Digilent 410-274P-KIT - Manual (Page 23)

Nexys4™ FPGA Board Reference Manual
Copyright Digilent, Inc. All rights reserved.
Other product and company names mentioned may be trademarks of their respective owners.
Page 23 of 29
12.1 I2C Interface
The ADT7420 chip acts as a slave device using the industry standard I2C communication scheme. To communicate
with ADT7420 chip, the I2C master must specify a slave address (0x4B) and a flag indicating whether the
communication is a read (1) or a write (0). Once specifications are made for communication, a data transfer takes
place. For ADT7420, the data transfer should consist of the address of the desired device register followed by the
data to be written to the specified register. To read from a register, the master must write the desired register
address to the ADT7420, then send an I2C restart condition, and send a new read request to the ADT7420. If the
master does not generate a restart condition prior to attempting the read, the value written to the address
register will be reset to 0x00.
As some registers store 16-bit values as 8-bit register pairs, the ADT7420 will automatically increment the address
register of the device when accessing certain registers, such as the temperature registers and the threshold
registers. This allows for the master to use a single read or write request to access both the low and high bytes of
these registers. A complete listing of registers and their behavior can be found in the ADT7420 datasheet available
on the Analog Devices web site.
12.2 Open Drain Outputs
The ADT7420 provides two open drain output signals to indicate when pre-set temperature thresholds are
reached. If the temperature leaves a range defined by registers TLOW (0x06:0x07) and THIGH (0x04:0x05), the INT
pin can be driven low or high based upon the configuration of the device. Similarly, the CT pin can be driven low or
high if the temperature exceeds a critical threshold defined in TCRIT (0x08:0x09). Both of these pins need internal
FPGA pull-ups when used.
For details on the electrical specifications and configuration of the INT and CT pins, refer to the ADT7420
datasheet.
12.3 Quick Start Operation
When the ADT7420 is powered up, it is in a mode that can be used as a simple temperature sensor without any
initial configuration. By default, the device address register points to the temperature MSB register, so a two byte
read without specifying a register will read the value of the temperature register from the device. The first byte
read back will be the most significant byte (MSB) of the temperature data, and the second will be the least
significant byte (LSB) of the data. These two bytes form a two’s complement 16-bit integer. If the result is shifted
to the right three bits and multiplied by 0.0625, the resulting signed floating point value will be a temperature
reading in degrees Celsius.
For information on reading and writing to the other registers of the device, as well as notes on the accuracy of the
temperature measurements, refer to the ADT7420 datasheet.