I trying to understand what is taking place when and input is added to itself on the output of an and gate. I can see on a scope frequency changes and pulse duration when switching between Jtag and swd. I need to know why this happens when the jumper is installed 


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