Remote/Local Temperature Sensor
with SMBus Serial Interface
MAX1617A
the MAX1617A can be forced to perform A/D conver-
sions via the one-shot command, despite the RUN/STOP
bit being high.
Activate hardware standby mode by forcing the
STBY
pin low. In a notebook computer, this line may be con-
nected to the system SUSTAT# suspend-state signal.
The
STBY
pin low state overrides any software conversion
command. If a hardware or software standby command is
received while a conversion is in progress, the conversion
cycle is truncated, and the data from that conversion is not
latched into either temperature reading register. The previ-
ous data is not changed and remains available.
Supply-current drain during the 125ms conversion peri-
od is always about 450µA. Slowing down the conver-
sion rate reduces the average supply current (see
Typical Operating Characteristics).
Between conver-
sions, the instantaneous supply current is about 25µA
due to the current consumed by the conversion rate
timer. 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.
Write Byte Format
S
ADDRESS
7 bits
Slave Address: equiva-
lent to chip-select line of
a 3-wire interface
WR
ACK
COMMAND
8 bits
Command Byte: selects which
register you are writing to
ACK
DATA
8 bits
ACK
P
1
SMBus Digital Interface
From a software perspective, the MAX1617A appears 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 tempera-
ture data and write control bits and alarm threshold data.
Each A/D channel within the device responds to the
same SMBus slave address for normal reads and writes.
The MAX1617A employs 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 instruction. Use
caution with the shorter protocols in multi-master systems,
since a second master could overwrite the command
byte without informing the first master.
The temperature data format is 7 bits plus sign in two’s
complement form for each channel, with each data bit rep-
resenting 1°C (Table 2), transmitted MSB first. Measure-
ments are offset by +1/2°C to minimize internal rounding
errors; for example, +99.6°C is reported as +100°C.
Data Byte: data goes into the register
set by the command byte (to set
thresholds, configuration masks, and
sampling rate)
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
Slave Address: repeated
due to change in data-
flow direction
RD
ACK
DATA
8 bits
Data Byte: reads from
the register set by the
command byte
///
P
Send Byte Format
S
ADDRESS
7 bits
WR
ACK
COMMAND
8 bits
Command Byte: sends com-
mand with no data, usually
used for one-shot command
ACK
P
Receive Byte Format
S
ADDRESS
7 bits
RD
ACK
DATA
8 bits
Data Byte: reads data from
the register commanded
by the last Read Byte or
Write Byte transmission;
also used for SMBus Alert
Response return address
///
P
S = Start condition
P = Stop condition
Shaded = Slave transmission
/// = Not acknowledged
Figure 3. SMBus Protocols
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