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How JTAG take control of ARM core?

Ding Li
Note: This was originally posted on 29th July 2010 at http://forums.arm.com

Hello,

I have a question from customer regarding how to disable JTAG in their final product to prevent their application code from explosion from JTAG interface.

Since I am not familiar with JTAG debug module and ARM core interaction mechanism, I would like to know if there is any document talking about the JTAG debug detail procedure.

(1) We have a plan to disable the JTAG port during reset stage of the MCU, and enable it with the built-in ROM code in certain stage. I am not sure if it is workable, any attention needed?

(2) Is the system reset a must when entering JTAG debug mode? If not, please help explain the detail procedure on how JTAG take control of core.


Thanks a lot!
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  • Note: This was originally posted on 5th August 2010 at http://forums.arm.com

    Thanks for your comments.

    But I still have a concern: in production devices, the ICE will be disabled. Does that means there are no way to enable it again, and there is no way to control the device through JTAG port anymore.

    I am still looking for a method to dynamically control the access of JTAG interface, by password, or something else. I am stiil not sure if it workable that we disable the JTAG interface in reset stage and enable it by ROM code in later stage.

    Thanks!



    Joseph is right. JTAG is Test port and not debug port, It is just that  ICE use it.
    Production devices are ment to follow proper boot sequence to start working (Security purpose), hence they fuse ICE(remove it).  What you are suggesting happens on 'test chip'(Chips manufactured b4 production chips for testing purpose) were debug(ICE) automatically gets enable after reset.
    For 'Dynamic control Option' please check the hardware on which ur working or going to work.

    Regards,
    Kedar Kulkarni
    [color="#FF8C00"][color="#008000"]One day all digial world will be in my ARMs[/color][/color].
Reply
  • Note: This was originally posted on 5th August 2010 at http://forums.arm.com

    Thanks for your comments.

    But I still have a concern: in production devices, the ICE will be disabled. Does that means there are no way to enable it again, and there is no way to control the device through JTAG port anymore.

    I am still looking for a method to dynamically control the access of JTAG interface, by password, or something else. I am stiil not sure if it workable that we disable the JTAG interface in reset stage and enable it by ROM code in later stage.

    Thanks!



    Joseph is right. JTAG is Test port and not debug port, It is just that  ICE use it.
    Production devices are ment to follow proper boot sequence to start working (Security purpose), hence they fuse ICE(remove it).  What you are suggesting happens on 'test chip'(Chips manufactured b4 production chips for testing purpose) were debug(ICE) automatically gets enable after reset.
    For 'Dynamic control Option' please check the hardware on which ur working or going to work.

    Regards,
    Kedar Kulkarni
    [color="#FF8C00"][color="#008000"]One day all digial world will be in my ARMs[/color][/color].
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