Multichannel Remote/Local
Temperature Sensors
MAX1668/MAX1805/MAX1989
†
sion cycle is truncated, and the data from that conversion
is not latched into either temperature-reading register. The
previous data is not changed and remains available.
In standby mode, supply current drops to about 3µA.
At very low supply voltages (under the power-on-reset
threshold), the supply current is higher due to the
address pin bias currents. It can be as high as 100µA,
depending on ADD0 and ADD1 settings.
tion. Use caution with the shorter protocols in multimaster
systems, since a second master could overwrite the com-
mand byte without informing the first master.
The temperature data format is 7 bits plus sign in two’s-com-
plement form for each channel, with each data bit represent-
ing 1°C (Table 2), transmitted MSB first. Measurements are
offset by +0.5°C to minimize internal rounding errors; for
example, +99.6°C is reported as +100°C.
SMBus Digital Interface
From a software perspective, the MAX1668/MAX1805/
MAX1989 appear as a set of byte-wide registers that
contain temperature data, alarm threshold values, or
control bits. A standard SMBus 2-wire serial interface is
used to read temperature data and write control bits and
alarm threshold data. Each A/D channel within the
devices responds to the same SMBus slave address for
normal reads and writes.
The MAX1668/MAX1805/MAX1989 employ four standard
SMBus protocols: write byte, read byte, send byte, and
receive byte (Figure 3). The shorter receive byte protocol
allows quicker transfers, provided that the correct data
register was previously selected by a read byte instruc-
Write Byte Format
S
ADDRESS
7 bits
Slave Address: equiva-
lent to chip-select line of
a 3-wire interface
Read Byte Format
S
ADDRESS
7 bits
Slave Address: equiva-
lent to chip-select line
WR
ACK
COMMAND
8 bits
Command Byte: selects
which register you are
reading from
ACK
S
ADDRESS
7 bits
RD
WR
ACK
COMMAND
8 bits
Command Byte: selects which
register you are writing to
ACK
Alarm Threshold Registers
Ten (six for MAX1805) registers store alarm threshold
data, with high-temperature (T
HIGH
) and low-tempera-
ture (T
LOW
) registers for each A/D channel. If either
measured temperature equals or exceeds the corre-
sponding alarm threshold value, an
ALERT
interrupt is
asserted.
The power-on-reset (POR) state of all T
HIGH
registers of
the MAX1668 and MAX1805 is full scale (0111 1111, or
+127°C). The POR state of the channel 1 T
HIGH
register
of the MAX1989 is 0110 1110 or +110°C, while all other
channels are at +127°C. The POR state of all T
LOW
reg-
isters is 1100 1001 or -55°C.
DATA
8 bits
ACK
P
1
Data Byte: data goes into the register
set by the command byte (to set
thresholds, configuration masks, and
sampling rate)
ACK
DATA
8 bits
///
P
Slave Address: repeated
due to change in data-
flow direction
Receive Byte Format
Data Byte: reads from
the register set by the
command byte
Send Byte Format
S
ADDRESS
7 bits
WR
ACK
COMMAND
8 bits
Command Byte: sends com-
mand with no data
ACK
P
S
ADDRESS
7 bits
RD
ACK
DATA
8 bits
///
P
S = Start condition
P = Stop condition
Shaded = Slave transmission
/// = Not acknowledged
Data Byte: This command only
works immediately following a
Read Byte. Reads data from the
register commanded by that last
Read Byte; also used for SMBus
Alert Response return address
Figure 3. SMBus Protocols
10
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
