Unused bits in a program counter should be zero. However, sometimes they are not. And in fact, since the bits are unused, their value does not matter, as long as nobody from the outside, such as a debugger, reads them. So, can this cause problems for AVR microcontrollers?
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Stop-and-Go
One typical debugging activity is setting breakpoints and then running the program from breakpoint to breakpoint, inspecting the program’s internal state at each breakpoint. While this sounds simple, it gets complicated when one looks behind the curtain, which we will do in this blog post.
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Interrupted and Very Long Single-Steps
It often happens in embedded debugging that you suddenly end up in the interrupt dispatch table while single-stepping through your code. Another unrelated problem is that sometimes, single steps can take an eternity. In this blog post, I address both issues and describe how to circumvent them in a gdbserver implementation.
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Arduino IDE 2 Meets dw-link
Debugging classic AVRs in the Arduino IDE 2 is finally possible! It took a while to implement this feature, but now it is just a piece of cake to enable debugging and start using the debugger.
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Debugging 2.0
What keeps people from using a debugger? Well, it is mostly that one has initial costs in terms of setting up the debugging environment and of learning how to use the debugging tool. Hopefully, the next iteration of my hardware debugging tool dw-link, which is able to debug classic ATtinys and ATmegaX8s, will somewhat ease that burden, in particular, because you can buy the accompanying hardware now at Tindie.
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That’s One Small Step for a Man, One Giant Leap for a Debugger: On Single-Stepping and Interrupts
You want to make a single step in your program, but the debugger takes you to some unknown area of the program. This was, in fact, my first experience when I tried out Microchip’s MPLAB X IDE debugger on the innocent blinking sketch. Is this a bug or a feature?
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dw-link: A New Hardware Debugger for ATtinys and Small ATmegas
As mentioned in an earlier blog post this year, hardware debuggers are the premier class of embedded debugging tools. However, until today, there were only very few relatively expensive tools around supporting the debugWIRE interface that is used by the classic ATtinys and a few ATmega MCUs.
The good news is that now you can turn an Arduino UNO, Nano, or Pro Mini into a debugWIRE hardware debugger that communicates with avr-gdb, the AVR version of the GNU project debugger.
Surprise, Surprise!
When you develop a tool for a protocol that is undocumented, it is not surprising that you will encounter situations you will not have be anticipated. This was exactly what I experienced developing the hardware debugger dw-link, which connects debugWIRE MCUs to the GDB debugger. Although a substantial part of the debugWIRE protocol has been reverse-engineered, I encountered plenty of surprising situations: Split personality MCUs, stuck-at-one bits in program counters, secret I/O addresses, half-legal opcodes, and more.
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Debugging a Debugger With Itself
The above xkcd comic, which is titled Debugger, alludes to the concern that when you try to apply a particular method to itself, you might not get what you asked for. Turing’s Halting problem is a very famous example of this, i.e., you cannot algorithmically decide whether an algorithm terminates on an input. So, does that issue apply to debuggers as well? In particular, I asked myself whether it makes sense to debug the hardware debugger I am developing with itself.
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Link-Time Optimization and Debugging of Object-Oriented Programs on AVR MCUs
Link-time optimization (LTO) is a very powerful compiler-optimization technique. As I noticed, it does not go very well together with debugging object-oriented programs under GCC, at least for AVR MCUs. I noticed that in the context of debugging an Arduino program, and it took me quite a while to figure out that LTO is the culprit.
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