Display Data on your Microcontroller

Take the wallet-balance polling from the previous lesson and put it on a screen.

Hardware

This lesson assumes the Cheap Yellow Display (CYD) with its built-in TFT. If you have a small I2C OLED instead (typical SSD1306 0.96", 128×64), the wiring and library are different - see the dropdown below.

Reference: small I2C OLED display (SSD1306)

If your hardware is an ESP32-C3 with a small SSD1306 OLED instead of the CYD, here's the minimum-viable display sketch using Adafruit_SSD1306 over I2C.

Install libraries in the Arduino IDE Library Manager:

• Adafruit SSD1306 (by Adafruit)
• Adafruit GFX Library (dependency)

Wiring (ESP32-C3):

OLED pin	ESP32-C3 pin
VCC	3.3V
GND	GND
SDA	GPIO 8
SCL	GPIO 9

GPIO 8 / 9 are the default I2C pins on the ESP32-C3. If your board differs, adjust the I2C_SDA / I2C_SCL defines.

/*
 * ESP32-C3 I2C OLED Display Example (SSD1306)
 *
 * Display text and shapes on a 0.96" 128x64 OLED.
 *
 * Connections:
 *   OLED VCC -> ESP32 3.3V
 *   OLED GND -> ESP32 GND
 *   OLED SDA -> ESP32 GPIO 8
 *   OLED SCL -> ESP32 GPIO 9
 */

#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

#define SCREEN_WIDTH 128
#define SCREEN_HEIGHT 64
#define OLED_RESET -1
#define SCREEN_ADDRESS 0x3C  // or 0x3D — use the I2C scanner if unsure

#define I2C_SDA 8
#define I2C_SCL 9

Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, OLED_RESET);

void setup() {
  Serial.begin(115200);
  delay(1000);

  Wire.begin(I2C_SDA, I2C_SCL);

  if (!display.begin(SSD1306_SWITCHCAPVCC, SCREEN_ADDRESS)) {
    Serial.println("SSD1306 allocation failed");
    for (;;);
  }

  Serial.println("OLED Display initialized!");
}

void loop() {
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);

  display.setTextSize(1);
  display.setCursor(0, 0);
  display.println("Small Text");

  display.setTextSize(2);
  display.setCursor(0, 15);
  display.println("Medium");

  display.setTextSize(3);
  display.setCursor(0, 40);
  display.println("Big!");

  display.drawRect(95, 5, 30, 25, SSD1306_WHITE);

  display.display();
  delay(2000);
}

To adapt the rest of this lesson for the OLED: replace each tft.* call with the corresponding display.* call (display.fillRect, display.setCursor, display.println, then display.display() to push the buffer).

Introduction to TFT_eSPI

TFT_eSPI is a powerful library for driving TFT displays from ESP32 / ESP8266. It supports many display chips and gives you fast text, shapes, image and sprite rendering with rotation support.

Installing and configuring the library

Step 1: Install TFT_eSPI

1. Open Arduino IDE.
2. Sketch → Include Library → Manage Libraries.
3. Search for TFT_eSPI.
4. Install the library by Bodmer.

Step 2: Configure for the CYD

TFT_eSPI needs to be configured for your specific display. Easiest path is to drop in a CYD-specific User_Setup.h:

1. Download the CYD User_Setup.h from the ESP32-Cheap-Yellow-Display repo.
2. Find your TFT_eSPI library folder:
   • Windows: Documents\Arduino\libraries\TFT_eSPI\
   • macOS: ~/Documents/Arduino/libraries/TFT_eSPI/
   • Linux: ~/Arduino/libraries/TFT_eSPI/
3. Replace the existing User_Setup.h with the downloaded one.
4. Restart Arduino IDE so the new config takes effect.

Without this step, the display won't work

The CYD-specific User_Setup.h configures: ILI9341_2_DRIVER, 240×320 resolution, the right GPIO pins, 55 MHz SPI, and GPIO 21 for backlight.

Testing the display

Before fetching wallet data, verify the display works with a Hello World - centred white text on a blue background.

// Include TFT display library
#include <TFT_eSPI.h>
#include <SPI.h>

// Create TFT display object
TFT_eSPI tft = TFT_eSPI();

// Define custom gray color (RGB565 format: 5 bits red, 6 bits green, 5 bits blue)
#define TFT_GRAY 0x7BEF

void setup() {
    // Initialize serial communication for debugging
    Serial.begin(115200);

    // Initialize TFT display
    tft.init();

    // Set display rotation (1 = landscape)
    tft.setRotation(1);

    // Invert display colors (required for some CYD displays)
    tft.invertDisplay(true);

    // Fill entire screen with blue background
    tft.fillScreen(TFT_BLUE);

    // Set text properties
    tft.setTextColor(TFT_WHITE, TFT_BLUE);  // White text on blue background
    tft.setTextSize(3);                      // Large text size

