6.2. ETMv2 trace packets

The TRACEPKT pins output all trace information that is not encoded by the PIPESTAT pins. This information is organized into packets, each of one or more bytes in length.

There are three possible scenarios for information output on the TRACEPKT pins, depending on the number of pins in use:

Four pins

A byte is output over two cycles on TRACEPKT[3:0]. In the first cycle bits [3:0] are output and in the second cycle bits [7:4] are output.

Eight pins

A byte is output in a single cycle on TRACEPKT[7:0].

Sixteen pins

Up to two bytes can be output per cycle. If there is only one valid byte, it is output on TRACEPKT[7:0], and the signal on TRACEPKT[15:8] is 0x66. If there are two bytes to output, the first is output on TRACEPKT[7:0] and the second on TRACEPKT[15:8].

The TRACEPKT pins are used to output the following types of information:

Trace packets

Trace packets are output when the ETM is collecting trace data.

Note

  • In this document, a packet is a discrete quantity of trace information comprising one or more bytes. In previous versions of this document, the word packet and byte were used interchangeably.

  • Multiple packets can be placed in the FIFO in one cycle. Each packet begins with a header that indicates whether or not more packets follow. The only exception to this is branch addresses. A branch address is always the last packet to be placed in the FIFO in any cycle.

  • Types of data packet include the following:

    • a branch address

    • a normal data packet

    • a Context ID packet.

  • Each packet is identified by a packet header, see Trace packet headers.

Trace FIFO Offset (TFO) and TFO packets

TFO packets are occasionally output as part of the trace synchronization mechanism. Trace synchronization is described in Trace synchronization in ETMv2.

Trigger information

When a TR (Trigger) pipeline status occurs, the pipeline status that it replaces is output on the TRACEPKT[3:0] pins. For more information, see Trigger PIPESTAT signals.

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