Raspberry Pi Pico with Custom Chip: How It Works

Yes, you can use a Raspberry Pi Pico with a custom chip, but the Pico itself is not designed to have its silicon replaced—it is a complete microcontroller board. Instead, “custom chip” usually means one of three things: adding an external chip over SPI/I2C, using a Pico as the brain for a custom PCB you design, or programming the Pico’s own RP2040 microcontroller to act like a custom chip. This article explains each path, what is involved, and how to choose the right approach for your project.

What the Pico Actually Is

The Raspberry Pi Pico is a low-cost development board built around the RP2040 microcontroller, designed by Raspberry Pi Ltd. The RP2040 is a dual-core ARM Cortex-M0+ processor running at up to 133 MHz, with 264 KB of SRAM and 2 MB of flash memory on the board. Unlike a single-board computer like the Raspberry Pi 4, the Pico has no operating system; you write bare-metal C/C++ or MicroPython code that runs directly on the chip.

Because the RP2040 is a microcontroller, it is meant to interact with the physical world through its GPIO pins, ADCs, PWM, and communication peripherals. When people say “custom chip,” they often mean an ASIC (Application-Specific Integrated Circuit) or an FPGA (Field-Programmable Gate Array). The Pico cannot host an ASIC internally, but it can communicate with one externally, or you can use the Pico as the controller for a custom circuit board that includes your own chip.

Option 1: Add an External Custom Chip via SPI or I2C

The most common way to use a custom chip with a Pico is to connect it as an external peripheral. The RP2040 supports standard communication protocols:

• SPI – fast, full-duplex, uses 4 wires (MOSI, MISO, SCK, CS). Good for high-speed data, like reading from a custom sensor or memory chip.
• I2C – two wires (SDA, SCL), slower but simpler, good for connecting multiple devices on the same bus.
• UART – serial communication, often used for debugging or talking to modules.

To connect a custom chip, you need to know its datasheet: the voltage levels, register map, and protocol timing. Then you write a driver in C/C++ or MicroPython that sends the correct commands. For example, if you have a custom temperature sensor chip that outputs data over I2C, you would wire SDA and SCL to the Pico’s I2C pins (GP4 and GP5 by default), pull-up resistors to 3.3V, and then use the machine.I2C library in MicroPython to read the sensor’s registers.

This approach works with almost any chip that speaks a standard protocol, including ASICs, ADCs, DACs, and memory chips. The main limitation is that the Pico runs at 3.3V logic, so if your custom chip needs 5V, you’ll need level shifting.

Option 2: Design a Custom PCB with the RP2040

Instead of using the ready-made Pico board, you can design your own printed circuit board that includes the RP2040 chip directly. This is often what people mean when they say “custom chip”—they want the microcontroller integrated into their own hardware, not a separate dev board.

To do this, you need to design a PCB with the RP2040, its required support components (crystal oscillator, flash memory, voltage regulator, decoupling capacitors), and your custom chip. The RP2040 is available as a bare chip (QFN-56 package) and is designed to be soldered onto a PCB. Raspberry Pi provides the hardware design files and a reference design for the Pico, which you can adapt.

This route requires experience with PCB design (e.g., using KiCad or Altium), soldering fine-pitch components, and understanding the RP2040’s power and boot requirements. The advantage is that you get a compact, production-ready board tailored to your application, with the custom chip integrated on the same PCB. The downside is the learning curve and cost of prototyping.

Option 3: Program the RP2040 to Act Like a Custom Chip

Sometimes the “custom chip” you need is just a specific function, like a custom protocol converter or a signal generator. In that case, you can program the Pico to emulate that behavior in software. The RP2040 is powerful enough to handle many tasks that would otherwise require a dedicated chip.

For example, you could use the PIO (Programmable I/O) state machines on the RP2040 to generate custom waveforms or implement non-standard serial protocols. The PIO is a unique feature that lets you create custom hardware-like logic in software, running at high speed. This is ideal for tasks like driving LED strips, reading unusual sensors, or implementing a custom bus protocol.

