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PicoRuby for IoT:
Connecting to the Cloud with MQTT
April 24, 2026 / RubyKaigi 2026
Yuhei Okazaki(@Y_uuu)
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self.inspect
● Yuhei Okazaki
● @Y_uuu(X), @y-uuu.net(BlueSky)
● @yuuu(GitHub)
● IoT Cloud Engineer at Fusic Co., Ltd.
● Mentor at Fjord Boot Camp
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What is PicoRuby?
● PicoRuby is an extremely small
implementation of Ruby designed to run on
microcontrollers.
● It has been proven to work on several
popular microcontroller boards, such as
RP2040 and ESP32.
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What is PicoRuby?
● PicoRuby is an extremely small
implementation of Ruby designed to run on
microcontrollers.
● It has been proven to work on several
popular microcontroller boards, such as
RP2040 and ESP32. ← I am contributing
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What is ESP32?
● ESP32 is a microcontroller well
suited for IoT applications, with
built-in Wi-Fi and Bluetooth
support.
● It provides a TCP/IP stack
compatible with the BSD Sockets
API, offering a solid foundation
for network communication.
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What is R2P2-ESP32?
● R2P2-ESP32 is a project that
runs R2P2 (Ruby Rapid Portable
Platform) on the ESP32.
● It includes a shell and file system
that can run on a
microcontroller, making it easy
to get started with PicoRuby.
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Ref: Porting PicoRuby to Another Microcontroller: ESP32
● At RubyKaigi 2025, I talked about how to port PicoRuby to the ESP32.
● If you're interested in running PicoRuby on another microcontroller,
please check it out.
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https://rubykaigi.org/2025/presentations/Y_uuu.html
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What should I do next?
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I want to build an IoT system
● Because I'm an IoT Cloud Engineer.
● With I2C and UART support, I can already connect to sensors.
● Next, I want to send sensor data to the cloud (e.g., AWS).
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AWS IoT Core
● A managed MQTT broker provided by AWS.
● Communicates via MQTTS (MQTT + TLS).
● Data can be passed to various AWS services.
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What is MQTT?
● MQTT is a lightweight publish/subscribe
messaging protocol designed for
resource-constrained devices and
low-bandwidth, high-latency, or unstable
network environments.
● Like HTTP, it is a protocol that sits on top of
TCP.
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MQTT communicates via a broker
● Clients Publish to or Subscribe to a broker.
● A "topic" is specified as the destination for sending or receiving
messages.
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MQTT communicates via a broker
● When a client Publishes data to a specific topic:
● The data is sent to all clients currently Subscribing to that topic.
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MQTT communicates via a broker
● Each client can be both a Publisher and a Subscriber.
● When multiple clients subscribe to the same topic, the message is
broadcast to each of them.
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PicoRuby needs an MQTT client
● An MQTT client will enable the creation of IoT systems.
● I want to make it possible to use a client class similar to ruby-mqtt for
CRuby.
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Let's start developing
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Status Check
● To communicate via MQTTS(MQTT+TLS), lower-level protocols were
necessary.
● At the start of development, PicoRuby on ESP32 couldn't even connect
to Wi-Fi, let alone perform TCP communication.
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Feature RP2040 ESP32
Wi-Fi ✓ -
TCPSocket ✓ -
SSLSocket ✓ -
MQTTClient - -
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Development Steps
I proceeded with the work in the following order:
1. Add a Wi-Fi class for ESP32.
2. Port the Socket classes ( TCPSocket , SSLSocket ) to ESP32.
3. Implement the MQTTClient class from scratch.
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First challenge:
Connecting the microcontroller to Wi-Fi
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Wi-Fi class for RP2040
By tracing through the source code, I
found that an mrbgem called
picoruby-cyw43 handles the Wi-Fi
connection logic.
※CYW43 is the Wi-Fi/Bluetooth chip
used in the Raspberry Pi Pico W and
similar boards.
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CYW43 (Wi-Fi chip)
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ESP32 has Wi-Fi built into the SoC
Created an mrbgem called
picoruby-esp32. Defined an
ESP32::WiFi class with methods
aligned to the CYW43 interface.
