5.3.1. Synchronization frequency

Most trace sources output synchronization information periodically during tracing, and often each time tracing restarts after a gap. If older trace has been overwritten, the system cannot decompress any trace before the first synchronization point that remains in the buffer.

A TPA captures a large amount of trace and many synchronization points, so only a small proportion of the trace is lost in this way. An ETB captures fewer synchronization points, so a larger proportion of the trace might be lost. Figure 5.6 shows this effect.

Figure 5.6. Effect of frequency compared with infrequent synchronization points

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When you use an ETB, you must ensure that the synchronization frequency of trace sources is set to provide sufficient synchronization points. There must be enough points to make the amount of undecompressible trace small compared to the size of the ETB RAM. When you use a TPA, the size of the TPA buffer is usually large enough that the synchronization frequency is not significant.

Note

ETM9CS, ETM11CS, and HTM all have software-programmable synchronization periods.

Tracing more than one source

When they trace multiple sources, some trace sources can generate synchronization points much more frequently than others. If a trace source only uses a small portion of the bandwidth, then there might not be enough synchronization points for that trace source. You can program such trace sources to increase their synchronization frequencies to compensate for this.

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