3.16.47.  About the lock registers

The lock registers control memory-mapped software access to all other registers, including the ETMCR. If you lock the PTM using this feature, it ignores memory-mapped software writes. JTAG accesses, coprocessor accesses, memory-mapped debugger accesses, and all reads are unaffected.

Note

  • Any implementation of the PFT architecture that implements memory-mapped access to PTM registers must implement the lock access mechanism.

  • When the lock access mechanism is implemented, a PTM reset locks the PTM.

You can use this feature to prevent accidental modification of the PTM registers by software being debugged. For example, software that accidentally initializes unwanted areas of memory might disable the PTM, making it impossible to trace such software. To prevent this, on-chip software that accesses the PTM must access the PTM registers as follows:

  1. Unlock the PTM by writing 0xC5ACCE55 to the ETMLAR.

  2. Access the other PTM registers.

  3. Lock the PTM by writing any other value, for example 0x0, to the ETMLAR.

The following sections describe the lock registers:

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