4.14. Timestamping

You enable timestamping by setting the Timestamp enable bit in the Main Control Register to 1. See Main Control Register, ETMCR. On power up, or after a reset, this bit is set to 0, disabling timestamping.

Timestamp packet describes the timestamp packet. The T-sync packet is a special case of the timestamp packet, that always outputs all seven bytes of the timestamp.

If you have enabled timestamping, the following events request the generation of a timestamp packet:

When it receives a timestamp request, the PTM generates a timestamp packet only if at least one of the following applies:

The PTM might not generate the timestamp packet immediately it receives the request. It can delay generating the packet to a point that is convenient to the PTM.

In PFTv1.0, the timestamp indicates the time the packet is generated, not the time the request was made. This means that a timestamp does not indicate the time when the requesting event occurred. A timestamp is only a time indicator inserted into the trace stream near the event that requested a timestamp.

In PFTv1.1, the timestamp indicates the time of the last traced waypoint.

If the PTM receives multiple timestamp requests close together, it might not generate a timestamp packet for each request. However:

If you have enabled timestamping, timestamps are generated based on the value of the timestamp event. If the timestamp event is TRUE for many consecutive cycles, the PTM might generate many timestamp packets in close succession. Therefore, ARM recommends that, when programming the Timestamp Event Register, you use event resources that are active for only a single cycle at a time.

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