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Why are CoreSight Narrow Timestamp (NTS) components unsuitable for delivering the System Count value (CNTVALUEB) to processors?

Information in this article applies to:

  • CoreSight Debug and Trace
  • Cortex-A

Problem/Question

Why are CoreSight Narrow Timestamp (NTS) IP components unsuitable to deliver the System Count value (CNTVALUEB) to processors?

Scenario

This knowledge article is relevant for SoC designers who have access to CoreSight NTS IP components. This IP is used to compress a 64-bit trace timestamp value to save wires and reduce routing. A designer might recognize that the System Count value (CNTVALUEB) is also a 64-bit value, and therefore might consider using NTS IP to distribute the System Count value to one or more processors on chip. This article explains why that cannot be done.

Answer

NTS components cannot guarantee all the requirements that the Arm architecture requires for the System Count value. NTS components trade off some accuracy to save wires and simplify routing. The reduction in accuracy is acceptable for trace timestamps, but not for a System Count value that software relies on to provide an accurate timebase for Generic Timer functions.


Workaround

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Example

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Related Information

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