Arduino Uno ATmega328P Schematic Tutorial

Overview

This tutorial guides you through designing a complete Arduino Uno compatible board using tscircuit. The Arduino Uno is one of the most popular microcontroller boards, and understanding its circuit design is foundational for electronics engineers.

System Architecture

The Arduino Uno consists of four main subsystems:

MCU: ATmega328P — the heart of the board
USB Interface: ATmega16U2 — USB-to-serial conversion
Power: 5V regulator and auto-switching (USB/external)
I/O: Digital/analog pin headers, ICSP connectors

Components

Ref	Part	Description	Footprint
U1	ATmega328P-PU	Main MCU, 32KB Flash, 2KB SRAM	`dip-28`
U2	ATmega16U2	USB-to-Serial interface	`tqfp-32`
U3	NCP1117-5.0	5V LDO regulator	`sot-223`
J1	USB-B	USB Type-B connector	`usb-b`
Y1	16 MHz Crystal	MCU clock source	`hc49`
R1	Reset pull-up	10kΩ resistor	`0805`
C1-C3	Decoupling caps	100nF ceramic	`0805`
C4	Crystal load (×2)	22pF ceramic	`0805`
LED_P	Power LED	Green indicator	`0805`
LED_L	D13 LED	Yellow activity LED	`0805`
JP1-JP4	Pin headers	Digital/Analog I/O	`pin_header_1x15`

Step 1: ATmega328P MCU

The ATmega328P is an 8-bit AVR microcontroller with 32KB of flash memory:

<chip
  name="U1"
  footprint="dip-28"
  pcbX={0}
  pcbY={0}
/>

Key Pin Assignments

Pin	Function	Description
1	RESET	Active-low reset, pulled up to 5V
2-4	PD0-PD2	Digital I/O 0-2 (RX, TX, INT0)
5-6	PD3-PD4	Digital I/O 3-4 (INT1, XCK)
7-8	VCC/GND	Power supply
9-10	PB6-PB7	XTAL1/XTAL2 (crystal oscillator)
11-13	PB0-PB2	Digital I/O 8-10 (SPI)
14-17	PB3-PB5	Digital I/O 11-13 (SPI, PWM)
18-19	XTAL2/GND	Crystal and ground
20	AVCC	ADC power supply (connect to VCC)
21	AREF	ADC reference voltage
22-27	PC0-PC5	Analog inputs A0-A5
28	VCC	Power supply

Step 2: 16 MHz Crystal Oscillator

The ATmega328P requires an external 16 MHz crystal for clock generation:

<crystal
  name="Y1"
  footprint="hc49"
  frequency="16MHz"
  pcbX={-5}
  pcbY={-10}
/>

<capacitor name="C_Y1A" footprint="0805" capacitance="22pF" pcbX={-6} pcbY={-11} />
<capacitor name="C_Y1B" footprint="0805" capacitance="22pF" pcbX={-4} pcbY={-11} />

<trace from=".Y1 .1" to=".U1 > .XTAL1" />
<trace from=".Y1 .2" to=".U1 > .XTAL2" />
<trace from=".C_Y1A .pos" to=".Y1 .1" />
<trace from=".C_Y1B .pos" to=".Y1 .2" />
<trace from=".C_Y1A .neg" to="net.GND" />
<trace from=".C_Y1B .neg" to="net.GND" />

Step 3: Reset Circuit

The reset circuit ensures the MCU starts in a known state:

<resistor name="R1" footprint="0805" resistance="10k" pcbX={5} pcbY={-10} />
<capacitor name="C_RST" footprint="0805" capacitance="100nF" pcbX={7} pcbY={-10} />

<trace from="net.5V" to=".R1 .pos" />
<trace from=".R1 .neg" to=".U1 > .RESET" />
<trace from=".R1 .neg" to=".C_RST .pos" />
<trace from=".C_RST .neg" to="net.GND" />

The pull-up resistor keeps RESET high during normal operation. The capacitor provides noise filtering.

Step 4: Power Regulation

The Arduino can be powered via USB or an external barrel jack (7-12V):

<chip name="U3" footprint="sot-223" pcbX={-15} pcbY={-10} />

<trace from=".U3 .vin" to="net.VIN" />
<trace from=".U3 .gnd" to="net.GND" />
<trace from=".U3 .vout" to="net.5V" />

<capacitor name="C_IN" footprint="0805" capacitance="10uF" />
<capacitor name="C_OUT" footprint="0805" capacitance="10uF" />

<trace from="net.VIN" to=".C_IN .pos" />
<trace from="net.5V" to=".C_OUT .pos" />
<trace from=".C_IN .neg" to="net.GND" />
<trace from=".C_OUT .neg" to="net.GND" />

Step 5: LED Indicators

<led name="LED_P" color="green" footprint="0805" pcbX={-15} pcbY={10} />
<resistor name="R_LED_P" footprint="0805" resistance="330" pcbX={-17} pcbY={10} />

<trace from="net.5V" to=".R_LED_P .pos" />
<trace from=".R_LED_P .neg" to=".LED_P .pos" />
<trace from=".LED_P .neg" to="net.GND" />

<led name="LED_L" color="yellow" footprint="0805" pcbX={-10} pcbY={10} />
<resistor name="R_LED_L" footprint="0805" resistance="330" pcbX={-12} pcbY={10} />

