? Maxim Integrated Products 6
MAX31826
1-Wire Digital Temperature Sensor
with 1Kb Lockable EEPROM
Detailed Description
The MAX31826 digital thermometer provides 12-bit
temperature measurements and communicates over a
1-Wire
M
bus that by definition requires only one data line
(and ground) for communication with a central microcon-
troller. The data line requires a weak pullup resistor since
all devices are linked to the bus through a three-state
or open-drain port (i.e., the MAX31826s DQ pin). Four
location address inputs simplify mapping of individual
devices to specific locations.
Each device has a unique 64-bit serial code, allowing
multiple devices to function on the same 1-Wire bus.
Therefore, it is simple to use one microcontroller to con-
trol many devices distributed over a large area. In this
bus system, the microcontroller identifies and addresses
devices on the bus using each devices unique 64-bit
code. Because each device has a unique code, the
number of devices that can be addressed on one bus
is virtually unlimited. The 1-Wire bus protocol, including
detailed explanations of the commands and time slots, is
described in the 1-Wire Bus System section.
The Scratchpad 1 memory contains the 2-byte tem-
perature register that stores the digital output from the
temperature sensor. An additional 128 bytes (1Kb) of
general-purpose EEPROM is included for storage of sys-
tem data. The EEPROM can be locked to permanently
prevent any further data writes.
The device can also operate without an external power
supply. Power is instead supplied through the 1-Wire
pullup resistor through DQ when the bus is high. The
high bus signal also charges an internal capacitor (C
PP
),
which then supplies power to the device when the bus is
low. This method of deriving power from the 1-Wire bus
is referred to as parasite power. Alternatively, a power
supply on V
DD
can also power the device.
Measuring Temperature
The devices core functionality is its direct-to-digital tem-
perature sensor. The resolution of the temperature sensor
is 12 bits, corresponding to a least significant bit value
of 0.0625NC. The device powers up in a low-power idle
state. To initiate a temperature measurement, the master
must issue a Convert T command. Following the conver-
sion, the resulting thermal data is stored in the 12-bit tem-
perature register in the Scratchpad 1 memory and the
device returns to its idle state. If the device is powered by
an external supply, the master can issue read time slots
(see the 1-Wire Bus System section) after the Convert T
command, and the device responds by transmitting 0
while the temperature conversion is in progress and 1
when the conversion is done. If the device is powered
with parasite power, this notification technique cannot be
used because the bus must be pulled high by a strong
pullup during the entire temperature conversion. The
bus requirements for parasite power are explained in the
Powering the MAX31826 section.
The temperature data (in NC) is stored as a 16-bit sign-
extended twos complement number in the temperature
register (see the Temperature Register Format). The sign
bits (S) indicate if the temperature is positive or negative;
for positive numbers S = 0 and for negative numbers
S = 1. Table 1 gives examples of digital output data and
the corresponding temperature readings.
1-Wire is a registered trademark of Maxim Integrated Products, Inc.
Temperature Register Format
BIT 15
BIT 14
BIT 13
BIT 12
BIT 11
BIT 10
BIT 9
BIT 8
MSB
S
S
S
S
S
2
6
2
5
2
4
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
LSB
2
3
2
2
2
1
2
0
2
-1
2
-2
2
-3
2
-4