16 Protocol Diagnostics and Bus Design

Scenario: You’re building an indoor air quality monitor with 5 sensors on a single I2C bus connected to an ESP32:

1. BME280 (temperature, humidity, pressure) - Address: 0x76
2. BH1750 (light sensor) - Address: 0x23
3. CCS811 (VOC-derived equivalent CO2, eCO2, and equivalent total VOC, eTVOC; not direct CO2) - Address: 0x5A
4. SSD1306 (OLED display) - Address: 0x3C
5. Second BME280 (outdoor sensor) - Address: 0x77 (SDO pin to VCC)

Before solving the five-device bus, inspect Figure 16.1 to see what one listed address hides. The breakout shares SDA and SCL but also exposes wake, reset, and interrupt signals that affect integration and recovery.

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Read Figure 16.1, locate the sensing package first, then read the labelled I2C pads and the extra control pins around it. Unique addresses prevent one class of conflict, while pull-up strength, capacitance, power sequencing, and control-line handling complete the dependable bus design developed below.

Design Challenges to Solve:

1. Pull-Up Resistor Calculation

The I2C bus needs pull-up resistors on both SDA and SCL lines. Too weak (high resistance) causes slow rise times and communication errors. Too strong (low resistance) wastes power and may damage devices.

Formula: R_pullup = t_rise / (0.8473 × C_bus)

Where:

• t_rise = maximum rise time (1000 ns for standard mode 100 kHz, 300 ns for fast mode 400 kHz)
• C_bus = total bus capacitance (trace capacitance + device capacitance)

Step-by-step Calculation:

a) Estimate bus capacitance:

• PCB trace: ~30 pF per 30cm wire (assume 60cm total) = 60 pF
• BME280: 10 pF per device × 2 = 20 pF
• BH1750: 10 pF
• CCS811: 10 pF
• SSD1306: 400 pF (worst-case with long flex cable; typical breakout boards add 100-150 pF)
• Total C_bus: 60 + 20 + 10 + 10 + 400 = 500 pF

b) Choose I2C speed:

• Standard mode (100 kHz): lower power, simpler
• Fast mode (400 kHz): faster data, needed if polling sensors frequently
• Choice: Fast mode 400 kHz (t_rise = 300 ns max)

c) Calculate maximum pull-up resistance for fast mode:

R_max = t_rise / (0.8473 × C_bus)
R_max = 300 ns / (0.8473 × 500 pF)
R_max = 300 × 10^-9 / (0.8473 × 500 × 10^-12)
R_max = 708 Ω (approximately 0.708 kΩ)

d) Calculate minimum pull-up resistance: Minimum limited by maximum sink current (3 mA per I2C spec) and V_OL_max (0.4V):

R_min = (V_CC - V_OL) / I_OL = (3.3V - 0.4V) / 3mA = 0.97 kΩ

e) Evaluate resistor selection for 400 kHz:

• Maximum resistance from rise time: 708 Ω
• Minimum resistance from current limit: 0.97 kΩ
• Problem: No standard resistor value exists between 708 Ω and 0.97 kΩ that satisfies both constraints!
• Actual rise time with 1.0 kΩ: 1kΩ × 500pF × 0.8473 = 424 ns (exceeds 300 ns limit for 400 kHz)

Conclusion: 400 kHz is not achievable with 500 pF bus capacitance!

Let’s recalculate for 100 kHz standard mode instead:

Revised Calculation for 100 kHz:

t_rise_max = 1000 ns (standard mode specification)
R_max = 1000 ns / (0.8473 × 500 pF) = 2.36 kΩ (maximum resistance allowed)
R_min = (3.3V - 0.4V) / 3mA = 0.97 kΩ (minimum resistance from current limit)

Acceptable range: 0.97 kΩ to 2.36 kΩ

Test with 4.7 kΩ resistors (common value):

t_rise = 4.7kΩ × 500pF × 0.8473 = 1991 ns
Result: FAILS (exceeds 1000 ns maximum for 100 kHz)

Final Decision: A 2.2 kΩ pull-up meets the calculated rise-time and LOW-current bounds at 100 kHz, but the assumed 500 pF exceeds the 400 pF Standard-mode bus-load limit (NXP UM10204, Table 11). Reduce or segment the capacitance, or qualify an extended-load design under the I²C specification, before calling this bus reliable:

t_rise = 2.2kΩ × 500pF × 0.8473 = 932 ns ✓ (safely under 1000 ns limit)
Current draw = 3.3V / 2.2kΩ = 1.5 mA ✓ (within ESP32 limits)

2. Address Conflict Resolution

One BME280 at default address 0x76, second at alternate 0x77:

