Datasheet

Alarm Signaling
After the device performs a temperature conversion, the
temperature value is compared to the user-defined two’s
complement alarm trigger values stored in the 1-byte T
H
and T
L
registers (see T
H
and T
L
Register Format). The
signbit(S) indicatesifthevalueispositiveornegative;
forpositivenumbersS=0andfornegativenumbersS
= 1. The T
H
and T
L
registers are nonvolatile (EEPROM)
so they retain data when the device is powered down.
T
H
and T
L
can be accessed through bytes 2 and 3 of the
scratchpad, as explained in the Memory section.
Onlybits11:4ofthetemperatureregisterareusedinthe
T
H
and T
L
comparison since T
H
and T
L
are 8-bit registers.
If the measured temperature is lower than or equal to T
L
or higher than or equal to T
H
, an alarm condition exists
and an alarm flag is set inside the device. This flag is
updatedaftereverytemperaturemeasurement;therefore,
if the alarm condition goes away, the flag is turned off after
the next temperature conversion.
The master device can check the alarm flag status of all
MAX31820PAR devices on the bus by issuing an Alarm
Search[ECh]command.Anydeviceswithasetalarmflag
respond to the command, so the master can determine
exactly which devices have experienced an alarm condi-
tion. If an alarm condition exists and the T
H
or T
L
settings
have changed, another temperature conversion should be
done to validate the alarm condition.
T
H
and T
L
Register Format
Parasite Power
The device’s parasite-power circuit allows it to operate
without a local power supply. Parasite power is very
useful for applications that require remote temperature
sensing, or those that are very space constrained.
Figure 1 shows the device’s parasite-power control
circuitry, which “steals” power from the 1-Wire bus
through the DQ pin when the bus is high. The stolen
charge powers the device while the bus is high, and
some of the charge is stored on the parasite-power
capacitor (C
PP
) to provide power when the bus is low.
In parasite-power mode, the 1-Wire bus and C
PP
can
provide sufficient current to the device for most opera-
tions as long as the specified timing and voltage require-
ments are met (see the DC Electrical Characteristics and
AC Electrical Characteristics
tables).However,whenthe
device is performing temperature conversions or copy-
ing data from the scratchpad memory to EEPROM, the
operating current can be as high as 1.5mA. This current
can cause an unacceptable voltage drop across the weak
1-Wire pullup resistor and is more current than can be
supplied by C
PP
. To ensure that the device has sufficient
supply current, it is necessary to provide a strong pullup
on the 1-Wire bus whenever temperature conversions
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
Figure 1. Supplying the Parasite-Powered MAX31820PAR During Temperature Conversions
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
S 2
6
2
5
2
4
2
3
2
2
2
1
2
0
µP
V
PU
4.7kΩ
V
PU
1-Wire BUS
TO OTHER 1-Wire DEVICES
MAX31820PAR
GND DQ
MAX31820PAR 1-Wire, Parasite-Power,
Ambient Temperature Sensor
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