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LR Contents upon IRQ Handler in Cortex-M3

Muhammad Ahsan
Note: This was originally posted on 16th June 2011 at http://forums.arm.com

Hi,
I am with Cortex-M3 and I have implemented a function that calcultes CRC using DMA and upon the completion of this calculation and Interrupt is generated. As soon as the program jumps to the IRQ handler, the content of LR register becomes 0xFFFFFFF9 which causes the program to get lost upon the complete execution of IRQ handler.
In order to verify, whether the LR is loaded with same 0xFFFFFFF9 in case of entering some other IRQ, I have implemented another function, in which FRT counter counts and upon overflow jumps to the corresponding IRQ handler. I have observed that the LR contents are also changed to 0xFFFFFFF9.
Is there anything that I am missing in regard to implement and handle interrupts in Cortex-M3?
Suggestions will highly be appreciated!!!
Regards,
Parents
  • Note: This was originally posted on 16th June 2011 at http://forums.arm.com


    the content of LR register becomes 0xFFFFFFF9 which causes the program to get lost upon the complete execution of IRQ handler.

    This is not the reason. In case of ISR execution, LR does not contain the return address, but a magic EXC_RETURN value. The return address is on the stack at sp-24.


    Is there anything that I am missing in regard to implement and handle interrupts in Cortex-M3?

    Yes. Please have a look at the official documentation (ARMv7-M ARM, Cortex-M3 TRM), and/or Joseph Yiu's excellent book.

    Kindly
    Marcus
Reply
  • Note: This was originally posted on 16th June 2011 at http://forums.arm.com


    the content of LR register becomes 0xFFFFFFF9 which causes the program to get lost upon the complete execution of IRQ handler.

    This is not the reason. In case of ISR execution, LR does not contain the return address, but a magic EXC_RETURN value. The return address is on the stack at sp-24.


    Is there anything that I am missing in regard to implement and handle interrupts in Cortex-M3?

    Yes. Please have a look at the official documentation (ARMv7-M ARM, Cortex-M3 TRM), and/or Joseph Yiu's excellent book.

    Kindly
    Marcus
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