Nonintrusive debugging using the JTAG interface of FPGA-based prototypes

The IEEE Std. 1149.1, also known as JTAG, defines a serial interface to access test-dedicated logic embedded in integrated circuits, although it is also being used as an FPGA programming interface. This paper makes an analysis of the possibilities of reusing this infrastructure in debugging applications implemented in FPGAs while in the prototype validation phase, with emphasis on nonintrusive methods. Commercially available FPGAs may offer from basic JTAG implementations, to complex ones. Depending on these features, the paper discusses different methods for monitoring, tracing, debugging and profiling the execution of programs running on a microprocessor. Some of these methods require ad-hoc modules to be inserted, like embedded in-circuit emulators or trace-capable blocks. A tool is presented that demonstrates the possibility of automatically inserting and connecting debug-oriented blocks, and controlling them through the JTAG interface. Application examples are provided, showing the results of the use of the tool with some industrial and academic microprocessor system implementations.