If an ETM is powered-down and Locked (that is, the default state out of reset if the Lock Access mechanism is enabled), then the write to the ETM Control register fails as expected. However, when RealView Debugger attempts to read and display all of the ETM registers, this causes the APB to lock up.

To prevent the APB from locking up, set the RVD_CS_ETM_LOCK environment variable to the hexadecimal value that represents the password required to unlock the ETM hardware. The value is written to the CS_LOCKACCESS register on connect before the ETM is powered up.