Designing Embedded Hardware

JTAG

As mentioned earlier, the JTAG port (sometimes also known as a Test Access Port, or TAP) provides access to the internals of the processor and, through it, the rest of the computer system. JTAG is defined under IEEE standard 1149.1a-1993 Standard Test Access Port and Boundary Scan Architecture. Commercially available test suites use JTAG to provide in-circuit debug capability. The adventurous among you can also drive JTAG “manually,” using the information in that standard document.

JTAG uses a technique known as boundary scan to probe the circuit connections between peripherals and memories and the microprocessor. It does this by asserting outputs (independently of the CPU) and reading the response from external devices on input pins. It is useful for testing not only interconnections on the PCB but also design verification and even correct timing. JTAG can operate independently of the CPU, “manually” driving outputs, and can interrogate the processor as to its manufacturer, processor type, and revision number. JTAG can also be used to disable output pins while a board is undergoing test. Motorola has added functionality to the JTAG interface through an On-Chip Emulation module, or OnCE. The OnCE can let the processor run and watch system activity in response to the executing software. It can retrieve or set parameters in registers or memory, provide a host of debugging features (such as setting breakpoints, single-stepping, and instruction tracing).

A JTAG port provides ...