Global variables use 9 bytes (0%) of dynamic memory, leaving 2039 bytes for local variables. Sketch uses 444 bytes (1%) of program storage space. The USB-serial chip on official Mega 2560 and faithful clones use a programmable USB-serial chip which has a VID/PID specific to the Mega 2560 so the Arduino IDE is able to label the Mega's port in the Tools > Port menu. Your Nano uses a USB-serial chip (CH340) with a generic VID/PID so there is no way for the Arduino IDE to identify that COM5 is a Nano. I only see in a box 'Serial ports, Cim1, Com3 and Com 5', but no confirmation that the Nano is recognized.

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Click on the black console pane at the bottom of the Arduino IDE window.

Check the checkbox next to Show verbose output during: upload.

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Select File > Preferences from the Arduino IDE's menus.

Select Tools > Port > COM5 from the Arduino IDE's menus.

But in the large program, when debugging it, in the place signalled by 'Here', index i becomes 2 instead of 1 the first time, and then it continues 3, 4. This is a complete program, and runs perfectly. Typedef struct // For any parameter input As the program is lage, I extracted from it the part originating the problem, as follows: While debugging it I got to a point where the index of a 'for' seems not to behave correctly.

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