Datasheet MAX1618 (Maxim) - 8

Remote Temperature Sensor with SMBus Serial Interface Low-Power Standby Mode Table 1. Data Format (Two’s Complement)
Standby mode disables the ADC and reduces the sup- ply-current drain to 3µA (typ). Enter standby mode by
DIGITAL OUTPUT ROUNDED TEMP.
forcing the STBY pin low or through the RUN/STOP bit in
DATA BITS TEMP. (°C)
the configuration-byte register. Hardware and software
(°C) SIGN MSB LSB
standby modes behave almost identically; all data is +130.00 +127 0 111 1111 retained in memory, and the SMB interface is alive and listening for reads and writes. The only difference is that +127.00 +127 0 111 1111 in hardware-standby mode, the one-shot command
MAX1618
+126.50 +127 0 111 1111 does not initiate a conversion. +126.00 +126 0 111 1111 Standby mode is not a shutdown mode. Activity on the +25.25 +25 0 001 1001 SMBus draws extra supply current (see Typical +0.50 +1 0 000 0001 Operating Characteristics). In software-standby mode, the MAX1618 can be forced to perform A/D conversions +0.25 0 0 000 0000 through the one-shot command, despite the RUN/STOP 0.00 0 0 000 0000 bit being high. -0.25 0 0 000 0000 Enter hardware standby mode by forcing the STBY pin -0.50 0 0 000 0000 low. In a notebook computer, this line may be connect- ed to the system SUSTAT# suspend-state signal. -0.75 -1 1 111 1111 -1.00 -1 1 111 1111 The STBY pin low state overrides any software conver- sion command. If a hardware- or software-standby com- -25.00 -25 1 110 0111 mand is received while a conversion is in progress, the -25.50 -25 1 110 0111 conversion cycle is truncated, and the data from that -54.75 -55 1 100 1001 conversion is not latched into either temperature-read- ing register. The previous data is not changed and -55.00 -55 1 100 1001 remains available. -65.00 -65 1 011 1111 Supply-current drain during the 62ms conversion period -70.00 -65 1 011 1111 is always about 450µA. Between conversions, the instantaneous supply current is about 25µA due to the The temperature data format is 7 bits plus sign in two’s current consumed by the conversion-rate timer. In complement form for each channel, with each data bit standby mode, supply current drops to about 3µA. With representing +1°C (Table 1), transmitted MSB first. very low supply voltages (under the power-on reset Measurements are offset by +1/2°C to minimize internal threshold), the supply current is higher due to the rounding errors; for example, +99.6°C is reported as address input bias currents. It can be as high as 160µA, +100°C. depending on ADD0 and ADD1 settings.
Alarm Threshold Registers SMBus Digital Interface
Two registers, a high-temperature (THIGH) and a low- From a software perspective, the MAX1618 appears as a temperature (TLOW) register, store alarm threshold set of byte-wide registers that contains temperature data, data. If a measured temperature equals or exceeds the alarm threshold values, or control bits. Use a standard corresponding alarm threshold value, an ALERT inter- SMBus 2-wire serial interface to read temperature data rupt is asserted. and write control bits and alarm threshold data. The power-on reset (POR) state of the THIGH register is The MAX1618 employs four standard SMBus protocols: full scale (0111 1111 or +127°C). The POR state of the Write Byte, Read Byte, Send Byte, and Receive Byte TLOW register is 1100 1001 or -55°C. (Figure 2). The two shorter protocols (Receive and Send)
Thermostat Mode
allow quicker transfers, provided that the correct data Thermostat mode changes the function of the ALERT register was previously selected by a Write or Read Byte output from a latched interrupt-type output to a self- instruction. Use caution with the shorter protocols clearing thermostat for fan control. This output simply in multimaster systems, since a second master could responds to the current temperature (Figure 3). If the overwrite the command byte without informing the first current temperature is above T master. HIGH, ALERT activates
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