How Do You Interface a Digital Temperature Sensor?

Detailed Explanation

Digital temperature sensors integrate the measurement circuit, ADC, and calibration data in a single IC, reducing design complexity compared to an NTC thermistor voltage divider. The two most widely used types in embedded design are the DS18B20 (1-Wire bus) and the MCP9808 (I2C bus).

Analog vs Digital Temperature Sensing

Before selecting a digital sensor, understand the trade-offs against an NTC thermistor circuit:

Use a digital temperature sensor when: accuracy matters more than cost; you have no free ADC channels; you need multiple sensors on one wire; or you need a configurable alert interrupt.

For industrial applications requiring ±0.1–0.5°C accuracy over a wide range (−200°C to +850°C), platinum RTD sensors such as the PT100 and PT1000 are the standard choice — but they require an analog excitation source, Wheatstone bridge, and instrumentation amplifier rather than a simple digital bus connection.

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DS18B20 — 1-Wire Protocol

Hardware Interface

The DS18B20 requires only two connections in powered mode (three if you count VDD):

MCU GPIO (e.g. PA5) ──── 4.7 kΩ ──── VCC (3.3 V or 5 V)
                    │
              DS18B20 DQ pin
              DS18B20 VDD pin ──── VCC
              DS18B20 GND pin ──── GND

The 4.7 kΩ pullup resistor is mandatory — the 1-Wire bus is open-drain and the pullup defines the idle high state.

Parasite power mode (VDD tied to GND, two wires only): possible but not recommended for new designs — see FAQ above.

1-Wire Protocol Basics

1-Wire is a serial protocol that encodes bits using precise timing on the single data line. The master (MCU) initiates all communication; the slave (DS18B20) responds by pulling the line low.

Key timing sequences:

• Initialization: Master pulls line low for 480–960 µs (reset pulse), releases; sensor pulls low for 60–240 µs (presence pulse)
• Write 1: Master pulls low < 15 µs, releases; line recovers to high for remainder of 60 µs slot
• Write 0: Master pulls low for 60–120 µs
• Read: Master pulls low < 15 µs; sensor holds low (bit 0) or releases (bit 1) — master samples at 15 µs

These timing requirements mean you must either use a hardware UART (which can implement 1-Wire timing in oversampled UART mode) or bit-bang with precise microsecond delays. Most MCU platforms have a OneWire library that handles this automatically.

Reading Temperature from DS18B20

Typical firmware sequence (simplified):

// 1. Issue Reset, then Skip ROM (0xCC) to address all sensors
onewire_reset();
onewire_write(0xCC);  // Skip ROM

// 2. Start temperature conversion
onewire_write(0x44);  // Convert T command

// 3. Wait for conversion (750 ms for 12-bit)
delay_ms(750);

// 4. Issue Reset, Skip ROM again, then Read Scratchpad (0xBE)
onewire_reset();
onewire_write(0xCC);
onewire_write(0xBE);

// 5. Read 9 bytes of scratchpad data
for (int i = 0; i < 9; i++) {
    scratchpad[i] = onewire_read();
}

// 6. Extract temperature from bytes 0 and 1 (little-endian 16-bit signed)
int16_t raw = (scratchpad[1] << 8) | scratchpad[0];
float temp_C = raw / 16.0f;  // For 12-bit resolution (0.0625°C per LSB)

The 9-byte scratchpad includes: temperature LSB, temperature MSB, TH and TL alarm registers, configuration register, and CRC. Always verify the CRC (last byte) to confirm data integrity — especially important for long cable runs.

DS18B20 Resolution Configuration

The configuration register's R1:R0 bits set resolution:

For temperature monitoring with 1°C reporting resolution, 9-bit is sufficient and 8× faster than 12-bit.

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MCP9808 — I2C Interface

The MCP9808 from Microchip is a ±0.25°C accuracy digital temperature sensor using a standard I2C bus. It is well-suited to applications requiring tighter accuracy than the DS18B20's ±0.5°C.

Hardware Interface

MCU SCL ──── SCL (MCP9808)
MCU SDA ──── SDA (MCP9808)
             VDD ──── 2.7 V to 5.5 V (typically 3.3 V)
             GND ──── GND
             A0, A1, A2 ──── GND or VDD (set I2C address)
             ALERT (open-drain) ──── 10 kΩ to VDD ──── MCU GPIO input

I2C pull-up resistors (4.7 kΩ to 3.3 V) are required on SCL and SDA — typically already present if other I2C devices are on the bus. See what is I2C? for pull-up sizing details.

I2C Address

The MCP9808 has a 7-bit base address of 0x18, with address bits A2:A1:A0 set by pins. This allows up to 8 MCP9808 devices on a single I2C bus, which is sufficient for multi-zone thermal monitoring in a product enclosure.

Reading Temperature

The ambient temperature register is at register address 0x05. It contains a 13-bit signed value in units of 0.0625°C per LSB, with flag bits for threshold violations.