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ESP32 with Wi-Fi
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Implementing the Wi-Fi connection in picoruby-esp32
Implemented based on the sample code from the ESP-IDF Wi-Fi
component (esp_wifi).
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mrbgems/picoruby-esp32/ports/esp32.c
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Implementing the Wi-Fi connection in picoruby-esp32
Implemented based on the sample code from the ESP-IDF Wi-Fi
component (esp_wifi).
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mrbgems/picoruby-esp32/ports/esp32.c
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Problem: Linker error occurred
A linker error occurred around Mbed TLS during the build.
What happened?
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What is Mbed TLS?
Mbed TLS is a lightweight SSL/TLS library suitable for embedded
environments.
It is used for password encryption in Wi-Fi processes.
It is also essential for MQTTS (MQTT+TLS) communication.
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Mbed TLS directory structure
PicoRuby includes Mbed TLS source code
within the picoruby-mbedtls mrbgem.
In the ESP32 development environment
ESP-IDF, esp_wifi has an indirect
dependency on Mbed TLS.
Therefore, we ended up with two copies
of Mbed TLS.
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Mbed TLS conflict
At the time (July 2025), the versions were different. Some functions shared
the same name but had different signatures, leading to linker errors.
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Solution: Refactoring picoruby-mbedtls
I extracted the Mbed TLS-dependent
code within picoruby-mbedtls and
moved it to the ports directory.
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Wi-Fi connection successful on R2P2-ESP32
Now, Wi-Fi can be connected by writing the following Ruby program.
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Second challenge:
Porting the Socket class
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Socket Classes for RP2040
● The picoruby-socket mrbgem
provides the Socket classes.
● It includes not only TCPSocket
and UDPSocket, but also
TCPServer and SSLSocket.
● I will port this mrbgem to ESP32.
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Structure of picoruby-socket
● The src directory contains the
bindings for mruby or mruby/c.
● The ports directory contains
implementation-specific code.
○ A new esp32 directory was
created here to add the
ESP32-specific code.
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Example: TCPSocket_send() (RP2040)
● The RP2040 implementation uses LwIP's raw APIs.
○ LwIP (LightWeight IP) is a lightweight TCP/IP stack for embedded
systems.
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mrbgems/picoruby-socket/ports/rp2040/tcp_socket.c
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Example: TCPSocket_send() (ESP32)
● ESP-IDF, the ESP32 development framework, supports LwIP.
○ Unlike RP2040, the Socket API is also available.
○ Code can be written just like socket programming on a PC.
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mrbgems/picoruby-socket/ports/esp32/tcp_socket.c
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Also Ported UDPSocket and TCPServer
● Neither is strictly required.
○ With UDPSocket, time
synchronization via NTP
becomes possible.
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Also Ported SSLSocket
● SSLSocket is required since communication with AWS IoT Core uses
MQTTS (MQTT+TLS).
● On ESP32, the Socket API is available, so we only need to call Mbed
TLS functions directly.
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mrbgems/picoruby-socket/ports/esp32/ssl_socket.c
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Also Ported SSLSocket
● The original SSLSocket did not support X.509 client certificates, so I
added the implementation.
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mrbgems/picoruby-socket/ports/esp32/ssl_socket.c
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Verifying TCPSocket
● Testing a connection to example.com.
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Verifying SSLSocket (1)
● Testing a connection to example.com.
● Prepare an SSLContext and a TCPSocket, then call SSLSocket.new.
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Verifying SSLSocket (1)
● Establish a connection with SSLSocket#connect.
● Call SSLSocket#write then SSLSocket#read.
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Verifying SSLSocket (2)
● Testing a connection to AWS IoT
Core.
○ Set the X.509 client
certificate.
○ Connect to port 8883 of the
AWS IoT Core endpoint.
● The connection is established
without any errors.
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Third challenge:
Implementing an MQTT client class
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Background
● Someone actually tried to
implement an MQTT client class
before me.
● @ryosk7, who gave a talk about
this yesterday.
● After some discussion, this Pull
Request was closed without
being merged.
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https://github.com/picoruby/picoruby/pull/190
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Implementation approach
● Do not use LwIP's MQTT client.
