3.8. Power Down support

ETMv3.3 introduces power-down support for the macrocell. This support enables the entire ETM state of the macrocell to be saved before it is powered down, and restored when it is powered up again.

Note

The ETM state is held in the ETM trace registers.

The main features of power-down support are:

It is implementation defined whether this power-down support is included in an ETM implementation. However, you can always read the ETMOSLSR to find whether this feature is implemented.

When power down support is implemented, a Direct JTAG interface to the ETM registers is not permitted. Access to the ETM registers from an external debugger must use the ARM Debug Interface v5. For more information see the ARM Debug Interface v5 Architecture Specification.

In a system that supports multiple power domains, the ETM might be split into two domains:

This power domain split enables the processor and core power domain of the ETM to be dynamically powered down while permitting a debugger to maintain communication with the ETM, and to determine that part of the ETM that is powered down.

Typically, the ETM core power domain is the same power domain as the processor. However, some implementations might separate the ETM core power domain from the processor power domain to enable the ETM core power domain to be powered down when the ETM is not in use.

In a typical CoreSight system, the ETM Debug domain is the same power domain as the other debug and trace components. This permits an implementation to power down all the debug and trace logic when not in use.

Table 3.94 shows how the ETMOSLSR indicates the power down support that is implemented.

Table 3.94. Determining the level of power down support

ETM ArchitectureETMOSLSR[3]ETMOSLSR[0]Power down support
ETMv3.3 or ETMv3.400SinglePower, see SinglePower in ETMv3.3 and ETMv3.4.
ETMv3.3 or ETMv3.401Full Support, see Full Power Down Support in ETMv3.3 and ETMv3.4.
ETMv3.500SinglePower, see SinglePower in ETMv3.5
ETMv3.510Full Support, see Full Power Down Support in ETMv3.5.

Values not shown are Reserved.

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