#include <Adafruit_BME280.h>

Adafruit_BME280 bme_indoor;  // 0x76
Adafruit_BME280 bme_outdoor; // 0x77

void setup() {
    Wire.begin(21, 22); // SDA=21, SCL=22 on ESP32

    // Initialize with explicit addresses
    if (!bme_indoor.begin(0x76)) {
        Serial.println("Could not find indoor BME280");
    }
    if (!bme_outdoor.begin(0x77)) {
        Serial.println("Could not find outdoor BME280");
    }
}

3. Bus Scanning for Debugging

Before writing sensor-specific code, scan the bus to verify all devices are present:

void scanI2CBus() {
    Serial.println("\n--- I2C Bus Scan ---");
    byte count = 0;

    for (byte addr = 1; addr < 127; addr++) {
        Wire.beginTransmission(addr);
        byte error = Wire.endTransmission();

        if (error == 0) {
            Serial.print("Device found at 0x");
            if (addr < 16) Serial.print("0");
            Serial.print(addr, HEX);

            // Identify common sensors
            switch(addr) {
                case 0x23: Serial.print(" (BH1750 Light)"); break;
                case 0x3C: Serial.print(" (SSD1306 OLED)"); break;
                case 0x5A: Serial.print(" (CCS811 Air Quality)"); break;
                case 0x76: Serial.print(" (BME280 #1)"); break;
                case 0x77: Serial.print(" (BME280 #2)"); break;
                default: Serial.print(" (Unknown)"); break;
            }
            Serial.println();
            count++;
        }
    }

    Serial.print("Total devices found: ");
    Serial.println(count);

    if (count != 5) {
        Serial.println("WARNING: Expected 5 devices!");
    }
}

Expected Output:

--- I2C Bus Scan ---
Device found at 0x23 (BH1750 Light)
Device found at 0x3C (SSD1306 OLED)
Device found at 0x5A (CCS811 Air Quality)
Device found at 0x76 (BME280 #1)
Device found at 0x77 (BME280 #2)
Total devices found: 5

4. Power Sequencing for CCS811

The CCS811 air quality sensor has specific power-up requirements:

void initCCS811() {
    pinMode(CCS811_WAKE_PIN, OUTPUT);
    digitalWrite(CCS811_WAKE_PIN, LOW); // Wake sensor

    delay(50); // Wait for sensor wake-up

    if (!ccs811.begin(0x5A)) {
        Serial.println("CCS811 not found!");
        return;
    }

    // available() waits for one data-ready sample, not burn-in. On first use allow 60 min in modes 1–3; after selecting a mode allow 20 min conditioning before treating eCO2/eTVOC as accurate.
    while (!ccs811.available()) {
        delay(500);
    }

    ccs811.setDriveMode(CCS811_DRIVE_MODE_1SEC); // Read every 1 second
}

5. Complete Reading Sequence

Optimized order to minimize blocking:

void readAllSensors() {
    // Read fast sensors while CCS811 processes
    float lux = lightMeter.readLightLevel();

    // Read BME280s (I2C read ~5ms each)
    bme_indoor.takeForcedMeasurement();
    float temp_in = bme_indoor.readTemperature();
    float hum_in = bme_indoor.readHumidity();
    float press_in = bme_indoor.readPressure() / 100.0F;

    bme_outdoor.takeForcedMeasurement();
    float temp_out = bme_outdoor.readTemperature();
    float hum_out = bme_outdoor.readHumidity();

    // Set current environmental data before checking for a fresh CCS811 sample
    ccs811.setEnvironmentalData(hum_in, temp_in);

    // Persist these start times and update modeSelectedMs whenever the drive mode changes.
    extern uint32_t firstPowerOnMs;
    extern uint32_t modeSelectedMs;
    const bool ccs811ConditioningComplete =
        static_cast<uint32_t>(millis() - firstPowerOnMs) >= 60UL * 60UL * 1000UL &&
        static_cast<uint32_t>(millis() - modeSelectedMs) >= 20UL * 60UL * 1000UL;
    bool ccs811ReadingsValid = false;
    if (ccs811.available() && ccs811.readData() == 0) {
        eCO2 = ccs811.geteCO2();
        eTVOC = ccs811.getTVOC();
        ccs811ReadingsValid = ccs811ConditioningComplete;
    }

    // Withhold estimates during conditioning; the display should show a conditioning label when invalid.
    updateDisplay(temp_in, hum_in, ccs811ReadingsValid ? eCO2 : NAN, ccs811ReadingsValid ? eTVOC : NAN, lux);
}

Bill of Materials (BOM):

Key Lessons:

1. Bus capacitance matters - high-capacitance devices (OLED) require stronger pull-ups
2. Scan before coding - verify all addresses are correct before writing application code
3. Read datasheets carefully - CCS811 has specific initialization and environmental compensation requirements
4. Optimize read order - start slow sensors first, read fast ones during wait times
5. Pull-up calculation is critical - incorrect values cause intermittent failures that are hard to debug