● Use TCPSocket and SSLSocket from picoruby-socket.
● Implement a pure Ruby MQTT client.
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Implementing the MQTT Client
● @hasumikin had already had Claude implement it.
○ "What we want is usually already built by @hasumikin" by
@hachiblog
● The MQTT protocol specification is publicly available, making it
straightforward to implement as a pure Ruby program.
○ https://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.h
tml
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MQTT Packet Example: Connect
● When an MQTT Client connects to an MQTT broker, it sends a packet
like the following over TCP (excerpt).
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I Did the Debugging
● It was calling methods that exist in CRuby but not in PicoRuby.
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mrbgems/picoruby-net-mqtt/mrblib/mqtt.rb
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I Did the Debugging
● In CRuby, pack / unpack are part of the standard library, but in
PicoRuby, an explicit require is needed.
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mrbgems/picoruby-net-mqtt/mrblib/mqtt.rb
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I Did the Debugging
● Insufficient nil checks were generating Steep warnings.
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mrbgems/picoruby-net-mqtt/mrblib/mqtt.rb
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Verifying MQTT Communication
● Since only Ruby code is written and it has no device dependencies, it
can be tested on a PC.
● Verified using Eclipse Mosquitto, an OSS MQTT Broker.
● Once it works on a PC, verify it works on the device as well.
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Example: Publishing to MQTT Broker
First, I verified Publish using PicoRuby's Net::MQTT::Client.
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Example: Publishing to MQTT Broker
Subscribe in advance with the following command
Run the following program on a PC or device to verify publish behavior
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Example: Publishing to MQTT Broker
Next, I verified Subscribe using PicoRuby's Net::MQTT::Client.
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Example: Subscribing to MQTT Broker
Run the following program on a PC or device to verify subscribe behavior
Publish with the following command
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Supporting MQTTS (MQTT+TLS)
● Finally connecting to AWS IoT Core.
● Added support for passing ssl, ca_file, cert_file, and key_file as
optional arguments
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Verifying MQTTS Communication
● Verifying MQTTS communication.
● Use AWS IoT Core as the MQTT Broker instead of Mosquitto.
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Verifying MQTTS Communication
● MQTTS Publish succeeded on both PC and devices (ESP32) 🎉
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Demo:
An IoT system using MQTT communication
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Let's Play with Marble Run
● I built a toy where a small ball
climbs stairs and rolls down.
● A microcontroller board is
connected to a motor, and it
starts moving when the program
is launched.
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System Configuration
● Microcontroller: ESPr®
Developer S3 Type-C
○ ESP32-S3-WROOM-1
■ FlashROM: 16MB
■ PSRAM: 8MB
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System Configuration
● Motor driver: TB67H450
○ Outputs a HIGH signal to
spin the motor
○ Speed control via waveform
output with picoruby-pwm
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System Configuration
● ToF Sensor: Unit ToF VL53L0X
○ Measures the distance to an
object.
○ Uses picoruby-i2c ported for
ESP32 by @bash to
communicate via I2C.
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Can You Imagine?
● Doesn't this look like a
production line in an automated
factory?
● I named this device pico-factory.
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Demo
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Architecture
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Conclusion
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ToDo
● The work discussed today has only been verified on FemtoRuby (VM:
mruby/c) so far.
○ I still need to verify it on PicoRuby (VM: mruby).
● picoruby-mqtt does not yet support MQTT 5.
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Summary
● We have set up the communication classes available for R2P2-ESP32.
● PicoRuby on ESP32 is finally IoT-Ready.
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Feature RP2040 ESP32
Wi-Fi ✓ ✓
TCPSocket ✓ ✓
SSLSocket ✓ ✓
MQTTClient ✓ ✓
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Summary
● Please also visit the repositories!
○ https://github.com/picoruby/picoruby
○ https://github.com/picoruby/picoruby-esp32
○ https://github.com/picoruby/R2P2-ESP32
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PR: I'm Selling an MQTT Book
● I self-published a book and am
selling it at the RubyKaigi 2026
bookstore. → SOLD OUT!!
● If you're interested in MQTT,
please look for it online!
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end
Thank you for coming!
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