7.7.4. Timestamp packet

Timestamping enables correlation between multiple trace streams, and is provided by timestamp packets. See Timestamping, ETMv3.5. The timestamp packet consists of:

The Timestamp size field of ETMCCER specifies the maximum size of the timestamp packet, as either eight bytes or ten bytes. See Configuration Code Extension Register, ETMCCER, ETMv3.1 and later

Figure 7.40 shows the format of the 48-bit timestamp packet.

Figure 7.40. 48-bit timestamp packet

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Figure 7.41 shows the format of a 64-bit timestamp packet.

Figure 7.41. 64-bit timestamp packet

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The fields in the timestamp packet are:

R

The R bit in the timestamp packet header is set to 1 if the clock frequency of the processor or timestamp generator has changed since the last timestamp packet, and is 0 otherwise. The ETM protocol does not give a precise indication of when the clock speed changes.

Timestamp

The timestamp header is always followed by at least one byte of timestamp. The timestamp value is compressed, so that the ETM generates only enough bytes of timestamp to output the most significant bit that changes. This is a similar compression mechanism to that used for address values, where the value of the bits that are not output have not changed since the last time they were output.

A value of zero indicates that the timestamp is unknown. This might also indicate that the implementation does not fully support timestamping.

C

The C bit indicates if another byte of timestamp information follows this byte. If the C bit is 1 then there is another byte of timestamp information in the packet. If the C bit is 0, then this is the last byte of information in the timestamp packet.

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