Using Segger J-tag on Nucleo-G431RB loss of Vt

BTrem.1 · ‎2020-09-04

I want to do an experiment with the Nucleo-G431RB board on a jtag bus but I'm already seeing something strange. This board he has a built in ST-Link and also a jtag connector (CN4) for an external debugger. The manual says the external debugger can be used with the ST-Link disabled by putting a reset jumper on J1. But when I do this the the +3.3V to Vt goes to 0, even while I'm leaving the ST-Link USB cable still plugged in. This of course prevents the Segger j-link from connecting.

If I don't hold the ST-link in reset the Segger J-link does connect in jtag mode and does appear to work. However, I note in my Keil debugger the interface see's two devices, one the STMG431RB and the other unknown.

I eventually want to chain a 2nd Nucleo-G431 with this first one to see if I can individually control two G431's on a chained j-tag bus. Do I need to find a way to disable the ST-links? Why would a reset (conn J1) cause 3.3V to shut down.

In my final application the ST-Links won't be on the target boards.

Thanks,

BTrem.1 · ‎2020-09-08

Adding some additional notes:

This is in the UM2505 Manual on the G431 Nucleo board:

#1 does work but the j-tag see's two devices, the G431 and an "unknown"

#2 does NOT work. With JP1 in place there is no +3.3V at CN4-3 (Vt).

Is +3.3V at CN4-3 supplied from the SEGGER debugger, or is it just sampled? If sampled, where is +3.3V sourced from?

My problem persists ... The J-LINK mini does not see the 2nd G431, only see's the first in the chain.

Can anyone offer advice on what to pursue next? I'm stuck ....

BTrem.1 · ‎2020-09-14

Answering my own question,

With the JP1 reset jumper in place I don't think the +3.3 supply gets the enable signal from the ST-LINK MCU.

I changed the JP5 jumper to +5V_CHRGR and left JP1 with the reset. Now both MCU's power up. The J-Tag still only recognizes one MCU however. As a note the J-Tag see's two devices in that MCU which I understand is the M4 core and the FPU. So chaining two G431 MCU's should result in seeing 4 J-Tag devices.

Still trying to get it to see the 2nd MCU. Will report back when I get more info.