5.2.5. Using additional buffers in trace systems

Adding supplemental FIFOs within a trace system can help to alleviate narrowing of bandwidths across a system and reducing the risk of losing trace data. Although trace sources are designed with FIFOs to mitigate the large burst nature, there are still situations when the ATB bandwidth can limit the amount of information that can be traced.

You can use a trace link component that has a large FIFO, such as the ETF that uses an SRAM for data storage, to compensate for differing bandwidth specification of devices on the inputs and output of the link. The FIFO enables for averaging of the ATB activity over larger periods of time than would be practical within a trace source, either by sharing the resources of a large buffer or because the conditions for it are not applicable in all ASICs where the trace source is implemented.

The following areas exist where an ETF can be fitted to reduce the effects that can cause loss of data:

Figure 5.5 shows four potential locations for fitting an ETF in a trace system. In such a system, not all the ETFs have to be fitted depending on the constraints of the ATB interfaces for the trace sinks and trace sources.

Figure 5.5. Using additional buffers for trace system

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An ETF with a large amount of SRAM are not going to solve all situations where FIFO overflows occur. Restrictions in bandwidth when crossing asynchronous boundaries and widths of ATB can alter the characteristics of the ATB before and after the change in bandwidth. For example, if an ETF is fitted after an ETM, to absorb large quantities of trace data produced during data-value trace, if the ATB interface is running significantly slower that the of the core, then the trace source is still liable to overflow as the bandwidth from the output of the ETM's internal FIFO is restricted between the ETM and ETF to enable the trace data to drain from the FIFO before more data is internally created.

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