Hi, I got some news.
I’ve been thinking about the capabilities of simavr that aren’t still supported, like the VCD dumps. I found that not only gtkwave support those files, but also the sigrok suite, which is an amazing tool for debugging electronic systems by looking at the signals they generate.
Now, to make simavr generate those VCD files, some parameters must be passed first, and right now there are only two ways to do it.
The first and most documented way requires passing the elf file directly to simavr as this:
simavr -m atmega328p -f 16000000 .pio/build/nanoatmega328new/firmware.elf
Then, add a block like this example into source code:
#include <avr_mcu_section.h>
AVR_MCU(F_CPU, "atmega328");
const struct avr_mmcu_vcd_trace_t _mytrace[] _MMCU_ = {
{ AVR_MCU_VCD_SYMBOL("TX"), .mask = (1 <<2), .what = (void*)&PORTD, }
};
Also the following must be added to platformio.ini
build_flags =
-Wl,--undefined=_mmcu,--section-start=.mmcu=0x910000
Once done a file with the vcd extension should appear which can be open with this command:
pulseview -I vcd -i capture.vcd
The second method requires neither the library or the build_flags and is easier to set up, but
like the previous one a parameter that must be added to the simavr call. Said command isn’t documented and I just found about it by reviewing its source code. It goes this way:
simavr -m atmega328p -f 16000000 --add-vcd-trace name=kind@addr/mask
Where:
name is a tag that will be used to identify the trace.
kind is related to the AVR_MMCU_TAG_VCD_IRQ, AVR_MMCU_TAG_VCD_TRACE and AVR_MMCU_TAG_VCD_PORTPIN macros and should be trace for what matters.
addr and mask are the memory region and bit mask for the data being traced, and they are expressed in hex, being the former 16bits and the latter 8bits long.
So the call equivalent to the first example would be like this:
simavr -m atmega328p -f 16000000 --add-vcd-trace TX=trace@0x002B/0x04
It should be noticed that both ways give the simulator the same kind of information about the memory that we want to be traced, and both of them require extra parameters for the simavr call, yet only the first one requires the extra compilation flags and code modification.
Now, what is so amazing about sigrok and that VCD file that might be worth the hassle of setting all of that?
The VCD dump has information of all the transitions or changes in the traced memory region as if a tool like a logic analyzer or oscilloscope was connected, but as is a simulated system it isn’t limited to just an IO pin, but it can trace everything even regular variables.
On the other hand, sigrok not only can show those changes but also decode them, so you can see all the bits, their timing, and meaning. A picture is a better way to show this 
@valeros @ivankravets
Is there any way to pass additional arguments to the simavr call?
If not, would it be possible to create an option in the platformio.ini file? Something like debug_extra_args maybe.