RP2040 Minimal Circuit Design: Build Your Own Custom Board (2025)

Introduction

The RP2040 is Raspberry Pi's first silicon, and it's become incredibly popular for custom hardware projects. While the Raspberry Pi Pico board is great for prototyping, many projects require custom form factors, different I/O configurations, or cost-optimized designs for production.

This guide provides everything you need to design your own RP2040-based circuit board from scratch. We'll cover the minimal circuitry required, explain each design decision, and provide JLCPCB-ready part numbers so you can go from schematic to assembled board quickly.

Why Design with the RP2040?

Minimum Circuit Requirements

Unlike many microcontrollers that can run with just power and a few capacitors, the RP2040 has specific requirements for a functional system:

System Block Diagram

Before diving into each subsystem, here's how all the pieces connect:

Power Supply Design

Power Architecture Overview

The RP2040 requires two voltage rails to operate:

• IOVDD (3.3V) - Powers all I/O, USB PHY, and peripherals
• DVDD (1.1V) - Powers the digital core (ARM cores, SRAM)

The RP2040 includes an internal linear regulator that generates 1.1V from the 3.3V supply. This simplifies external power design - you only need to provide 3.3V.

Internal 1.1V Regulator

The internal regulator requires:

• VREG_VIN - Connect to 3.3V with 1µF capacitor
• VREG_VOUT - Connect to all DVDD pins with 1µF capacitor

External 3.3V Supply Options

For USB-powered designs, you need to convert 5V USB to 3.3V:

Decoupling Capacitors

Proper decoupling is critical for RP2040 stability. The chip has multiple power pins that each need their own capacitor:

Crystal Oscillator

While the RP2040 has an internal oscillator, an external 12MHz crystal is required for accurate USB timing and stable operation.

Crystal Selection

The crystal must meet these specifications:

• Frequency: 12.000 MHz
• Load Capacitance: Typically 10-20pF
• ESR: <100Ω recommended
• Frequency Tolerance: ±30ppm or better

Load Capacitor Calculation

The load capacitors (CL1 and CL2) must be calculated based on the crystal's specified load capacitance and PCB stray capacitance:

C_load = (CL1 × CL2) / (CL1 + CL2) + C_stray

For equal capacitors (CL1 = CL2 = C):
C = 2 × (C_load - C_stray)

Example with 20pF load capacitance and 3pF stray:
C = 2 × (20pF - 3pF) = 34pF

Nearest standard values: 33pF or 27pF

Crystal Layout Guidelines

• Place crystal as close as possible to XIN/XOUT pins (pins 20/21)
• Keep load capacitors immediately adjacent to crystal pads
• Route XIN/XOUT traces away from high-speed signals (especially QSPI)
• Consider a ground guard ring around the crystal for noise immunity
• Keep the crystal on the opposite side of the chip from QSPI flash

QSPI Flash Memory

The RP2040 has no internal flash memory - all program code runs from an external QSPI flash chip. This is a critical component that must be chosen and connected carefully.

Flash Chip Selection

The RP2040 bootloader is optimized for Winbond W25Q series flash. While other chips may work, Winbond is the safest choice:

Flash Connections

QSPI_SS Pull-up: Add a 10kΩ pull-up resistor from QSPI_SS to 3.3V. This ensures the flash chip-select is high during power-up, preventing the flash from entering an undefined state.

Flash Layout Guidelines

• Place flash as close as possible to RP2040 QSPI pins
• Maximum trace length: 20mm (shorter is better)
• Recommended trace width: 0.15mm (6 mil)
• Keep clock trace as the longest among all QSPI signals
• Add 100nF decoupling capacitor directly at flash VCC pin
• Maintain solid ground plane under flash and traces

USB Interface

USB is the primary programming interface for the RP2040, enabling the convenient UF2 drag-and-drop firmware upload.

