JTAG connection of STM32G474RET6

Hello

I am new user of a STM products.

Now I am developing a board based on STM32G474RET6 MCU.

I want to connect STDC14  connector for JTAG programming through STLINK-V3SET.

I understand that pull-up and pull down resistors on JTCK, JTMS and JTDI are integrated inside MCU.

But what about connection of series low value resistors (20-100 ohm) to place on a mentioned above lines + JTDO?

These resistors on a high-speed signals helpful for a signal integrity and etc..

I see these resistors on STM32G4 DPOW Discovery board and STM32G4 Nucleo-64 board of STM32G474RET6 between MCU and embedded STLINK-V3E, but not from MCU and STDC14 connector?

Additional question: the T_NRST (pin 12 of STDC14 connector) - is an output of reset signal produced on a board to external debugger, or an input of evaluation board from the debugger?

Thank you.

[GET]

Hello AlexHalf,

Please find answers below:

Yes the JTAG serial resistors must be present only on the direct path between STLINK-V3E to MCU/STDC14 , and should be as close as possible from STDC14 (constraint from STLINK-V3E integration with MCU to avoid stubs and for signal integrity and EMC) . 

The ESD protections should be as close as possible to STDC14.

T_NRST (pin 12 STDC14) : When a debugger is plugged into STDC14, the debugger can reset the Target MCU and other connected peripherals present on extension connectors thanks to T_NRST.

Best Regards,

Hello Get

Following your answers, I want to clarify this issue more.

1. The series resistors on the JTAG path do not present on both evaluation boards on the path between MCU and STDC14 connector. They present only between MCU and integrated STLINK-V3E IC. If I want to use external STLINK-V3SET (programmer/debugger) through STDC14 connector - internal STLINK-V3E IC will be tri-stated. Or they present also inside external STLINK-V3SET (MB1441)?

2. Series resistors recommended to place close to the source - from this it seems that resistor of JTDO should be placed near MCU while another 3 (on JTCK, JTDI & JTMS) near (or inside) STLINK-V3SET?

3. If T_NRST is also an output from the debugger - probably it is also can be output from integrated STLINK-V3E IC? In this case why R18 resistor on Nucleo board is with 0402 1/16W size?

When B2 reset switch is pressed on this resistor fall almost 1/4W (3.3V / 47-ohm x 3.3V = 0.23W)?

 Thank you.

Alex

[GET]

Hello AlexHalf,

1. Not sure I well catch your point, but while using a debugger on STDC14, you have to put the B-G474E-DPOW1 on-board STLINK-V3E in Reset mode (move JP7 on JP6) and to change the JP5 Power configuration according to your need, you can even optionally disconnect all signals with SW1 switches (all OFF , High physical position). Using STLINK-V3SET should not be a problem in that configuration, but please check also next point 2.

2. The on-board serial resistors included on JTAG for B-G474E-DPOW1 are only there to interface STM32G4 MCU with on-board STLINK-V3E. Please refer to the documentation of STLINK-V3SET to check if specific conditions/constraints apply Vs interfacing to a STDC14 header interface and associated conditions on a target board.

3. T_NRST has pull-up inside STM32G4 MCU, thus there is no issue to pull low T_NRST from the on board STLINK-V3E or from the Reset button, neither from STDC14 debugger providing it not bring high level directly to T_NRST signal (open drain is ok). I think serial resistor on T_NRST is here to have no risk Vs any error programmation during development of tools.

Hope this answers to your questions. Best Regards.