LM75A Temperature Sensor

Overview

The LM75A is the most common temperature sensor found in Antminer hash boards. It is a digital temperature sensor with an I2C interface, capable of measuring temperatures from -55°C to +125°C with ±2°C accuracy. Multiple manufacturers produce LM75A-compatible parts (NXP, TI, ON Semiconductor, STMicroelectronics), all following the same register map and protocol.

On Antminer hash boards, LM75A sensors are placed at several positions along the chip array to monitor ASIC die temperatures. The readings are used by firmware to control fan speed, adjust chip frequency, and trigger thermal protection shutdowns.

Specifications

I2C Address Configuration

The LM75A I2C address is configurable via three address pins (A0, A1, A2), allowing up to 8 sensors on a single I2C bus:

In Antminer hash boards, the most common configuration uses addresses 0x48 through 0x4B (4 sensors per board, with A2 tied to GND).

Register Map

The LM75A has four accessible registers:

Temperature Register (0x00)

The temperature register contains a 9-bit two's complement value in the upper 9 bits of the 16-bit register:

Byte 0 (MSB):  [D8] [D7] [D6] [D5] [D4] [D3] [D2] [D1]
Byte 1 (LSB):  [D0] [x]  [x]  [x]  [x]  [x]  [x]  [x]

Temperature = (Byte0 << 1 | Byte1 >> 7) × 0.5°C

Example readings:

Configuration Register (0x01)

Reading Temperature via PIC Bridge (Antminer)

On Antminer hash boards, temperature sensors are accessed through the PIC16F1704 I2C bridge. The reading sequence is:

TX: [55 AA] [05] [3C] [sensor_addr] [02] [00] [checksum]

RX: [06] [3C] [01] [MSB] [LSB] [checksum]

raw = (msb << 8) | lsb
if raw & 0x8000:  # Negative temperature
    raw = raw - 65536
temp_celsius = (raw >> 7) * 0.5

Common Failure Modes

1. Sensor Not Responding

Symptoms: Miner reports "temp sensor error", temperature shows as 0°C or -1°C.

Causes:

• Failed LM75A IC (internal failure)
• Broken I2C trace (SDA or SCL) between the sensor and the PIC bridge
• Missing 3.3V supply to the sensor
• PIC bridge failure (appears as sensor failure but affects all sensors)

Diagnosis: Check 3.3V at the sensor VCC pin. If power is present, check I2C continuity. If multiple sensors fail simultaneously, suspect the PIC bridge rather than individual sensors.

2. Inaccurate Reading

Symptoms: Temperature reading is significantly higher or lower than actual board temperature.

Causes:

• Sensor not in good thermal contact with the monitoring area
• Sensor operating at incorrect supply voltage
• Environmental interference (nearby hot component affecting the reading)

3. Stuck at Power-On Value

Symptoms: Temperature reads a constant value that never changes.

Causes:

• Sensor in shutdown mode (configuration register bit 0 set)
• I2C bus partially broken (reading cached data)
• Clock line (SCL) stuck, preventing new conversions

Testing Procedures

Replacement Notes

• The LM75A is available in SOIC-8 and SOT-23-5 packages — verify the package type before ordering replacements
• Match the address pin configuration (A0, A1, A2) to the original sensor — incorrect addressing means the firmware cannot find the sensor
• LM75A parts from different manufacturers (NXP, TI, ST) are interchangeable
• Use a soldering iron for SOIC-8 or fine-tip for SOT-23-5
• After replacement, verify the sensor responds at the correct I2C address and reads a reasonable temperature

Found In These Miners

• Antminer S19 — 4 sensors per board (0x48-0x4B)
• Antminer S19 Pro — 4 sensors per board
• Antminer S19 XP — 4 sensors per board
• Antminer S19j Pro — 4 sensors per board
• Antminer S9 — 2-4 sensors per board (older LM75 variant)

Related Pages

• Temperature Sensors Overview
• TMP451 Remote Temperature Sensor
• NCT218 Temperature Sensor
• PIC16F1704 I2C Bridge
• LDO Regulators (3.3V supply)
• Cooling & Thermal Management
• UART, SPI & I2C Explained