In-situ FPGA Debug Driven by On-Board Microcontroller

Often we are faced with the situation that the behavior of a circuit changes in an unpredictable way when a chassis cover is attached or the system is not easily accessible. For instance, in a deployed environment, such as space, hardware can malfunction in unpredictable ways. What can a designer do to ascertain the cause of the problem? Register interrogations only go so far, and sometimes the problem being debugged is register transactions themselves, or the problem lies in the FPGA programming. This work provides a solution; namely, the ability to drive a JTAG chain via an on-board microcontroller and support a read/write register interface running a logic analyzer core. This is achieved without the use of a JTAG cable or any external interface. We have demonstrated the functionality of the prototype system using a Xilinx Spartan 3E FPGA and a microchip PIC18f2550 microcontroller. This paper will discuss the implementation details as well as present case studies describing how the tools have aided satellite hardware development at Los Alamos National Laboratory.