JTAG Adapter Hardware
Adapter hardware varies widely. When not integrated into a development board, it involves a short cable to attach to a JTAG connector on the target board; a connection to the debugging host, such as a USB, PCI, or Ethernet link; and enough electronics to adapt the two communications domains (and sometimes provide galvanic isolation). A separate power supply may be needed. There are both "dumb" adapters, where the host decides and performs all JTAG operations; and "smart" ones, where some of that work is performed inside the adapter, often driven by a microcontroller. The "smart" adapters eliminate link latencies for operation sequences that may involve polling for status changes between steps, and may accordingly offer faster throughput.
At this writing (mid-2009) adapters with full speed USB links are probably the most common approach, and new products often include high speed USB support. Higher end products often support Ethernet, with the advantage that the debug host can be quite remote. Adapters which support high speed trace ports generally include several megabytes of trace buffer and provide high speed links (USB or Ethernet) to get that data to the host.
Personal computer parallel port adapters are simple and inexpensive, but they are relatively slow because they use the host CPU to change each bit ("bit banging"). They have declined in usefulness because newer computers do not have parallel port hardware. Driver support is also a problem, because the adapter electronics varied so widely.
Serial port adapters also exist, and are similarly declining in usefulness. They generally involve either slower bitbanging than a parallel port, or a microcontroller translating some command protocol to JTAG operations. Such serial adapters are also not fast, but their command protocols could generally be reused on top of higher speed links.
With all JTAG adapters, software support is a basic concern. Many vendors do not publish the protocols used by their JTAG adapter hardware, limiting their customers to the tool chains supported by those vendors. This is a particular issue for "smart" adapters, some of which embed significant amounts of knowledge about how to interact with specific CPUs.
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