The ETM9 provides an output called PWRDOWN. When HIGH this indicates that the ETM is not currently enabled, so you can stop the CLK input and hold the other ETM9 signals stable. You can use this to reduce system power consumption when trace is not being used. When a TAP reset (nTRST) occurs, PWRDOWN is automatically forced HIGH until the ETM9 control register has been programmed.

You can use the PWRDOWN output directly to gate the ETM9 CLK input. This is shown in Figure 3.8.

The PWRDOWN signal is changed synchronously to TCK. Because PWRDOWN changes many cycles before trace is enabled, this does not cause any metastability problems if you use PWRDOWN to gate the ETM9 clock. If using PWRDOWN in this way causes problems with static timing analysis, you can synchronize PWRDOWN to CLK before using it to gate the ETM9 clock.

The PWRDOWN output is controlled by the ARM debug tools, and is automatically cleared at the start of a debug session.

The ARM920T (Rev 1), and ARM922T macrocells implement this logic internally by providing an ETMPWRDOWN input to the trace interface, causing the clock and data outputs to the ETM to be stopped.

You must connect the PWRDOWN output from ETM9 to the ETMPWRDOWN input to these cores, as shown in Figure 3.9.

The following cores support the generic trace interface:

On these cores, the trace outputs are prevented from switching using an ETMEN core input. You must tie the ETMEN input to the inverted PWRDOWN output from ETM9, as shown in Figure 3.10.

For more information, see the Technical Reference Manual for your processor.The ETMEN output from ETM9 must only be used to disable the trace port, not to disable ETM9 itself. See Dual-processor tracing for more information. Table 3.3 lists the possible combinations of ETMEN and PWRDOWN.