Running TLS on a Tiny MCU – RP2040 + W5500 with Only 32 kB RAM & 128 kB Flash

Introduction

Let’s start with the question everyone asks: How small can an MCU be and still run proper TLS? The answer is surprisingly small — an RP2040 + W5500 runs full TLS 1.3 with only 32 kB RAM and 128 kB flash.

WIZnet Product Integration

The W5500 is the star here. It handles the complete TCP/IP stack in hardware, so the RP2040 only communicates over SPI. Almost no RAM or CPU time is spent on networking — perfect for adding TLS on tiny microcontrollers.

Technical Implementation

Hardware wiring is extremely simple:

• Pico SPI0 → W5500 (CS=GP5, SCK=GP2, MOSI=GP3, MISO=GP4, RST=GP6)
• 3.3 V + GND

Software: just copy mongoose.c and mongoose.h, then enable TLS with one line in mongoose_custom.h.

Reproduction Guide

1. Clone https://github.com/cesanta/mongoose
2. Open examples/arduino/w5500-http in Arduino IDE
3. Select Raspberry Pi Pico as board
4. In mongoose_custom.h add: #define MG_ENABLE_MBEDTLS 1
5. Build & upload
6. Check Serial Monitor for IP → open https://[IP] in browser

You now have a working HTTPS server.

Core Features and Performance

• Plain HTTP: ≈ 17 kB RAM
• HTTPS (TLS 1.3): ≈ 25 kB RAM (+8 kB for TLS)
• TLS handshake: ~600 ms on 133 MHz Cortex-M0+
• Extra flash for TLS: ≈ 20 kB
• Confirmed minimum: 32 kB RAM / 128 kB flash

Code Snippet

Only change needed:

// mongoose_custom.h
#define MG_ENABLE_MBEDTLS 1        // enable TLS
#define MG_TLS MG_TLS_MBEDTLS      // use mbedTLS backend

Applications and Extensions

• Secure sensor dashboards
• Remote firmware updates over HTTPS
• MQTT over TLS on very low-cost boards
• Industrial devices that need encryption with < 64 kB RAM (planned: client certificates, PSK mode for even lower RAM)

Conclusion

If you thought TLS was impossible on small MCUs, this demo proves otherwise. RP2040 + W5500 + Mongoose = production-grade HTTPS server in under 30 kB RAM.

Try it today — everything is ready here: https://github.com/cesanta/mongoose/tree/master/examples/arduino/w5500-http