3.4.2 Debugging Details

The figure below illustrates the MPLAB Snap In-Circuit Debugger system when it is ready to begin debugging.

To find out whether an application program will run correctly, a breakpoint is typically set early in the program code. When a breakpoint is set from the user interface of MPLAB X IDE, the address of the breakpoint is stored in the special internal debug registers of the target device. Commands on PGC and PGD communicate directly to these registers to set the breakpoint address.

Next, the Debug > Debug Main Project function is usually selected in MPLAB X IDE. The debugger tells the debug executive to run. The target starts from the Reset vector and executes until the Program Counter reaches the breakpoint address that was stored previously in the internal debug registers.

After the instruction at the breakpoint address is executed, the in-circuit debug mechanism of the target device “fires” and transfers the device’s program counter to the debug executive (much like an interrupt) and the user’s application is effectively halted. The debugger communicates with the debug executive via PGC and PGD, gets the breakpoint status information and sends it back to MPLAB X IDE. MPLAB X IDE then sends a series of queries to the debugger to get information about the target device, such as the file register contents and the state of the CPU. These queries are performed by the debug executive.

The debug executive runs like an application in program memory. It uses some locations on the stack for its temporary variables. If the device does not run, for whatever reason (no oscillator, faulty power supply connection, shorts on the target board, etc.), then the debug executive cannot communicate to the MPLAB Snap In-Circuit Debugger, and MPLAB X IDE will issue an error message.

Another way to set a breakpoint is to select Debug > Pause. This toggles the PGC and PGD lines so that the in-circuit debug mechanism of the target device switches the Program Counter from the user’s code in program memory to the debug executive. Again, the target application program is effectively halted, and MPLAB X IDE uses the debugger communications with the debug executive to interrogate the state of the target device.