Week 11 - Getting Two Boards to Talk

What we had to do

This week we needed to send a message between two projects. Sounds simple enough, right?
Well, it took us a few tries to get it working properly, but we figured it out in the end!

What we decided to build

We wanted to make something pretty cool - press a button on one board and have LEDs light up on BOTH boards. The idea was to use two XIAO RP2040 boards and make them communicate using serial communication (UART). We thought this would be easier than trying to do wireless stuff, and honestly, we were right!

How we made them talk - UART Serial

Instead of going with WiFi or Bluetooth (which seemed complicated), we decided to use good old UART serial communication. It's basically like having a conversation between two boards using just two wires. Pretty neat actually!

How UART works (in simple terms)

Think of it like walkie-talkies, but with wires:

• TX (Transmit): The "mouth" - where one board talks
• RX (Receive): The "ear" - where the other board listens
• The trick: You cross the wires! TX from board 1 goes to RX on board 2, and vice versa
• Ground: Both boards need to share the same ground (like holding hands)
• Speed: Both need to talk at the same speed - we used 9600 bps

Building the hardware

We used our custom XIAO RP2040 boards from previous weeks. Each one already had an LED and button wired up, which made things easier.

How we wired everything

Board 1 Pin	Board 2 Pin	What it does
TX (Pin 0)	RX (Pin 1)	Board 1 talks to Board 2
RX (Pin 1)	TX (Pin 0)	Board 2 talks back to Board 1
GND	GND	Shared ground (super important!)
LED on D2	LED on D2	LEDs that light up
Button on D3	Button on D3	Buttons to press

Writing the code

The programming part was actually pretty straightforward once we figured out the logic. Both boards run almost the same code, just with tiny differences.

The basic setup stuff

#define LED_PIN D2
#define BTN_PIN D3
// These were the same on both boards

Setting up the communication

Serial1.begin(9600);  // Start talking at 9600 baud
// We tried other speeds but 9600 worked best

Sending the message

if (digitalRead(BTN_PIN) == LOW) {
  Serial1.println("LED_ON");  // Tell the other board to turn on its LED
  digitalWrite(LED_PIN, HIGH); // Turn on our own LED too
}

Listening for messages

if (Serial1.available()) {
  String message = Serial1.readString();
  if (message.indexOf("LED_ON") >= 0) {
    digitalWrite(LED_PIN, HIGH); // Turn on LED when we get the message
  }
}

Testing (and debugging!)

Let's be honest - it didn't work the first time. We had to debug a few things:

1. First attempt: forgot to cross the TX/RX wires (oops!)
2. Second try: had different baud rates on each board
3. Third time: forgot the common ground connection
4. Fourth attempt: SUCCESS! Both LEDs lit up when we pressed either button

The big demo!

When we finally got it working, it was pretty satisfying. Press one button, both LEDs light up almost instantly. The communication was fast and reliable.

What we discovered

Things that worked really well:

• Super reliable: Once we got the wiring right, it never failed
• Really fast: Almost no delay between button press and LED response
• Both directions work: Either board can talk to the other
• Easy to understand: The code is pretty simple to follow

Important lessons we learned:

• Wire crossing is crucial: TX → RX, RX → TX (not TX → TX!)
• Ground matters: Without shared ground, nothing works
• Speed matching: Both boards MUST use the same baud rate
• Clear messages: Simple text commands work best

Why we chose UART over other options

Communication Method	Good stuff	Not so good stuff
UART (what we used)	Simple, reliable, fast, low power	Need wires, short distance only
ESP-NOW	No wires, works far apart	Way more complicated, need ESP32 boards
I2C	Can connect many devices	More complex addressing system
SPI	Super fast	Need more wires, overkill for our project

What we learned

This project taught us a lot about communication between microcontrollers:

• UART serial is perfect for simple, reliable communication between nearby boards
• Getting the wiring right is half the battle - that TX/RX crossover is critical
• Don't forget the ground connection! (We did, initially)
• XIAO RP2040 boards are great for these kinds of projects
• Simple message protocols work better than complex ones
• Serial communication is ideal when you don't mind having wires

Overall, this was a really fun project! We managed to get two boards talking to each other using serial communication. It might not be as fancy as wireless, but it's reliable and easy to understand. Plus, seeing both LEDs light up when you press one button never gets old!

Pro tip: Always check your wiring twice before blaming the code. We learned this the hard way!