DS75 Datasheet, PDF (6/13 Pages) Dallas Semiconductor – 2.Wire Thermal Watchdog

DS75
Configuration Register Programming
The configuration register is accessed if the DS75 pointer is currently set to the 01h location. Writing to
or reading from the register is determined by the R/W bit of the 2–wire control byte (See “2–wire Serial
Data Bus” section). Data is read from or written to the configuration register MSb first. The format of the
register is illustrated below in Figure 3. The effect each bit has on DS75 functionality is described below
along with the power–up state of the bit. The user has read/write access to all bits in the configuration
register. The entire register is volatile, and thus it will power–up in the default state.
CONFIGURATION/STATUS REGISTER
Figure 3
0
R1
R0
F1
F0 POL TM SD
MSb
LSb
SD = Shutdown bit. If SD is “0”, the DS75 will continuously perform temperature conversions and store
the last completed result in the thermometer register. If SD is changed to “1”, the conversion in progress
will be completed and stored; then the device will revert to a low–power standby mode. The O.S. output
will be cleared if the device is in the interrupt mode and remain unchanged in the comparator mode. The
2–wire port remains active. The power–up default state is “0” (continuous conversion mode).
TM = Thermostat mode. If TM=“0”, the DS75 is in the comparator mode. TM=“1” sets the device to the
interrupt mode. See “OPERATION–Thermostat Control” section for a description of the difference
between the two modes. The power–up default state of the TM bit is “0” (comparator mode).
POL = O.S. Polarity Bit. If POL =“1”, the active state of the O.S. output will be high. A “0” stored in
this location sets the thermostat output to an active low state. The user has read/write access to the POL
bit, and the power–up default state is “0” (active low).
F0, F1 = O.S. Fault Tolerance bits. The fault tolerance defines the number of consecutive conversions
returning a temperature beyond limits is required to set the O.S. output in an active state. This may be
necessary to add margin in noisy environments. Table 4 below defines the four settings. The DS75 will
power up with F0=F1=“0”, such that a single occurrence will trigger a fault.
Fault Tolerance Configuration Table 4
F1 F0 Consecutive conversions beyond limits
to generate fault
00
1
01
2
10
4
11
6
R0, R1 = Thermometer resolution bits. Table 5 below defines the resolution of the digital thermometer,
based on the settings of these two bits. There is a direct trade-off between resolution and conversion time,
as depicted in the AC Electrical Characteristics. The default state is R0=0 and R1=0 (9–bit conversions).
“0” = Reserved Location. The master can write to this bit, but it will always read out as a “0”. The
power–on default state is “0”.
6 of 13