7.3.2. Shared trace port

Some applications might not have enough pins available for trace, so you might have to multiplex trace signals with other functions. This has the effect of increasing the load on the trace signals, unless a specific trace-only development board is built.

When an ETM output pin is multiplexed with other functions, the addition of the TPA target header can add a stub to the PCB track on the target system. When this happens, the following additional constraints apply (The goal is to minimize the effect of the TPA target header on non TPA-based signal usage and maintain the integrity of the trace measurements.):

Signal does not require termination in normal operation or is parallel-terminated

This means that a full voltage swing signal travels down the track. Ensure that the propagation delay of the stub added for the TPA target header is 20 per cent or less of the overall rise and fall time of the signal.

Signal is series terminated for normal operation

This means that a one-half voltage swing signal begins each transition on the track and propagates down the track until it is terminated at the target node. This case is potentially very problematic. The one-half voltage swing signal can maintain the TPA input at its threshold voltage for longer than the required rise and fall time. To prevent this, you must move the TPA target header to within one fifth of the rise time of the target end of the track. If this is not possible, you must slow down the rise and fall times until this requirement can be met.

Sometimes, board layout constraints make it impossible to keep the stubs to the trace target header to less than 20 per cent of the rise and fall time. If length and speed requirements do not allow the rise and fall times to be increased to meet the design requirements in this case, you can adjust the thresholds used by the TPA or LA, if supported. The target system must be able to provide:

  • sufficient noise margin around an altered threshold

  • sufficient setup and hold times because these will now be reduced.

Note

It is unlikely that any TPA supplier will guarantee support for this mode of operation.

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