This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

AM625: JTAG TRST signal pull down.

Part Number: AM625
Other Parts Discussed in Thread: AM623,

I'm designing the system based on the AM625 / AM623 SoCs.

I noticed that the TRSTn signal (pin B10) is pulled down with a 4.7k resistor.

I understand it was done to properly initialize the JTAG controller integrated into SoC during the system's power-up.

But according to the following document https://www.jtag.com/wp-content/uploads/2019/04/0020-Board-DFT-E-1.pdf

It is recommended to pull up the TRSTn signal

Also, in the IEEE Std 1149.1-2001, section 4.6.2

the pull-up resistor on the TRST pin is recommended

Many JTAG debuggers have the TRST signal pins configured as an open drain and they can't initialize the AM625 JTAG interface

Is there any workaround that will allow me to do the following?

- Properly initialize the JTAG interface during the power-up stage

- Make the TRST pulled up after the power-up stage (JTAG interface will be enabled by default)

Should I expect any issues if the JTAG interface will be enabled by default?

That is the recommendation that I found in the JTAG specification and a few other places

How I can implement it on the AM625 SoC?

 

  • I agree the pull-down recommendation is not consistent with the IEEE 1149 standard.

    The 1149 standard was initially developed to support testing of complete PCB assemblies via boundary scan, but this peripheral was later adopted as a way to control code execution when trying to debug system software.

    TI deviated from the 1149 standard many years ago because the primary purpose for the JTAG peripheral on TI processors was to support debug functions rather than boundary scan. The recommendation for holding the TRSTn input low during normal operation was to prevent any chance of noise on the other JTAG signals from accidently causing the debug subsystem from altering code execution during normal operation. The recommendation remains the same today. 

    Most debuggers support an option to operate their TRST output as open-drain or push-pull. The TRST output of the debugger will need to be configured to operate as a push-pull output so the TRST signal with a pull-down can be driven high when communicating with the TAP controller in the AM62x processor.

    Regards,
    Paul