Low-Power Standby Mode
Standby mode disables the ADC and reduces the sup-
ply-current drain to 3µA (typ). Enter standby mode by
forcing the STBY pin low or through the RUN/STOP bit in
the configuration-byte register. Hardware and software
standby modes behave almost identically; all data is
retained in memory, and the SMB interface is alive and
listening for reads and writes. The only difference is that
in hardware-standby mode, the one-shot command
does not initiate a conversion.
Standby mode is not a shutdown mode. Activity on the
SMBus draws extra supply current (see Typical
Operating Characteristics). In software-standby mode,
the MAX1618 can be forced to perform A/D conversions
through the one-shot command, despite the RUN/STOP
bit being high.
Enter hardware standby mode by forcing the STBY pin
low. In a notebook computer, this line may be connect-
ed to the system SUSTAT# suspend-state signal.
The STBY pin low state overrides any software conver-
sion command. If a hardware- or software-standby com-
mand 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-read-
ing register. The previous data is not changed and
remains available.
Supply-current drain during the 62ms conversion period
is always about 450µA. Between conversions, 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. With
very low supply voltages (under the power-on reset
threshold), the supply current is higher due to the
address input bias currents. It can be as high as 160µA,
depending on ADD0 and ADD1 settings.
SMBus Digital Interface
From a software perspective, the MAX1618 appears as a
set of byte-wide registers that contains temperature data,
alarm threshold values, or control bits. Use a standard
SMBus 2-wire serial interface to read temperature data
and write control bits and alarm threshold data.
The MAX1618 employs four standard SMBus protocols:
Write Byte, Read Byte, Send Byte, and Receive Byte
(Figure 2). The two shorter protocols (Receive and Send)
allow quicker transfers, provided that the correct data
register was previously selected by a Write or Read Byte
instruction. Use caution with the shorter protocols
in multimaster 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
representing +1°C (Table 1), transmitted MSB first.
Measurements are offset by +1/2°C to minimize internal
rounding errors; for example, +99.6°C is reported as
+100°C.
Alarm Threshold Registers
Two registers, a high-temperature (T
HIGH
) and a low-
temperature (T
LOW
) register, store alarm threshold
data. If a measured temperature equals or exceeds the
corresponding alarm threshold value, an ALERT inter-
rupt is asserted.
The power-on reset (POR) state of the T
HIGH
register is
full scale (0111 1111 or +127°C). The POR state of the
T
LOW
register is 1100 1001 or -55°C.
Thermostat Mode
Thermostat mode changes the function of the ALERT
output from a latched interrupt-type output to a self-
clearing thermostat for fan control. This output simply
responds to the current temperature (Figure 3). If the
current temperature is above T
HIGH
, ALERT activates
MAX1618
Remote Temperature Sensor
with SMBus Serial Interface
8 _______________________________________________________________________________________
Table 1. Data Format (Two’s Complement)
DIGITAL OUTPUT
DATA BITS
0 111 1111+127+127.00
0 111 1111
0 111 1111+126+126.00
+127+126.50
0 001 1001
0 000 0001+1+0.50
0 000 0000
0 000 000000.00
ROUNDED
TEMP.
(°C)
TEMP.
(°C)
0+0.25
+25+25.25
0 000 0000
0 000 00000-0.50
1 111 1111
1 111 1111-1-1.00
-1-0.75
1 110 0111
1 110 0111-25-25.50
1 100 1001
1 100 1001-55-55.00
0-0.25
-55-54.75
-25-25.00
1 011 1111
1 011 1111-65-70.00
-65-65.00
SIGN MSB LSB
0 111 1111+127+130.00