    // Calculate text position to center it on screen
    String text = "Hello World!";
    int textWidth = text.length() * 6 * 3;   // Approximate width (6 pixels per char * text size)
    int textHeight = 8 * 3;                  // Approximate height (8 pixels * text size)
    int textX = (320 - textWidth) / 2;       // Center horizontally (320 is screen width)
    int textY = (240 - textHeight) / 2;      // Center vertically (240 is screen height)

    // Display the centered text
    tft.setCursor(textX, textY);
    tft.println("Hello World!");

    Serial.println("Display test complete!");
}

void loop() {
    // Nothing to do in the loop for this test
}

Source: Workshop-02/examples/display-hello-world/display-hello-world.ino

If you see "Hello World!" on a blue screen, you're good. If you see corrupted graphics or nothing at all, double-check that you replaced User_Setup.h.

Understanding the code

• tft.init() - initialises the display.
• tft.setRotation(1) - landscape orientation. Values 0–3 rotate by 90° each.
• tft.invertDisplay(true) - some CYDs have inverted colours; without this, blue may show as yellow.
• tft.fillScreen(TFT_BLUE) - fills the screen with a colour constant.
• tft.setTextColor(fg, bg) - text colour and background.
• tft.setTextSize(size) - size multiplier for the default font.
• tft.setCursor(x, y) - origin (0,0) is top-left.

Screen dimensions

The CYD is 320×240 in landscape (rotation 1). Use those dimensions when centring text.

Displaying the wallet balance

Now combine the previous lesson's Koios fetch with the display: white text on blue, balance in large type, a live timestamp updating every second, balance refreshed every 60 seconds.

// Include necessary libraries
#include <WiFi.h>              // WiFi connectivity
#include <HTTPClient.h>       // HTTP client for API calls
#include <ArduinoJson.h>      // JSON parsing
#include <TFT_eSPI.h>         // TFT display library
#include <SPI.h>               // SPI communication for display

// Create TFT display object
TFT_eSPI tft = TFT_eSPI();

// WiFi credentials - replace with your network details
const char* ssid = "Your SSID";
const char* password = "Your Password";

// Koios API endpoint for fetching account information
const char* apiUrl = "https://preprod.koios.rest/api/v1/account_info";

// Your Cardano stake address (Preprod Testnet)
String stakeAddress = "stake_test1...";

// Variables for timing balance checks
unsigned long lastCheck = 0;                    // Timestamp of last balance check
const unsigned long checkInterval = 60000;      // Check every 60 seconds
unsigned long lastFetchTime = 0;                // Timestamp of last successful fetch
unsigned long lastDisplayUpdate = 0;            // Timestamp of last display update
const unsigned long displayUpdateInterval = 1000; // Update display every 1 second

// Store current balance to detect changes
float currentBalance = 0.0;

void setup() {
    // Initialize serial communication for debugging
    Serial.begin(115200);

    // Initialize TFT display
    tft.init();
    tft.setRotation(1);        // Set to landscape orientation
    tft.invertDisplay(true);   // Invert colors for correct display
    tft.fillScreen(TFT_BLUE);  // Fill screen with blue background
    tft.setTextColor(TFT_WHITE, TFT_BLUE);  // White text on blue background
    tft.setTextSize(2);        // Set text size

    // Display startup message on screen
    tft.setCursor(10, 10);
    tft.println("Connecting...");

    // Start WiFi connection
    WiFi.begin(ssid, password);
    while (WiFi.status() != WL_CONNECTED) {
        delay(1000);
        Serial.println("Connecting to WiFi...");
    }

    Serial.println("Connected to WiFi");
    Serial.print("IP address: ");
    Serial.println(WiFi.localIP());

    // Clear screen and show connection success
    tft.fillScreen(TFT_BLUE);
    tft.setCursor(10, 10);
    tft.println("Connected!");
    delay(1000);

    // Perform initial balance fetch and display
    fetchStakeBalance();
}

void loop() {
    // Check if WiFi connection is still active
    if (WiFi.status() != WL_CONNECTED) {
        Serial.println("WiFi connection lost. Reconnecting...");
        WiFi.reconnect();

        while (WiFi.status() != WL_CONNECTED) {
            delay(1000);
            Serial.print(".");
        }
        Serial.println("Reconnected!");
    }

    // Get current time in milliseconds
    unsigned long currentMillis = millis();

    // Check if enough time has passed since last check
    if (currentMillis - lastCheck >= checkInterval) {
        fetchStakeBalance();
        lastCheck = currentMillis;  // Update last check timestamp
    }

    // Update the timestamp display every second
    if (currentMillis - lastDisplayUpdate >= displayUpdateInterval) {
        updateTimestamp();
        lastDisplayUpdate = currentMillis;
    }
}

void fetchStakeBalance() {
    // Only proceed if WiFi is connected
    if (WiFi.status() == WL_CONNECTED) {
        HTTPClient http;

        // Initialize HTTP client with API URL
        http.begin(apiUrl);