You would write a program in C/C++ or MicroPython that configures the PIO to do what you want. This is often easier and cheaper than designing a custom ASIC, especially for low-volume projects. However, it is still software, so it is limited by the clock speed and available memory.

How to Choose the Right Approach

Deciding which method to use depends on your project’s goals:

• If you have a specific external chip (like a sensor or a specialized IC), go with Option 1: connect it via SPI/I2C.
• If you want to mass-produce a product with the Pico’s brain and your custom chip, go with Option 2: design a custom PCB.
• If you need a custom function but no physical chip, go with Option 3: program the Pico to do it.

Also consider the cost and time. Option 1 is the fastest and cheapest for prototyping. Option 2 is more expensive upfront but can reduce cost per unit in volume. Option 3 is free in hardware but requires programming effort.

Common Problems and How to Avoid Them

Here are some typical issues when working with custom chips and the Pico:

• Voltage mismatch – The Pico is 3.3V logic. If your custom chip expects 5V, you need a level shifter or you risk damaging the Pico. Check the datasheet.
• Missing pull-up resistors – I2C requires pull-up resistors (typically 4.7kΩ) on SDA and SCL. Without them, communication will fail.
• Incorrect register addresses – When talking to a custom chip, you must use the exact register addresses from the datasheet. A common mistake is misreading the datasheet and writing to the wrong address.
• Power supply issues – Custom chips may draw more current than the Pico’s 3.3V regulator can supply. Use a separate power supply for the chip and common ground.
• Soldering problems – If you are assembling a custom PCB, fine-pitch QFN chips are hard to solder by hand. Use a hot-air rework station or consider using a breakout board.

Costs and Tools

The cost of using a custom chip with a Pico varies widely. The Pico board itself costs about $4. A custom PCB can cost anywhere from a few dollars for a simple prototype to hundreds for a complex multi-layer board. The custom chip itself could be anything from a $1 sensor to a $50 ASIC.

Tools you might need:

• Breadboard and jumper wires for prototyping (Option 1).
• PCB design software like KiCad (free) for Option 2.
• Soldering iron, solder, and possibly a hot-air station for assembling custom boards.
• Logic analyzer or oscilloscope to debug communication between the Pico and the chip.

Safety and Best Practices

When working with electronics, always follow basic safety:

• Disconnect power before changing wiring.
• Check polarity of power connections to avoid short circuits.
• Use a current-limiting resistor for LEDs and other components.
• Be careful with electrostatic discharge (ESD) when handling bare chips.

For custom PCBs, follow the manufacturer’s design rules and double-check your schematic before ordering. If you are unsure about a design, consult with an experienced engineer.

Frequently Asked Questions

Can I put a custom chip inside the Pico?

No, the Pico is a fixed board with the RP2040 soldered on. You cannot replace the chip. But you can connect an external custom chip via the GPIO pins.

Do I need to know C or MicroPython to use a custom chip?

Yes, you need to write code to communicate with the chip. MicroPython is easier for beginners; C is faster and gives more control. Both are supported on the Pico.

What is PIO and why is it useful for custom chips?

PIO (Programmable I/O) is a set of state machines on the RP2040 that can generate and read custom digital signals in real time. It lets you implement protocols that the chip doesn’t have built-in hardware for, making it easier to interface with unusual custom chips.

Can I use a 5V custom chip with the Pico?

You can, but you need to level-shift the signals. The Pico’s GPIO pins are not 5V tolerant, so feeding 5V into them can damage the RP2040. Use a level shifter or a voltage divider.

Is designing a custom PCB with the RP2040 hard?

It is challenging but doable. You need to follow the reference design, handle the QFN package, and ensure proper power decoupling. Many hobbyists have done it with KiCad and a good soldering setup.

What if my custom chip uses a protocol like CAN or USB?

The RP2040 has USB support, but for CAN you will need an external CAN controller chip. The Pico does not have built-in CAN. You can connect a CAN transceiver and controller via SPI, which is a common approach.

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