Datasheet

are taking place, or data is being copied from the scratch-
padtoEEPROM.Thiscanbeaccomplishedbyusinga
MOSFETtopullthebusdirectlytotherail,asshownin
Figure1. The 1-Wire bus must be switched to the strong
pullup within 10µs (max) after a Convert T [44h] or Copy
Scratchpad[48h]commandisissued,andthebusmust
be held high by the pullup for the duration of the conver-
sion (t
CONV
) or data transfer (t
WR
 = 10ms). No other
activity can take place on the 1-Wire bus while the pullup
is enabled.
The device can also be powered by the conventional
method of connecting an external power supply to the
V
DD
pin, as shown in Figure 2. The advantage of this
methodis thatthe MOSFETpullupisnotrequired,and
the 1-Wire bus is free to carry other traffic during the
temperature-conversion time.
The use of parasite power is not recommended for
temperatures above +100°C since the device may not be
able to sustain communications due to the higher leakage
currentsthatcanexistatthesetemperatures.Forapplica-
tions in which such temperatures are likely, it is strongly
recommended that the device be powered by an external
power supply.
In some situations the bus master might not know whether
the devices on the bus are parasite powered or powered
by external supplies. The master needs this information
to determine if the strong bus pullup should be used
during temperature conversions. To get this informa-
tion,themastercanissueaSkipROM[CCh]command
followed by a Read Power Supply [B4h] command,
followed byaread time slot. Duringtheread time slot,
parasite-powered devices pull the bus low, and exter-
nally powered devices let the bus remain high. If the
bus is pulled low, the master knows that it must supply
the strong pullup on the 1-Wire bus during temperature
conversions.
64-Bit Lasered ROM Code
Each device contains a unique 64-bit code stored in ROM
(
Figure3). The least significant 8 bits of the ROM code
contain the device’s 1-Wire family code, 28h. The next
48 bits contain a unique serial number. The most sig-
nificant 8 bits contain a cyclic-redundancy-check (CRC)
byte that is calculated from the first 56 bits of the ROM
code. A detailed explanation of the CRC bits is provided
in the CRC Generation section. The 64-bit ROM code and
associated ROM function control logic allow the device to
operate as a 1-Wire device using the protocol detailed in
the 1-Wire Bus System section.
Figure 2. Powering the MAX31820 with an External Supply
Figure 3.64-Bit Lasered ROM Code
µP
V
PU
4.7kΩ
1-Wire BUS
TO OTHER 1-Wire DEVICES
(EXTERNAL SUPPLY)V
DD
MAX31820
GND DQ V
DD
MSb
8-BIT
CRC CODE
48-BIT SERIAL NUMBER
MSb MSbLSb
LSb
LSb
8-BIT FAMILY CODE
(28h)
MSbLSb
MAX31820 1-Wire Ambient Temperature Sensor
www.maximintegrated.com
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