Building an Arduino Uno Schematic

Overview

This tutorial will walk you through building an Arduino Uno schematic using tscircuit. We'll cover the ATmega328P microcontroller, 16MHz crystal oscillator with load capacitors, reset circuit, power supply with an LDO voltage regulator, decoupling capacitors, and LED indicators — the essential components that make up the Arduino Uno.

The Arduino Uno is one of the most popular development boards in the maker community. At its heart is the ATmega328P microcontroller running at 16MHz with 32KB of Flash, 2KB of SRAM, and 1KB of EEPROM. This schematic-only design focuses on the core circuitry needed to get the ATmega328P up and running.

Arduino Uno Minimum System Requirements

The Arduino Uno's core circuit needs the following to operate reliably:

• Power Supply — The ATmega328P requires a stable 5V supply. We use a USB-C connector with an AMS1117-5.0 LDO to step down USB's 5V (VBUS) to a regulated 5V rail.
• Clock Circuit — A 16MHz ceramic resonator (or crystal with load capacitors) provides the system clock. Two 22pF capacitors are needed as load capacitors for a crystal oscillator.
• Reset Circuit — The RESET pin (PC6) must be pulled high via a 10k resistor. A push button pulls it low to reset the microcontroller.
• Decoupling Capacitors — A 100nF ceramic capacitor close to the ATmega328P's power pins filters high-frequency noise, while 10uF bulk capacitors stabilize the LDO input and output.
• LED Indicators — A green power LED indicates the board is powered, and a yellow LED on digital pin 13 (PB5/SCK) provides a user-controllable indicator.

Building the Circuit Step by Step

Step 1: ATmega328P Microcontroller

The ATmega328P-PU is the heart of the Arduino Uno. It comes in a 28-pin DIP package with 23 general-purpose I/O lines, 32KB of Flash memory, and operates at up to 20MHz (the Arduino Uno runs it at 16MHz).

We define it using the <chip> component with pin labels matching the ATmega328P-PU pinout. The left side contains the reset pin, port D pins, power, ground, and crystal pins. The right side contains the analog pins, AREF, AVCC, and port B pins (including the SPI and digital pins).

The schPortArrangement property controls how pins appear on the schematic symbol. Each side has a direction (top-to-bottom) and an array of pin names. The pin names use the format PORT/PIN_FUNCTION to show both the port name and the Arduino pin name.

Step 2: Power Supply (USB-C + LDO)

The Arduino Uno operates at 5V. We use a USB-C connector as the power source and an AMS1117-5.0 LDO (Low Dropout Regulator) to provide a stable 5V output. The SmdUsbC component provides the USB-C connector, and we add 10uF capacitors on both the input and output of the LDO for stability.

The AMS1117-5.0 takes the USB VBUS (5V) as input and outputs a regulated 5V. While this seems redundant (5V in, 5V out), the LDO provides a clean, stable voltage even when the USB supply fluctuates. In the actual Arduino Uno, the LDO input comes from the DC barrel jack (7-12V), but for simplicity we connect it to VBUS here.

Step 3: Connect Power to ATmega328P

Now we connect the 5V output from the LDO to the ATmega328P's power pins. The ATmega328P has two power pins: VCC (digital supply) and AVCC (analog supply). Both must be connected to 5V. It also has two GND pins. We add a 100nF decoupling capacitor close to the chip's power pins to filter high-frequency noise.

The 100nF decoupling capacitor (C3) is critical for reliable operation. It acts as a local energy reservoir that can supply instantaneous current when the ATmega328P switches its outputs, preventing voltage dips on the power rail that could cause erratic behavior.

Step 4: 16MHz Clock Circuit

The ATmega328P requires an external clock source for accurate timing. The Arduino Uno uses a 16MHz crystal oscillator with two 22pF load capacitors. The crystal connects to the XTAL1 (PB6) and XTAL2 (PB7) pins.

The 22pF load capacitors are required for the crystal to oscillate at the correct frequency. The capacitor values are specified by the crystal manufacturer and depend on the crystal's load capacitance specification. For a typical 16MHz crystal with 16pF load capacitance, two 22pF capacitors provide the correct load (accounting for stray PCB capacitance of about 5pF on each pin).

Step 5: Reset Circuit

The ATmega328P has an active-low reset pin (PC6/RESET). For normal operation, this pin must be held high. We use a 10k pull-up resistor to VCC and a push button to ground — pressing the button pulls RESET low, restarting the microcontroller.

The 10k pull-up resistor keeps RESET at 5V during normal operation. When the push button is pressed, it creates a direct path from RESET to GND, pulling the pin low and triggering a reset. In the actual Arduino Uno, a 100nF capacitor is also placed in parallel with the button for auto-reset during programming via the serial DTR signal, but we omit it here for simplicity.

Step 6: LED Indicators

The Arduino Uno has two important LEDs: a green power LED that indicates the board is powered, and a yellow LED connected to digital pin 13 (PB5/SCK) that serves as a built-in test indicator. Each LED has a 1k current-limiting resistor.

