Browse through this article if you’re interested in building your very own Arduino UNO.
Introduction
In the past blogs, you’ve learned several topics about the Arduino UNO. You’ve learned the bits and pieces of a basic UNO development board. You’ve also learned How to Program an ATMEGA328P stand-alone. Wouldn’t it be nice if you’ll be able to construct your own Arduino UNO from scratch?
Basic Components of an Arduino UNO
An Arduino UNO basically has these components to function well with the Arduino IDE.
- ATMEGA328P MCU
- 16 MHz Crystal oscillator
- 2 pcs. 22pF NPO ceramic capacitor for matching with the crystal
- A 5V voltage regulator (7805 or AMS117-5)
- A 3.3V voltage regulator (optional)
- LEDs for
- Power
- D13 (Builtin LED)
- Programming activity (SCK or TX/RX pins)
- USB-to-Serial converter board (CH340, FTDI, etc.) for bootloader mode programming
- Buffer transistor for signal LEDs
- Filter capacitors
- 6-pin AVR ISP header for direct/ISP programming
CONSTRUCTION on a Breadboard
It is possible to create your Arduino UNO on only a breadboard. This is a great way to start learning the parts and processes of this indispensable development board. Below is a summary of the different circuits involved.
Basic Circuit with Power Supply
Below is a circuit to power and run an Arduino UNO with 5V along with an optional 3.3V supply. You may use the 7805 and 78L33 voltage regulators for this. The DC jack should be supplied with a voltage 2V – 3V higher than 5V. Note that a 16 MHz crystal oscillator is attached along with some matching caps. The reset line is pulled up to Vcc via a 10K resistor. Note that the connections of the power and GND pins of the ATMEGA328P include the AVcc pin.
| Connection | ATMEGA328P | Comment |
|---|---|---|
| Vcc | pin 7 | Digital Supply (5V) |
| AVcc | pin 20 | Analog Supply (5V) |
| GND | pin 8 | Ground |
| GND | pin 22 | Ground |
| XTAL1 | pin 9 | via 22pF cap to GND |
| XTAL2 | pin 10 | via 22pF cap to GND |
| RESET PIN | pin 1 | pull up 10K Resistor |
The power port should match what an original Arduino UNO has. It has IOREF, RESET, 3.3V, 5V, GND (2X), and Vin. IOREF is used to match I/O voltage from board to board, where it is basically just 5V using the UNO and shields for it. The first port is reserved (NC).
The Ports (Analog and Digital)
Next, here is a separate view of the circuit connections of the Analog and Digital ports of the UNO. Basically, all the analog and digital lines are output, as well as I2C lines (which are also analog ports A4 and A5). The AREF port determines the ADC reference voltage.
| Port Function | ATMEGA328P Pin | Comment |
|---|---|---|
| A0 | pin 23 | Analog |
| A1 | pin 24 | Analog |
| A2 | pin 25 | Analog |
| A3 | pin 26 | Analog |
| A4 | pin 27 | Analog/I2C - SCL |
| A5 | pin 28 | Analog/I2C - SDA |
| D0 | pin 2 | Digital |
| D1 | pin 3 | Digital |
| D2 | pin 4 | Digital |
| D3 | pin 5 | Digital/PWM |
| D4 | pin 6 | Digital |
| D5 | pin 11 | Digital/PWM |
| D6 | pin 12 | Digital/PWM |
| D7 | pin 13 | Digital |
| D8 | pin 14 | Digital |
| D9 | pin 15 | Digital/PWM |
| D10 | pin 16 | Digital/PWM |
| D11 | pin 17 | Digital/PWM |
| D12 | pin 18 | Digital |
| D13 | pin 19 | Digital |
| AREF | pin 21 | ADC Referrence Voltage |
On an UNO you’ll see the ports as:
Programming POrts
On an UNO, you’ll see both a USB port (with a dedicated USB-to-Serial chip such as ATMEGA16u2, or CH340) and an ICSP port. These ports are used for programming the main ATMEGA328P chip. The USB-to-Serial chip is used for Bootloader, while the ICSP port is for In-Circuit Serial Programming mode. Learn more about the programming modes of the ATMEGA328P here. It’s best to have both modes available on your UNO circuit.
| USB-to-Serial Adapter | ATmega328P |
|---|---|
| VCC (5V) | VCC (Pin 7) |
| GND | GND (Pin 8) |
| AVCC (pin 20) | |
| GND (pin 22) | |
| TXD | RXD (Pin 2) |
| RXD | TXD (Pin 3) |
| DTR | RESET (Pin 1) via a 0.1µF capacitor and RESET (Pin 1) pulled up to Vcc via 10k resistor. |
| OSC1 (Pin 9) via XTAL and NPO Cap to GND | |
| OSC2 (Pin 10) via XTAL and NPO Cap to GND |
| 6-pin ISP Programmer | ATmega328P |
|---|---|
| VCC | VCC (Pin 7) |
| GND | GND (Pin 8) |
| MOSI | MOSI (Pin 17) |
| MISO | MISO (Pin 18) |
| SCK | SCK (Pin 19) |
| RESET | RESET (Pin 1) - pull up to VCC via 10K resistor |
| AVCC (Pin 20) | |
| GND (Pin 22) | |
| OSC1 (Pin 9) via XTAL and NPO Cap to GND | |
| OSC2 (Pin 10) via XTAL and NPO Cap to GND |
Signal LEDs
The signal LEDs provide visual information to the user about certain functions in the circuit. Information such as power, built-in LED, and programming activity. Sometimes, you’ll need a buffer (such as a transistor) to not disrupt the actual function of the port (such as during UART transmission).
| Function | ATMEGA328P Pin | Comment |
|---|---|---|
| Power | Vcc | 5V Power Indicator |
| Built-in LED | pin 19 | LED Port |
| Programming Indicator | pin 2 | UART RX port (Bootloader mode programming) |
Conclusion
You just learned about the different circuits and connections involved in an Arduino UNO development board. With this, you should be able to construct your own UNO. Why not try to layout this circuit on a printed circuit board?
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