Common Extensions
Microprocessor vendors have often defined their own core-specific debugging extensions. Such vendors include Infineon, MIPS with EJTAG, and more. If the vendor does not adopt a standard (such as the ones used by ARM processors; or Nexus), they need to define their own solution. If they support boundary scan, they generally build debugging over JTAG.
Freescale has COP and OnCE (On-Chip Emulation). OnCE includes a JTAG command which makes a TAP enter a special mode where the IR holds OnCE debugging commands for operations such as single stepping, breakpointing, and accessing registers or memory. It also defines EOnCE (Enhanced On-Chip Emulation) presented as addressing real time concerns.
ARM has an extensive processor core debug architecture (CoreSight) that started with EmbeddedICE (a debug facility available on most ARM cores), and now includes many additional components such as an ETM (Embedded Trace Macrocell), with a high speed trace port, supporting multi-core and multithread tracing. Note that tracing is non-invasive; systems do not need to stop operating to be traced. (However, trace data is too voluminous to use JTAG as more than a trace control channel.)
Nexus defines a processor debug infrastructure which is largely vendor-independent. One of its hardware interfaces is JTAG. It also defines a high speed auxiliary port interface, used for tracing and more. Nexus is used with some newer platforms, such as the Atmel AVR32 and Freescale MPC5500 series processors.
Read more about this topic: Joint Test Action Group
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