In the realm of DIY electronics, few challenges are as satisfying as designing a project that combines practical utility with hands-on learning. Among the many components hobbyists explore, the AN2131QC stands out as a versatile microcontroller built specifically for USB applications. Manufactured by Cypress Semiconductor, this chip is a part of the EZ-USB series and was designed to streamline the development of USB peripherals. While it's now somewhat dated, the AN2131QC still sees use in niche projects and vintage hardware repairs, making it a unique component for a dedicated DIY project.

This article outlines a complete project to build a USB-based data logger using the AN2131QC. This is not a general guide on how to use the component, nor is it a sweeping overview of USB communications or firmware writing. Instead, we’re diving into one specific project, focusing on the experience of bringing it to life, from initial concept to final assembly.

The Concept: Portable USB Data Logger

The idea for this project is simple: a small, USB-powered device that can be plugged into any system with a USB port to record sensor data over time. The USB interface, powered by the AN2131QC, handles communication with a host computer. A simple EEPROM allows firmware storage. The logger includes a temperature and humidity sensor and stores data onto a microSD card using a basic serial interface.

This project has practical real-world uses, such as tracking environmental conditions in a computer lab, a greenhouse, or even inside a shipping package. More importantly for us, it’s an ideal way to explore the capabilities of the AN2131QC and understand how USB interfaces can work in a DIY setting.

Planning the Build

Before diving into soldering or wiring, it’s crucial to lay out what this device will need. At the heart of the system is the AN2131QC microcontroller. It features an integrated USB 1.1 controller and an 8051-compatible core, making it ideal for custom USB device development. Paired with an I²C EEPROM, the microcontroller can be programmed with device firmware that runs on power-up.

The supporting cast of components includes:

● AN2131QC: the USB microcontroller

● 24LC64 EEPROM: for firmware storage

● DHT22: temperature and humidity sensor

● MicroSD breakout board: for data storage

● Voltage regulators and level shifters: to ensure safe power and data line levels

● USB connector: type B or micro-B, depending on form factor

● Capacitors and resistors: for signal conditioning and stability

● Custom PCB or perfboard: depending on whether the builder prefers prototyping or final production

With the block diagram mentally mapped out, the next phase involves physically assembling the core USB circuitry.

Setting Up the USB Core

The AN2131QC is a 48-pin chip that requires some precision in handling. In our case, we mount it on a breakout adapter, which makes it easier to interface with surrounding components using headers or direct soldering.

Powering the AN2131QC is straightforward—it operates at 3.3V for I/O, though it can tolerate 5V in some configurations. A linear regulator ensures the USB 5V is stepped down safely. Decoupling capacitors are installed near the power pins to smooth out any voltage ripples, which is standard practice for any microcontroller-based system.

The USB connection itself is made via a standard USB B or micro-B connector. The D+ and D− lines are routed carefully, with short, matched-length traces to maintain signal integrity. Proper resistors (typically 1.5k pull-ups and 68-ohm series resistors) are used on the USB lines to conform to USB full-speed requirements.

With this done, we now have a system that can connect to a host computer and be recognized as a USB device, assuming firmware is present.

Adding the Firmware Storage

The AN2131QC loads its firmware from an external EEPROM each time it powers up. For our project, we’re using the 24LC64—a 64-kilobit EEPROM accessed via I²C. It’s important to wire the SDA and SCL lines to the appropriate pins on the AN2131QC, and also pull them up with resistors to ensure reliable communication.

In a typical workflow, the firmware would be developed and uploaded to the EEPROM using a USB firmware loader tool provided by Cypress, often via the host USB connection itself. In our project context, the firmware contains basic logic to handle USB enumeration and initiate data collection from the DHT22 sensor.

Integrating the DHT22 Sensor

Next, we bring in the DHT22, which is a digital sensor capable of measuring both temperature and humidity. It operates at 3.3V or 5V, which makes interfacing simple. Its single-wire protocol means it only needs one data line connected to a general-purpose I/O pin on the AN2131QC.

Physically, the sensor is mounted away from the main PCB to avoid heat distortion from the microcontroller. A short cable can connect it to the board, and a pull-up resistor ensures clean digital transitions on the data line.

In firmware, polling the sensor at regular intervals (say, every 5 seconds) allows our logger to build a consistent data timeline.

Storing the Data on a MicroSD Card

To store the collected environmental data, we use a microSD card connected via a breakout board. These boards often use SPI for communication, which is compatible with many 8051-based microcontrollers like the one inside the AN2131QC.

A key design consideration is voltage level: SD cards require 3.3V logic, while some parts of our system may run at 5V. Level shifters (or voltage dividers for simplicity) are used to bridge this gap, especially on the MOSI, MISO, SCK, and CS lines.

We physically place the SD card socket at the edge of the PCB for easy access. Once wired, our firmware can write plain-text CSV files to the card, logging timestamps and sensor readings in a format that’s easy to analyze later on a PC.

Enclosure and Assembly

With the functional circuit ready, attention turns to packaging. We use a small plastic enclosure with cutouts for the USB port, microSD card slot, and an opening for the DHT22 sensor. For airflow, we drill small ventilation holes around the sensor port.

Mounting the PCB inside the enclosure involves plastic standoffs and screws. A small LED added to the board gives visual feedback—blinking during logging, steady-on during idle, or flashing rapidly in case of errors.

The entire build fits into a device no larger than a matchbox, making it highly portable and easy to tuck into any environment where monitoring is needed.

Real-World Usage and Testing

Once assembled and programmed, testing is straightforward. Plug the data logger into any USB port and ensure that it enumerates correctly. The host PC should recognize it as a USB device. Using terminal software or a simple Python script, we can send commands to the device and begin data logging.

Placing the logger in a greenhouse, a refrigerator, or even your backpack, the device begins recording ambient conditions at set intervals. Later, removing the microSD card allows you to pull the data for graphing or analysis in software like Excel or Google Sheets.

Challenges and Lessons Learned

Using the AN2131QC in a DIY project like this isn’t without challenges. For one, it’s not as beginner-friendly as modern Arduino-based microcontrollers. The 8051 architecture and firmware loading process require patience and attention to detail. USB debugging, in particular, can be a pain if you’re not familiar with protocol-level behaviors.

Also, the limited I/O on the AN2131QC means you need to plan your connections carefully. If you need additional sensors or interfaces, you’ll likely need to implement multiplexing or use I²C expanders.

That said, completing a USB data logger using this chip is a deeply rewarding experience. It forces you to think beyond canned libraries and introduces you to the inner workings of USB device design, a topic often abstracted away in modern development platforms.

Final Thoughts

This DIY USB data logger, built around the AN2131QC, is more than just a tool—it’s a gateway to understanding how low-level USB communication works, how microcontrollers load firmware from external memory, and how real-world signals can be captured, interpreted, and stored.

For hobbyists interested in digging deeper into embedded systems and USB development, this project provides a meaningful challenge. It marries hardware and firmware design, encourages careful component selection, and results in a practical, working device you can use or improve further.

So if you’re looking for a project that’s off the beaten path, consider reviving the AN2131QC and building your own data logger. It’s a small window into the fascinating world of USB device design—and a memorable journey in electronics craftsmanship.