USB-C Connector Design

For modern designs, USB-C is recommended over Micro-USB due to its reversibility and durability. Key connections:

ESD Protection

While not strictly required, ESD protection on USB lines is highly recommended for any product that will be handled by users:

USB Trace Layout

USB is a differential signal requiring controlled impedance:

• Target impedance: 90Ω differential
• Trace width: 0.8mm (for 1.6mm board thickness)
• Trace spacing: 0.15mm between D+ and D-
• 27Ω series resistors: Place close to RP2040 pins
• Length matching: Keep D+ and D- equal length

Boot Mode Circuit

The RP2040 uses a ROM-based bootloader that checks the BOOTSEL condition at power-up. Understanding the boot sequence is essential for designing a reliable programming interface.

BOOTSEL Button Design

The BOOTSEL function uses the QSPI_SS pin. When QSPI_SS is held low during reset, the RP2040 enters USB mass storage mode for firmware upload:

QSPI_SS (pin 57)
    |
    +--- 10kΩ pull-up to 3.3V (optional but recommended)
    |
    +--- 1kΩ series resistor --- BOOTSEL button --- GND

Boot Sequence:
1. Press and hold BOOTSEL button
2. Apply power (or press RESET while holding BOOTSEL)
3. RP2040 sees QSPI_SS low → enters USB boot mode
4. Release button
5. Drag UF2 file to the USB drive

Reset Circuit

The RUN pin (pin 26) is the RP2040's reset input (active-low):

RUN (pin 26)
    |
    +--- 10kΩ pull-up to 3.3V
    |
    +--- 100nF capacitor to GND (debounce/filter)
    |
    +--- RESET button --- GND

Typical use:
1. Power-on: RUN pulled high, chip starts
2. Press RESET: RUN pulled low, chip resets
3. Release: RUN returns high, chip restarts
4. For BOOTSEL mode: Hold BOOTSEL, press RESET

GPIO Breakout

The RP2040 provides 30 GPIO pins with flexible peripheral mapping. For a minimal design, consider which GPIOs to expose:

PCB Layout Guidelines

Layer Stackup

The official minimal design uses a 2-layer board, but 4-layer is recommended for better performance and easier routing:

Ground Plane Strategy

The RP2040's center ground pad is the only ground connectionfor the entire chip. This makes ground plane design critical:

• Use 9 vias minimum under the ground pad for thermal and electrical connection
• Create solid ground plane directly under the RP2040
• Connect all decoupling capacitor grounds to this plane with vias
• Maintain continuous ground under QSPI traces
• Use ground stitching vias around the perimeter

Critical Signal Routing

Bill of Materials

JLCPCB Basic Parts

Using JLCPCB Basic Parts minimizes assembly fees ($3 per Extended part type). Here are basic parts where possible:

Complete BOM

Estimated cost: ~$2-3 per board for components (5 board minimum), plus ~$5 assembly fee + ~$3 per extended part type.

Common Mistakes to Avoid

Testing and Debugging

After assembly, follow this testing sequence:

1. Visual inspection: Check for solder bridges, especially on the RP2040 and USB connector. The QFN-56 package is prone to bridging.
2. Power check (no USB): Measure 3.3V and 1.1V rails before connecting USB. Verify they're stable and correct voltage.
3. BOOTSEL test: Hold BOOTSEL, connect USB. Computer should show a new USB drive called "RPI-RP2".
4. Flash test: Drag a UF2 file (like the Pico blink example) to the drive. It should disappear and the code should run.
5. UART test: Connect a USB-UART adapter to GPIO0/GPIO1 and verify serial output works.

Conclusion

Designing a custom RP2040 board requires attention to detail in power supply, flash memory, crystal, and USB circuits. However, with the excellent documentation from Raspberry Pi and the growing community of open-source designs, it's an achievable project for anyone with basic PCB design experience.

The key points to remember:

• Use Winbond W25Q series flash for guaranteed compatibility
• Pay special attention to the center ground pad connection
• Keep QSPI traces short and crystal close to the chip
• Don't skimp on decoupling capacitors
• Include BOOTSEL and RESET buttons for easy programming

Download Raspberry Pi's official KiCad minimal design files as a starting point, and don't hesitate to reference other open-source RP2040 projects for inspiration.