TMP451 Remote Temperature Sensor

Overview

The TMP451 is a high-accuracy remote/local temperature sensor manufactured by Texas Instruments, found primarily in MicroBT Whatsminer hash boards. Unlike the LM75A which measures only local (on-die) temperature, the TMP451 can measure both its own die temperature (local) and the junction temperature of a remote semiconductor die via an external thermal diode — typically a dedicated sensing transistor built into the ASIC chip package.

This remote sensing capability allows the TMP451 to measure the actual ASIC junction temperature with higher accuracy than a local sensor mounted on the PCB surface, which only measures the board temperature near the chip rather than the chip itself.

Specifications

How Remote Sensing Works

The TMP451 measures remote temperature by passing a known current through an external diode-connected transistor (typically a PNP BJT or a dedicated thermal diode built into the ASIC). The forward voltage of this diode is temperature-dependent — it decreases by approximately 2mV per degree Celsius.

TMP451                         ASIC Chip
┌──────────┐                 ┌────────────┐
│          │  DXP  ──────────│  Thermal   │
│  Remote  │                 │   Diode    │
│  Channel │  DXN  ──────────│  (on-die)  │
│          │                 └────────────┘
└──────────┘

The TMP451 uses a dual-current technique to cancel out series resistance errors, making it more accurate than simpler single-current methods. This is particularly important in mining applications where high-current PCB traces near the sensor can introduce parasitic resistance.

I2C Interface

Default Address

The TMP451 has a fixed default I2C address of 0x4C. Some variants support an alternate address pin, but in mining hardware the default address is almost always used.

Key Registers

Reading Temperature

To read the full 12-bit remote temperature:

# Read remote temperature from TMP451 at 0x4C
msb = i2c_read(0x4C, 0x01)  # Integer part
lsb = i2c_read(0x4C, 0x10)  # Fractional part

# Combine: MSB is integer degrees, LSB upper 4 bits are fraction
temp = msb + (lsb >> 4) * 0.0625

# Example: msb=0x4B (75), lsb=0x80 (8 × 0.0625 = 0.5)
# Temperature = 75.5°C

Common Failure Modes

1. Remote Channel Open Circuit

Symptoms: Remote temperature reads -64°C or the status register shows the OPEN bit set. Local temperature still reads correctly.

Causes:

• Broken trace between TMP451 DXP/DXN pins and the ASIC thermal diode
• ASIC chip failure destroying the internal thermal diode
• Solder joint failure on the TMP451 or the diode connection

Diagnosis: Check continuity between the TMP451 DXP/DXN pins and the ASIC thermal diode pads. Read the status register (0x02) — bit 2 indicates remote diode open circuit.

2. Sensor Not Responding on I2C

Symptoms: No ACK at address 0x4C during I2C scan.

Causes:

• Failed TMP451 IC
• Missing supply voltage (check 3.3V rail from LDO)
• I2C bus held low by another failed device

Diagnosis: Verify 3.3V at VCC pin. Check SDA/SCL lines are not stuck LOW.

3. Inaccurate Remote Reading

Symptoms: Remote temperature significantly different from expected (more than 5°C off from comparable sensors).

Causes:

• Missing or incorrect calibration offset (register 0x11)
• PCB noise coupling into the DXP/DXN traces
• Wrong ideality factor for the particular ASIC's thermal diode

Diagnosis: Read the remote offset register. Compare local vs. remote readings — in equilibrium (board cold, no load), they should be within 2-3°C of each other.

Testing Procedures

Replacement Notes

• TMP451 comes in a SOT-23-6 package — use a fine-tip soldering iron or hot air at 260°C
• The DXP and DXN pins are sensitive — ensure clean solder joints with no bridging
• After replacement, verify the manufacturer ID register reads 0x55
• Check that firmware does not require a specific calibration offset to be programmed in the remote offset register
• If replacing due to remote channel failure, also inspect the ASIC thermal diode connection — the replacement sensor will fail the same way if the trace is broken

Found In These Miners

• Whatsminer M50 — 2-4 sensors per board
• Whatsminer M50S — 2-4 sensors per board
• Whatsminer M30S — 2 sensors per board
• Whatsminer M30S++ — 2 sensors per board

Related Pages

• Temperature Sensors Overview
• LM75A Temperature Sensor
• NCT218 Temperature Sensor
• LDO Regulators (sensor power supply)
• Cooling & Thermal Management
• UART, SPI & I2C Explained