// Read 2 bytes from register 0x05
i2c_write(MCP9808_ADDR, 0x05);  // Set register pointer
i2c_read(MCP9808_ADDR, buf, 2);  // Read MSB, LSB

// Extract temperature
uint8_t msb = buf[0] & 0x1F;   // Mask flag bits [7:5]
uint8_t lsb = buf[1];

int16_t raw;
if (msb & 0x10) {               // Sign bit set (negative temperature)
    raw = 0xF000 | (msb << 8) | lsb;
} else {
    raw = (msb << 8) | lsb;
}
float temp_C = raw * 0.0625f;

Alert Pin for Interrupt-Driven Temperature Monitoring

Connect the ALERT pin to a GPIO interrupt input on the MCU. Program threshold registers:

// Set TUPPER = 40.0°C, TLOWER = 20.0°C
i2c_write_reg(MCP9808_ADDR, 0x02, 0x02, 0x80);  // TUPPER = 40°C
i2c_write_reg(MCP9808_ADDR, 0x03, 0x01, 0x40);  // TLOWER = 20°C

// Enable alert output in CONFIG register (reg 0x01), enable alert output
i2c_write_reg(MCP9808_ADDR, 0x01, 0x00, 0x08);  // ALERT output enabled

Configure the MCU GPIO as an interrupt input on the falling edge (ALERT is active-low). The ISR reads the temperature register to clear the alert latch and records the event. This pattern keeps the MCU in a low-power sleep state and wakes it only when the temperature leaves the acceptable range — ideal for battery-powered products.

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Other Digital Temperature Sensors

TMP117 (TI, I2C): ±0.1°C accuracy (−20°C to +50°C), 16-bit resolution, one-shot mode for low power. Used in medical and HVAC applications where MCP9808 accuracy is insufficient.

HDC1080 / SHT40 (I2C): Combined temperature and humidity sensors. The SHT40 from Sensirion achieves ±0.2°C / ±1.8% RH accuracy. Used in environmental monitoring, weather stations, HVAC.

MAX31855 (SPI): Dedicated thermocouple-to-digital converter. Connects directly to a K-type thermocouple, performs cold-junction compensation internally, outputs temperature over SPI (14-bit, 0.25°C resolution). Used in high-temperature applications (300–1350°C range) such as reflow oven control, combustion monitoring.

MAX31865 (SPI): Dedicated RTD (PT100/PT1000)-to-digital converter. Handles excitation, ratiometric ADC, and fault detection internally; returns the RTD resistance code over SPI. The equivalent of the MAX31855 for platinum RTDs rather than thermocouples. See using the MAX31865 with a PT100 for wiring, register configuration, and temperature conversion.

For the signal conditioning and ADC interface considerations that apply when analog sensors feed into MCU ADC inputs, the digital sensor approach eliminates those concerns entirely — another reason to prefer digital ICs when accuracy and PCB simplicity matter more than component cost.

For thermal management design in a product, including sensor selection, placement, and alert threshold configuration, Zeus Design's electronics engineering team provides full sensor integration and firmware development support.

Design Considerations

• Place the sensor away from heat sources: PCB traces carrying significant current, voltage regulators, and power transistors all radiate heat that will bias the temperature reading. Mount the DS18B20 or MCP9808 on a separate section of PCB, or on a short extension cable, to measure the ambient environment rather than the board's self-heating.
• Add a 100 nF decoupling capacitor close to VDD: Digital temperature sensor ICs draw brief current spikes during I2C or 1-Wire communication. A 100 nF ceramic capacitor directly at the VDD pin prevents voltage dips on the power supply from causing spurious readings or communication errors.
• Validate 1-Wire CRC in firmware: Long cables, inductive loads nearby, and EMI can corrupt 1-Wire data. The DS18B20 CRC byte (byte 8 of the scratchpad) detects single-bit errors. Always check the CRC and retry the read if it fails rather than accepting corrupted temperature data.

Common Mistakes

• Wrong 1-Wire pullup resistor: The DS18B20 datasheet specifies 4.7 kΩ as the nominal pullup. A 10 kΩ resistor is too weak for buses longer than ~20 cm, causing intermittent communication failures that are difficult to diagnose. A 1 kΩ resistor creates excessive loading on the line. Use 4.7 kΩ as the default.
• Using 12-bit resolution when 9-bit suffices: Every polling cycle takes 750 ms at 12-bit resolution. For a thermostat controller reporting temperature once per second, this is fine. But if the firmware loop must run at 100 Hz for other reasons, 750 ms blocking wait causes missed cycles. Use 9-bit (93 ms) for coarse monitoring, or start the conversion asynchronously and read the result in the next loop iteration.
• Forgetting I2C address conflicts: The MCP9808's default address 0x18 can conflict with other I2C devices — check the full I2C device map for your design before deciding whether to use the address pins. If 0x18 is occupied, set A0=1 to move to 0x19.