I created these notes to help better understand the role of the components we are soldering on the MAKERbuino. They also contribute towards an understanding of the schematic. Please feel free to suggest changes.
The Second 10K Resistor (R3)
This resistor, according to the instructions, is “important for connecting the microcontroller to the serial UART programmer board”. Okay, I wouldn’t have worded it exactly like that. Its primary role appears to be a pull-up resistor for the reset line (connected to the reset pin of the microcontroller) and therefore is more important for keeping the reset off when not in use.
To recap the previous discussion of the reset pin: the reset pin of the ATmega328P is an active-low pin (denoted by the line above the word RESET on the datasheet). This means that in its unused state, we should give it a high voltage to indicate we are not resetting it. When we want to reset the microcontroller, we need to supply that pin with a low voltage momentarily. At all other times it should be at a high voltage and that is what this resistor achieves.
The 100nF capacitor we will connect later is more “important for connecting the microcontroller to the serial UART programmer board”.