The power LED (green) is connected between VCC and GND through a 1k resistor — it's always on when the board is powered. The D13 LED (yellow) is connected to PB5/SCK through a 1k resistor. When pin 13 is driven HIGH in your Arduino sketch, current flows from the pin through the resistor and LED to ground, lighting it up. This is the LED that blinks when you run the classic "Blink" sketch.

Step 7: Complete Schematic

Now we combine all the sub-circuits into the complete Arduino Uno schematic. This includes the USB-C power supply, AMS1117-5.0 LDO regulator, ATmega328P microcontroller, 16MHz crystal with load capacitors, decoupling capacitors, reset circuit, and both LED indicators.

import { SmdUsbC } from "@tsci/seveibar.smd-usb-c"

export default () => {
return (
  <board width="80mm" height="60mm" routingDisabled>
    <SmdUsbC
      name="J1"
      connections={{
        GND1: "net.GND",
        GND2: "net.GND",
        VBUS1: "net.VBUS",
        VBUS2: "net.VBUS",
      }}
      schX={-10}
      schY={4}
    />

    <chip
      name="U2"
      manufacturerPartNumber="AMS1117-5.0"
      schWidth={2}
      schHeight={3}
      schPortArrangement={{
        leftSide: {
          direction: "top-to-bottom",
          pins: ["GND", "VO"]
        },
        rightSide: {
          direction: "top-to-bottom",
          pins: ["VI"]
        }
      }}
      pinLabels={{
        pin1: ["pin1", "GND"],
        pin2: ["pin2", "VO"],
        pin3: ["pin3", "VI"]
      }}
      supplierPartNumbers={{ jlcpcb: ["C6187"] }}
      schX={-5}
      schY={4}
    />

    <chip
      name="U1"
      manufacturerPartNumber="ATmega328P-PU"
      schWidth={5}
      schHeight={8}
      schPortArrangement={{
        leftSide: {
          direction: "top-to-bottom",
          pins: ["PC6/RESET", "PD0/RXD", "PD1/TXD", "PD2/INT0", "PD3/INT1", "PD4", "VCC", "GND", "PB6/XTAL1", "PB7/XTAL2", "PD5", "PD6", "PD7"]
        },
        rightSide: {
          direction: "top-to-bottom",
          pins: ["PC5/A5/SCL", "PC4/A4/SDA", "PC3/A3", "PC2/A2", "PC1/A1", "PC0/A0", "GND2", "AREF", "AVCC", "PB5/D13/SCK", "PB4/D12/MISO", "PB3/D11/MOSI", "PB2/D10", "PB1/D9", "PB0/D8"]
        }
      }}
      supplierPartNumbers={{ jlcpcb: ["C82151"] }}
      schX={5}
      schY={0}
    />

    <chip
      name="Y1"
      manufacturerPartNumber="CSTCE16M0V53-R0"
      schWidth={2}
      schHeight={2}
      schPortArrangement={{
        leftSide: {
          direction: "top-to-bottom",
          pins: ["1"]
        },
        rightSide: {
          direction: "top-to-bottom",
          pins: ["3"]
        }
      }}
      supplierPartNumbers={{ jlcpcb: ["C126020"] }}
      schX={5}
      schY={-6}
    />

    <capacitor
      name="C1"
      capacitance="22pF"
      footprint="0402"
      supplierPartNumbers={{ jlcpcb: ["C1555"] }}
      schX={3}
      schY={-8}
    />

    <capacitor
      name="C2"
      capacitance="22pF"
      footprint="0402"
      supplierPartNumbers={{ jlcpcb: ["C1555"] }}
      schX={7}
      schY={-8}
    />

    <capacitor
      name="C3"
      capacitance="100nF"
      footprint="0402"
      supplierPartNumbers={{ jlcpcb: ["C307514"] }}
      schX={2}
      schY={-4}
    />

    <capacitor
      name="C4"
      capacitance="10uF"
      footprint="0805"
      supplierPartNumbers={{ jlcpcb: ["C15850"] }}
      schX={-7}
      schY={1}
    />

    <capacitor
      name="C5"
      capacitance="10uF"
      footprint="0805"
      supplierPartNumbers={{ jlcpcb: ["C15850"] }}
      schX={-3}
      schY={1}
    />

    <resistor
      name="R1"
      resistance="10k"
      footprint="0402"
      supplierPartNumbers={{ jlcpcb: ["C25744"] }}
      schX={0}
      schY={4}
    />

    <pushbutton
      name="SW1"
      footprint="pushbutton"
      schX={-2}
      schY={4}
    />

    <led
      name="LED_PWR"
      color="green"
      footprint="0603"
      supplierPartNumbers={{ jlcpcb: ["C72043"] }}
      schX={10}
      schY={4}
    />

    <resistor
      name="R2"
      resistance="1k"
      footprint="0402"
      supplierPartNumbers={{ jlcpcb: ["C11702"] }}
      schX={10}
      schY={6}
    />

