These are “Resistor Networks”, meaning a whole bunch of individual resistors bundled into a single package. Most of the resistors on the schematic are listed as “RN#” followed by a letter. Other things to note on the schematic that are maybe unusual for those that don’t look at these things day in and day out… If you’re not familiar with the way I’m listing the pins, “Serial Tx” is the function the pin that is important to us, “Pin #” is the actual pin number on the integrated circuit itself, and PORTD# is the way the pin would be referred to in the uC’s datasheet, and in actual C code. ![]() So, that means the uC section really breaks down into the USB-to-Serial interface controlled by the ATMEGA16U2, and the actual Arduino Brain ATMEGA328P-PU, that is the thing that we all think of as “Arduino”. Reset/Pin1 on the 328P is connected to Pin13/PORTD7 on the 16U2, Serial Rx/Pin2/PORTD0 on the 328P is connected to Serial Tx/Pin8/PORTD3 on the 16U2, and Serial Tx/Pin3/PORTD1 on the 328P is connected to Serial Rx/Pin8/PORTD2 on the 16U2. It turns out there are only three signals shared in common between the two uC’s. Going a little deeper, I want to see what the interconnection points are between the systems, besides power and ground - what signals are being communicated. There’s the uC section, comprised of two uC’s. There’s the voltage management section, which prevents voltage from being sourced from USB and the 9V jack simultaneously. There’s the power input from the 9V jack with voltage regulation. Looking at the printout, there are three obvious subsections. I’ll remember to do that when I get to the point of drawing my own schematic. I’m sure it would be a bit easier to read if it were color coded, or broken into hierarchical sheets. A lot of the reference designators (“R3”, “C1”, etc) are overwritten by part numbers, as are the interconnections themselves. I am incredibly grateful to for making these schematics public, but let us be real, these aren’t the easiest things to read. I think printing this thing out, dead tree style, is probably a good way to go. Of course there’s the microcontroller (henceforth referred to as the uC) as well. There’s a USB connector, so some USB/Serial thing will be on there. I know there’s a DC connector on there, so there will be some manner of voltage regulation. I think the first thing I’ll want to do is try to identify some of the major subsystems. In this way, hopefully others will be able to learn and play with the files and hopefully be able to build their own Arduino UNO R3 clone.īecause the Arduino is an open source hardware design, publishes the current schematic and is available at this link: Arduino UNO R3 Schematic I’ll also write all the steps necessary to go from a board with a pile of components in the shrink wrap (or cut tape as the case may be), to a fully functional Arduino. In the end, I’ll publish a full set of KiCad design files, production Gerbers that can be uploaded to OSHPark, and a BOM with all the components valid as of the time of production (not sure if I’ll maintain it long term, but we’ll see). ![]() I’ll need to pull up datasheets to verify assumptions made from reading the schematic, too and once the board is ready for fabrication, I’ll also start digging into the firmware.
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