2.10.1. ETMv1 and ETMv2 behavior

In ETMv1.x and ETMv2.x, the trace port protocol assumes that the trace port runs at the same frequency as the processor. This means that clocking modes in these devices are bandwidth invariant, because the number of pins in the trace port is varied to preserve the trace port bandwidth. The trace capture device must be aware of the mode in use and must reconstruct the trace to appear as if trace was captured at full speed from a normal port.

The modes are:


The trace port runs at processor clock speed.


The trace port runs at half the processor clock speed over twice the number of pins.


The trace port runs at twice the processor clock speed over half the number of pins.

Additionally, the trace port clock can be selected to run:

Trace must always be captured off both edges in multiplexed mode.

Two fields in the ETMCR select this behavior. See Main Control Register, ETMCR. The two fields are bit [13], Half-rate clocking, and bits [17:16], Port mode, as Table 2.15 shows.

Table 2.15. Clocking, port mode, port speed, and data pins in ETMv1 and ETMv2

Half-rate clockingPort mode NameTRACECLK edge

Clock ratio[a]

Data:Clock ratio[b]

Width ratio[c]

1b00Normal, half-rate clockingBoth1:21:11:1
1b10Demultiplexed, half-rate clockingBoth1:41:22:1

[a] Ratio of (trace port clock speed):(ETM clock speed).

[b] Ratio of (trace port data rate):(ETM clock speed).

[c] Ratio of (number of data port pins):(number of ETM pins).

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