Designing Embedded Hardware, 2nd Edition

JTAG

A JTAG (Joint Test Action Group) port 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. It is sometimes also known as a Test Access Port, or TAP. 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 the aforementioned document.

Using JTAG

To utilize JTAG, the processor you are using must specifically support it. In other words, the processor must have a JTAG interface inbuilt. JTAG is very rare in small microcontrollers, with most providing no support. However, more advanced microcontrollers and faster processors typically do include a JTAG interface, and this is an essential part of the debugging process for systems based on these chips.

The JTAG port allows for real-time debugging of hardware and software. It allows you to single-step or multi-step through code running directly on the target system. You can individually (and manually) toggle signal lines of the processor to test external subsystems in the computer (also known as boundary scan ). You can set breakpoints both at locations in code or for when a particular address (or device) is accessed. The JTAG port allows you to examine and modify registers and memory locations. To utilize the JTAG interface, you need to have support tools that are JTAG ...