<trace from=".U1 > .PB5" to=".R_LED_L .pos" />
<trace from=".R_LED_L .neg" to=".LED_L .pos" />
<trace from=".LED_L .neg" to="net.GND" />

Step 6: Pin Headers

<chip name="JP1" footprint="pin_header_1x15" pcbX={-25} pcbY={0} />
<chip name="JP2" footprint="pin_header_1x15" pcbX={25} pcbY={0} />
<chip name="JP3" footprint="pin_header_1x6" pcbX={-25} pcbY={-15} />
<chip name="JP4" footprint="pin_header_1x6" pcbX={25} pcbY={-15} />

Digital I/O Mapping

Header Pin	ATmega328P Pin	Arduino Pin
JP1:1	PD0	D0 (RX)
JP1:2	PD1	D1 (TX)
JP1:3	PD2	D2
JP1:4	PD3	D3 (PWM)
JP1:5	PD4	D4
JP1:6	PD5	D5 (PWM)
JP1:7	PD6	D6 (PWM)
JP1:8	PD7	D7

Complete Circuit

export default () => {
  return (
    <board width="70mm" height="55mm">
      {/* ATmega328P MCU */}
      <chip name="U1" footprint="dip-28" pcbX={0} pcbY={0} />
      
      {/* 16 MHz Crystal */}
      <crystal name="Y1" footprint="hc49" frequency="16MHz" pcbX={-5} pcbY={-10} />
      <capacitor name="C_Y1A" footprint="0805" capacitance="22pF" pcbX={-6} pcbY={-11} />
      <capacitor name="C_Y1B" footprint="0805" capacitance="22pF" pcbX={-4} pcbY={-11} />
      
      {/* Reset Circuit */}
      <resistor name="R1" footprint="0805" resistance="10k" pcbX={5} pcbY={-10} />
      <capacitor name="C_RST" footprint="0805" capacitance="100nF" pcbX={7} pcbY={-10} />
      
      {/* Power Regulator */}
      <chip name="U3" footprint="sot-223" pcbX={-15} pcbY={-10} />
      <capacitor name="C_IN" footprint="0805" capacitance="10uF" />
      <capacitor name="C_OUT" footprint="0805" capacitance="10uF" />
      
      {/* LEDs */}
      <led name="LED_P" color="green" footprint="0805" pcbX={-15} pcbY={10} />
      <resistor name="R_LED_P" footprint="0805" resistance="330" pcbX={-17} pcbY={10} />
      <led name="LED_L" color="yellow" footprint="0805" pcbX={-10} pcbY={10} />
      <resistor name="R_LED_L" footprint="0805" resistance="330" pcbX={-12} pcbY={10} />
      
      {/* Pin Headers */}
      <chip name="JP1" footprint="pin_header_1x15" pcbX={-25} pcbY={0} />
      <chip name="JP2" footprint="pin_header_1x15" pcbX={25} pcbY={0} />
      <chip name="JP3" footprint="pin_header_1x6" pcbX={-25} pcbY={-15} />
      <chip name="JP4" footprint="pin_header_1x6" pcbX={25} pcbY={-15} />
    </board>
  )
}

PCB Layout Guidelines

Power Traces

5V supply: Minimum 0.5mm width for 500mA current
3.3V supply: Minimum 0.3mm width
GND plane: Solid ground plane on bottom layer

Crystal Routing

Keep traces short: Under 10mm from crystal to MCU pins
Guard ring: Route ground pour around crystal traces
No vias: Avoid vias in crystal signal paths

Decoupling Capacitors

Placement: Within 2mm of MCU power pins
Trace width: 0.3mm minimum
Via placement: Use multiple vias for ground connections

Manufacturing Files

# Generate Gerbers
tsci build --gerber

# Generate BOM
tsci build --bom

# Generate Assembly
tsci build --pick-and-place

Cost Estimate

Component	Unit Cost	Qty	Total
ATmega328P-PU	$1.80	1	$1.80
ATmega16U2	$1.20	1	$1.20
NCP1117-5.0	$0.20	1	$0.20
USB-B Connector	$0.15	1	$0.15
16 MHz Crystal	$0.15	1	$0.15
Passives (×15)	$0.02	15	$0.30
Pin Headers (×4)	$0.10	4	$0.40
PCB (2-layer)	$1.00	1	$1.00
Total			$5.20

Summary

You've designed a complete Arduino Uno compatible board covering:

ATmega328P MCU configuration
16 MHz crystal oscillator circuit
Reset circuit with noise filtering
Power regulation and filtering
LED indicators for status and activity
Pin header layout matching Arduino Uno form factor
Manufacturing file generation

This design is fully compatible with the Arduino IDE and bootloader.

Overview​

System Architecture​

Components​

Step 1: ATmega328P MCU​

Key Pin Assignments​

Step 2: 16 MHz Crystal Oscillator​

Step 3: Reset Circuit​

Step 4: Power Regulation​

Step 5: LED Indicators​

Step 6: Pin Headers​

Digital I/O Mapping​

Complete Circuit​

PCB Layout Guidelines​

Power Traces​

Crystal Routing​

Decoupling Capacitors​

Manufacturing Files​

Cost Estimate​

Summary​