3.5.60.  About the lock registers, ETMv3.2 and later

From ETM architecture version 3.2, the lock registers control memory-mapped software access to all other registers, including the ETMCR. If you lock the ETM using this feature, it ignores memory-mapped software writes. Direct JTAG accesses, coprocessor accesses, memory-mapped debugger accesses, and all reads are unaffected.


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

  • When the lock access mechanism is implemented, an ETM reset locks the ETM.

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

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

  2. Access the other ETM registers.

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

The following sections describe the lock registers:

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