        // Set content type header for JSON request
        http.addHeader("Content-Type", "application/json");

        // Create JSON payload with stake address
        // Koios API expects stake addresses in an array under "_stake_addresses" key
        String jsonPayload = "{\\"";
        jsonPayload += "_stake_addresses";
        jsonPayload += "\\":[\\"";
        jsonPayload += stakeAddress;
        jsonPayload += "\\"]}";

        // Send POST request and get response code
        int httpResponseCode = http.POST(jsonPayload);

        // Check if request was successful (response code > 0)
        if (httpResponseCode > 0) {
            // Get response body as string
            String response = http.getString();
            Serial.println("HTTP Response Code: " + String(httpResponseCode));

            // Create JSON document to parse response (2048 bytes buffer)
            DynamicJsonDocument doc(2048);
            DeserializationError error = deserializeJson(doc, response);

            // Check if JSON parsing was successful
            if (!error) {
                // Verify response is an array with at least one element
                if (doc.is<JsonArray>() && doc.size() > 0) {
                    // Get first account info object from array
                    JsonObject accountInfo = doc[0];

                    // Extract total balance as string (Koios returns balance as string)
                    // total_balance includes delegated amount + rewards
                    const char* balanceStr = accountInfo["total_balance"];

                    // Convert string to long long (for large Lovelace values)
                    long long balanceLovelace = 0;
                    if (balanceStr != nullptr) {
                        balanceLovelace = atoll(balanceStr);
                    }

                    // Convert from Lovelace to tADA (test ADA)
                    // 1 tADA = 1,000,000 Lovelace
                    float balance = balanceLovelace / 1000000.0;

                    // Print account information
                    Serial.println("Stake Address: " + String(accountInfo["stake_address"].as<const char*>()));
                    Serial.println("Total Balance: " + String(balance, 6) + " tADA");

                    // Check if balance has changed since last check
                    if (balance != currentBalance) {
                        if (balance > currentBalance) {
                            Serial.println("✓ Balance increased!");
                        } else if (balance < currentBalance) {
                            Serial.println("✓ Balance decreased!");
                        }
                        // Update current balance
                        currentBalance = balance;
                    }

                    // Update last fetch time and refresh full display
                    lastFetchTime = millis();
                    updateDisplay();
                }
            } else {
                // Print error if JSON parsing failed
                Serial.print("JSON parsing failed: ");
                Serial.println(error.c_str());
            }
        } else {
            // Print error if HTTP request failed
            Serial.println("Error in HTTP request");
            Serial.println("HTTP Response Code: " + String(httpResponseCode));
        }

        // Close HTTP connection
        http.end();
    } else {
        Serial.println("WiFi not connected");
    }
}

void updateDisplay() {
    // Fill entire screen with blue background
    tft.fillScreen(TFT_BLUE);

    // Display title "Wallet Balance" in white
    tft.setTextSize(2);
    tft.setTextColor(TFT_WHITE, TFT_BLUE);
    tft.setCursor(10, 10);
    tft.println("Wallet Balance");

    // Display balance amount in large white text
    tft.setTextSize(4);
    tft.setTextColor(TFT_WHITE, TFT_BLUE);
    tft.setCursor(10, 50);
    tft.print(String(currentBalance, 2));  // Format to 2 decimal places

    // Display "ADA" unit
    tft.setTextSize(2);
    tft.println(" ADA");

    // Display initial timestamp
    updateTimestamp();
}

void updateTimestamp() {
    // Calculate seconds since last fetch
    unsigned long secondsAgo = (millis() - lastFetchTime) / 1000;

    // Clear the timestamp area (blue rectangle over the old text)
    tft.fillRect(10, 220, 200, 10, TFT_BLUE);

    // Display last update timestamp in lower left corner
    tft.setTextSize(1);
    tft.setTextColor(TFT_WHITE, TFT_BLUE);
    tft.setCursor(10, 220);
    tft.print("Updated ");
    tft.print(secondsAgo);
    tft.println("s ago");
}

Source: Workshop-02/examples/display-wallet-balance/display-wallet-balance.ino

Replace WiFi credentials and your stake address before uploading. You should see your tADA balance update every minute and the timestamp tick every second.

What's next?

You can now poll on-chain data and render it on a screen. From here, try other Koios endpoints, or build alternative visualisations on the same hardware. The next lessons in this workshop add hardware actuation: a relay (drive a real light bulb) and an LED ring (an Epoch Clock).

Further Resources

• TFT_eSPI on GitHub - the library.
• Adafruit GFX Graphics Library - TFT_eSPI builds on this.
• Arduino TFT_eSPI Reference - Arduino's library docs.
• ESP32-Cheap-Yellow-Display - community resources for the CYD.
• LVGL - much more featureful UI library if you outgrow TFT_eSPI.

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Adapted from the CardanoThings workshop series, originally produced under Project Catalyst Fund 11. Source code: github.com/CardanoThings/Workshops/Workshop-02.