ADT7410 Temperature Sensor

Contents

• ADT7410 Description
• ADT7410 Registers
• Temperature Measurement Modes
  • Continuous Mode
  • One-Shot Mode
  • One SPS Mode
• Reading the Temperature
• Temperature Watchdog
• Power Modes
• ADT7410 MicroPython Driver for micro:bit
• Index to Sensors

ADT7410 Sensor Description

The ADT7410 is a temperature only sensor chip from Analog Devices Inc.

Temperature is measured with a bandgap temperature sensor. This class of sensor relies upon the temperature dependent voltage characteristic of a silicon diode. This is common technology for this type of temperature sensor.

The supply voltage for this sensor is 2.7V to 5.5V. Thus it is suitable for both 3.3V and 5V microcontrollers.

The current draw for the ADT7410 is very low which is typical of this type of sensor. Average supply current (at 3.3V) is around 210 µA during a temperature conversion with the I²C interface inactive. The chip has a shutdown mode - discussed in more detail later - with a typical current draw (at 3.3V) of only 2 µA.

The measurable temperature range is a very respectable -55°C to 150°C. The sensor chip converts the raw analog value directly to °C in twos complement binary format. The user needs to convert this to the decimal equivalent - something usually done by the driver software.

There is no calibration or linear correction required by the user.

The default analog-to-digital conversion resolution is 13-bits (0.0625°C). However this is configurable with a 16-bit resolution (0.0078°C) also available.

Temperature accuracy to a 3σ confidence is quoted by the manufacturer as being ±0.33°C. Analog Devices also supplies a maximum accuracy (6σ ?) of ±0.5°C.

The ADT7410 is manufactured as an 8-lead, 5mm x 4mm SSOP (Standard Small Outline Package) package.

The ADT7410 has 8 pins arranged in two rows of four.

Pin descriptions:

• VDD, GND
  Positive power pin and Ground pin respectively.

• SDA, SCL
  I²C pins; serial data pin and serial clock pin respectively.

• INT
  Over-temperature and Under-temperature indicator.

• CT
  Critical Over-temperature indicator.

• A0, A1
  i²C address pins; connected independently to either VS or GND to give a choice of four different I²C addresses. That means that up to four of these sensors can be connected to the same I²C bus.

Breakout boards generally expose all of these pins.

The user communicates with the chip through a standard I²C interface. It accepts a maximum serial clock frequency of 400KHz.

As stated above there are four possible addresses that can be configured through the A0 and A1 pins. The possible address are 0x48, 0x49, 0x4A and 0x4B.

ADT7410 Registers

The ADT7410 contains 14 registers:

• 9 x temperature registers consisting of :
  • 2 x temperature measurement registers
  • 7 x Watchdog temperature registers
• 1 x Status register
• 1 x ID register
• 1 x Configuration register
• 1 x Address pointer register
• 1 x software reset

These are all well documented in the Datasheet.

Temperature Measurement Modes

There are three different modes, set with Bits[6:5] of the Configuration register.

Reading the Temperature

Obtaining a temperature reading is simply a matter of writing the Temperature register address (0x00) to set the register address pointer, followed by reading back two bytes.

The two bytes are assembled into a single 16-bit word that represents the °C temperature in two's complement format. If 13-bit resolution is being used then the 13 most significant bits are used. If in 16-bit resolution then all 16 bits are used.

The conversion to decimal is simple. The Datasheet provides the algorithms along with some helpful examples.

Temperature Watchdog

Many digital temperature-only sensors have a watchdog function. The ADT7410 is no exception; with watchdog functions that surpass many of its peers.

Basically, the watchdog monitors temperatures and sets interrupts if high setpoints are exceeded or the temperature drops below a low setpoint.

The ADT7410 has three separate watchdog setpoints, each comprised of 2 x 8-bit registers:

This setpoint stores the °C under-temperature limit value. When the temperature dips below this value the INT pin is activated.

This setpoint stores the °C over-temperature limit value. When this value is exceeded the INT pin is activated.

This setpoint stores the °C critical over-temperature limit value. When this value is exceeded the CT pin is activated.

Under-temperature and over-temperature events both set the INT pin. Which event caused the interrupt can be determined by reading the Status register.

There is also a Hysteresis 8-bit setpoint register. This value is subtracted from the HIGH and CRIT setpoints and added to the LOW setpoint. The calculated hysteresis temperatures determine when over-temperature or under-temperature events no longer exists.

The ADT7410 Watchdog provides several more 'tweaks'. Full details are provided in the excellent product Datasheet.

Power Modes

The ADT7410 has an Active Mode and a Shutdown Mode.  The mode is determined by the value of Bits[6:5] in the Configuration register.

Shutdown Mode significantly reduces power supply current. No temperature readings are taken so the watchdog is also inactive.

The I²C interface is still active, allowing the same read/write access to the sensor's registers as in Active Mode. This also means that the last temperature conversion value is also available (though it will not be updated).

ADT7410 MicroPython Driver for micro:bit

An ADT7410 MicroPython driver specifically for the micro:bit has been developed as part of this series on MicroPython for the microbit. The driver webpage also provides a detailed description of the driver's methods with sample code.

The driver allows a user to easily connect and read the temperature using these sensors from a micro:bit. All three temperature conversion modes plus the Shutdown mode are available.

The ADT7410 driver is implemented as a class. It's easy for a knowledgeable user to extend the class.

The driver does not implement the watchdog functions. The reasoning behind this omission is explained on the driver webpage.