Nuts and Volts - March 2018

Many of the low- to mid-range PICs have little or no internal debugging capability. Some (with no internal support) have low cost debug headers available from Microchip that replace the target device during development. Others have internal support along with re-writable program memory to support the ICD3 debug executive. In either case, these PIC devices generally have one hardware and one software breakpoint (BP). The hardware BP is always used by the ICD3 executive, leaving one measly BP for your debugging.

Once one BP is set in your code, you have single-stepping control only, and you lose the ability to step into or over functions/procedures/calls, etc. This means that navigating your code in a controlled manner while observing internal registers, values, and/or I/O behavior can be quite a challenge! It’s infuriating to single-step through some problem code only to step into a function call that throws all your monitored values out-of-scope. You can’t easily get back, and there is likely many cycles ahead before the function returns.

Certainly you can run a simulator like MPSIM and get quite detailed debugging control, but if your system interfaces with the real world, you are going to have to simulate real world peripherals and timing events that can be hard to develop and operate. You can also use oscilloscopes and logic analyzers, but to do so you must have access to the event you are studying.

Enhancing Your Debugger

Oftentimes, the purpose of a debug BP is to stop the processor to examine the internal and external machine state in hopes of understanding where a bug is hiding. You then step around your code observing system behavior. When you can’t do this due to resource limitations or event timing, you can often get what you want by leaking it out on an unused pin or on a used pin between I/O operations.

Think of this technique as a probe that can poke around inside the processor at a time and place of your choosing. Of course, this technique will not magically add breakpoint control to your debugger, but it will allow you access to internal state and register values on-the-fly to augment the limited BP features.

The Logic Analyzer

There was a time when a logic analyzer cost thousands of dollars and was as big as a suitcase. With the advent of low cost field-programmable gate arrays (FPGAs) that has all changed.

There are many USB connected eight- and 16-bit

I work with a broad spectrum of low- and mid-range PIC microcontrollers using MPLAB/X and a Microchip ICD2/3 debugger/ programmer as shown in Manchester debugging technique can be implemented in any language, but I will start with C for its structural clarity.