    <led
      name="LED_D13"
      color="yellow"
      footprint="0603"
      supplierPartNumbers={{ jlcpcb: ["C72044"] }}
      schX={10}
      schY={-2}
    />

    <resistor
      name="R3"
      resistance="1k"
      footprint="0402"
      supplierPartNumbers={{ jlcpcb: ["C11702"] }}
      schX={10}
      schY={0}
    />

    <trace from=".U2 .VI" to="net.VBUS" />
    <trace from=".U2 .VO" to="net.V5" />
    <trace from=".U2 .GND" to="net.GND" />
    <trace from=".C4 .pos" to="net.VBUS" />
    <trace from=".C4 .neg" to="net.GND" />
    <trace from=".C5 .pos" to="net.V5" />
    <trace from=".C5 .neg" to="net.GND" />
    <trace from=".U1 .VCC" to="net.V5" />
    <trace from=".U1 .AVCC" to="net.V5" />
    <trace from=".U1 .GND" to="net.GND" />
    <trace from=".U1 .GND2" to="net.GND" />
    <trace from=".C3 .pos" to="net.V5" />
    <trace from=".C3 .neg" to="net.GND" />
    <trace from=".Y1 .1" to=".U1 .PB6/XTAL1" />
    <trace from=".Y1 .3" to=".U1 .PB7/XTAL2" />
    <trace from=".C1 .pos" to=".U1 .PB6/XTAL1" />
    <trace from=".C1 .neg" to="net.GND" />
    <trace from=".C2 .pos" to=".U1 .PB7/XTAL2" />
    <trace from=".C2 .neg" to="net.GND" />
    <trace from=".R1 .pos" to="net.V5" />
    <trace from=".R1 .neg" to=".U1 .PC6/RESET" />
    <trace from=".SW1 .pin1" to=".U1 .PC6/RESET" />
    <trace from=".SW1 .pin2" to="net.GND" />
    <trace from=".R2 .pos" to="net.V5" />
    <trace from=".R2 .neg" to=".LED_PWR .pos" />
    <trace from=".LED_PWR .neg" to="net.GND" />
    <trace from=".R3 .pos" to=".U1 .PB5/D13/SCK" />
    <trace from=".R3 .neg" to=".LED_D13 .pos" />
    <trace from=".LED_D13 .neg" to="net.GND" />
  </board>
)
}

Component Reference

Component	Designator	JLCPCB Part #	Description
ATmega328P-PU	U1	C82151	8-bit AVR microcontroller (DIP-28)
AMS1117-5.0	U2	C6187	5V LDO voltage regulator
16MHz Crystal/Resonator	Y1	C126020	16MHz ceramic resonator
22pF Capacitor	C1, C2	C1555	Crystal load capacitors (0402)
100nF Capacitor	C3	C307514	ATmega328P decoupling capacitor (0402)
10uF Capacitor	C4, C5	C15850	LDO input/output bulk capacitors (0805)
10k Resistor	R1	C25744	Reset pull-up resistor (0402)
1k Resistor	R2, R3	C11702	LED current-limiting resistors (0402)
Green LED	LED_PWR	C72043	Power indicator LED (0603)
Yellow LED	LED_D13	C72044	D13 user indicator LED (0603)
Push Button	SW1	—	Reset button
USB-C Connector	J1	—	Power input (via SmdUsbC)

ATmega328P Pin Mapping

The ATmega328P-PU pin names map to Arduino pin numbers as follows:

ATmega328P Pin	Arduino Pin	Function
PC0	A0	Analog input 0
PC1	A1	Analog input 1
PC2	A2	Analog input 2
PC3	A3	Analog input 3
PC4	A4/SDA	Analog input 4 / I2C Data
PC5	A5/SCL	Analog input 5 / I2C Clock
PD0	D0/RX	Digital pin 0 / Serial Receive
PD1	D1/TX	Digital pin 1 / Serial Transmit
PD2	D2	Digital pin 2 / INT0
PD3	D3~	Digital pin 3 / INT1 / PWM
PD4	D4	Digital pin 4
PD5	D5~	Digital pin 5 / PWM
PD6	D6~	Digital pin 6 / PWM
PD7	D7	Digital pin 7
PB0	D8	Digital pin 8
PB1	D9~	Digital pin 9 / PWM
PB2	D10~	Digital pin 10 / PWM / SS
PB3	D11~	Digital pin 11 / PWM / MOSI
PB4	D12	Digital pin 12 / MISO
PB5	D13	Digital pin 13 / SCK / Built-in LED

Pins marked with ~ support PWM output via the analogWrite() function.

Next Steps

• Add a USB-to-Serial bridge — Add an ATmega16U2 or CH340G chip to enable programming the ATmega328P directly via USB
• Add pin headers — Add female pin headers to expose all I/O pins for prototyping, just like the real Arduino Uno
• Add ICSP header — Add a 2x3 pin header for In-Circuit Serial Programming
• Add a 3.3V regulator — Add an LP2985-3.3 to provide a 3.3V output for low-voltage peripherals
• Order the PCB — Follow the Ordering Prototypes guide to manufacture your board