Using a Tiny Code Reader on a Raspberry Pi Pico

Example code that shows how to interface the Raspberry Pi Pico to Useful Sensor's Tiny Code Reader board over I2C.

Introduction

The Tiny Code Reader is a small, low-cost hardware module that reads QR codes. It's designed to be a simple way to provision a system, for example by providing the wifi network name and password, or to provide input when there's no keyboard.

There's a detailed developer guide available, but this project has sample code that shows you specifically how to get the module up and running with a Raspberry Pi Pico using C.

Building

Make sure that you're able to build and run the standard Raspberry Pi Pico examples, like blink. You can find general instructions on this in the Pico Getting Started Guide, and here's a Colab notebook that might help. The main issues to watch out for are ensuring that PICO_SDK_PATH is set as an environment variable, and points to the right location.

Once you're ready, run the following commands from within this repository's folder to create the build files:

mkdir build
cd build
cmake ..

After that succeeds, you should be able to run the compilation stage:

make

There should now be a binary at build/code_reading_example.utf2. You can install this on your Pico board, though it won't work until you wire up the sensor to the right pins.

Wiring Information

Wiring up the device requires 4 jumpers, to connect VDD, GND, SDA and SCL. The example here uses I2C port 0, which is assigned to GPIO4 (SDA, pin 6) and GPIO5 (SCL, pin 7) in software. Power is supplied from 3V3(OUT) (pin 36), with ground attached to GND (pin 38).

Follow the wiring scheme shown below:

If you're using Qwiic connectors, the colors will be black for GND, red for 3.3V, blue for SDA, and yellow for SDC.

Running Code Detection

Once you have the module wired up, connect the Pico over USB while holding the bootsel button to mount it as a storage device, copy the face_detection_example.utf2 file over to it, and it should begin running. To see the logging output you'll need to set up minicom or a similar tool. Once that is done, you should start to see information about the codes it spots, or error messages. If you hold the sensor so that it's pointing at a QR code you should see output like this:

Found 'http://en.m.wikipedia.org'

To get this output, I opened an example QR code on my phone and held it about fifteen centimeters or six inches from the module, facing the camera.

Troubleshooting

Power

The first thing to check is that the sensor is receiving power through the VDD and GND wires. The simplest way to test this is to look at the LED on the front of the module. Whenever it has power, it should be flashing blue multiple times a second. If this isn't happening then there's likely to be a wiring issue with the power connections.

Communication

If you see connection errors when running the code detection example, you may have an issue with your wiring. To help track down what's going wrong, you can copy over the scan_i2c.utf2 file to the board, and this will display which I2C devices are available in the logs. Here's an example from a board that's set up correctly:

00 .  .  .  .  .  .  .  .  .  .  .  .  @  .  .  .
10 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .
20 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .
30 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .
40 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .
50 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .
60 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .
70 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .
Done.

The important entry is the first @ shown on line starting with 00. This indicates that there's a response on the address 0x0C, which is the fixed location of the person sensor. If the @ isn't present at this point in the grid then it means the sensor isn't responding to I2C messages as it should be. The most likely cause is that there's a wiring problem, so if you hit this you should double-check that the SDA and SCL wires are going to the right pins.

Running Wifi Provisioning

I often need to connect microcontrollers to Wifi, but that usually requires reprogramming them or at least editing a text file. QR codes make it possible to easily set the Wifi name and password at runtime.

On Android you can just go to Wifi settings, choose share, and it will display a code containing the name and password of the network you're currently connected to. It's also possible to do the same thing on iOS but it's a bit more fiddly.

This example application shows how to read a Wifi settings QR code and use it to automatically connect. To use it, copy the wifi_provisioning.uf2 file from build/ onto your Pico W. It will start the Tiny Code Reader, and then once you display a QR code generated using one of the methods above it should use the SSID name and password to connect to the network. You can verify that it worked by connecting to a serial port and looking for output like:

Trying to connect to <SSID>:<Password>
Connected to 'WIFI:S:<SSID>;T:WPA;P:<Password>;H:false;;'.

It's also possible to store the connection details in Flash once they've been read once so that they persist over reboots, but that's complex enough to be outside of the scope of this example.

Writing your own Applications

Hopefully the example code shown here should give you a good starting point for using the sensor in your own projects, but you can see more details about the interface in tiny_code_reader.h. This header contains the data structures used to return information from the peripheral, and functions to read and configure the device. If you are trying to port this code to a different board, you can check the developer guide to see if there's already support for your platform, and if not, the main differences are likely to be in the I2C initialization, reading, and writing implementations. If you can find examples of how to do the I2C bus setup on the new board, and then equivalents to the i2c_read_blocking and i2c_write_blocking functions, you should be able to reuse the rest of the data structures and logic.