Rainbow Electronics DS75LX - Manual (Page 5)

DS75LX: Digital Thermometer and Thermostat with Extended Addressing
5 of 13
OPERATION—MEASURING TEMPERATURE
The DS75LX measures temperature using a bandgap temperature-sensing architecture. An on-board delta-sigma
analog-to-digital converter (ADC) converts the measured temperature to a digital value that is calibrated in degrees
celsius; for Fahrenheit applications a lookup table or conversion routine must be used. The DS75LX is factory-
calibrated and requires no external components to measure temperature.
At power-up the DS75LX immediately begins measuring and converting its own temperature to a digital value. The
resolution of the digital output data is user-configurable to 9, 10, 11, or 12 bits, corresponding to temperature
increments of 0.5
°C, 0.25°C, 0.125°C, and 0.0625°C, respectively, with 9-bit default resolution at power-up. The
resolution is controlled via the R0 and R1 bits in the configuration register as explained in the Configuration
Register section of this data sheet. Note that the conversion time doubles for each additional bit of resolution.
After each temperature measurement and analog-to-digital conversion, the DS75LX stores the temperature as a
16-bit two’s complement number in the 2-byte temperature register (see Figure 2). The sign bit (S) indicates if the
temperature is positive or negative: for positive numbers S = 0 and for negative numbers S = 1. The most recently
converted digital measurement can be read from the temperature register at any time. Since temperature
conversions are performed in the background, reading the temperature register does not affect the operation in
progress.
Bits 3 through 0 of the temperature register are hardwired to 0. When the DS75LX is configured for 12-bit
resolution, the 12 MSbs (bits 15 through 4) of the temperature register contain temperature data. For 11-bit
resolution, the 11 MSbs (bits 15 through 5) of the temperature register contain data, and bit 4 reads out as 0.
Likewise, for 10-bit resolution, the 10 MSbs (bits 15 through 6) contain data, and for 9-bit the 9 MSbs (bits 15
through 7) contain data, and all unused LSbs will contain 0s. Table 1 gives examples of 12-bit resolution digital
output data and the corresponding temperatures.
Figure 2. Temperature, T
H
, and T
L
Register Format
bit 15 bit 14 bit 13 bit 12 bit 11 bit 10 bit 9 bit 8
MS Byte S 2
6
2
5
2
4
2
3
2
2
2
1
2
0
bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0
LS Byte 2
-1
2
-2
2
-3
2
-4
0 0 0 0
Table 1. 12-Bit Resolution Temperature/Data Relationship
TEMPERATURE (
°C)
DIGITAL OUTPUT
(BINARY)
DIGITAL OUTPUT
(HEX)
+125 0111 1101 0000 0000 7D00h
+25.0625 0001 1001 0001 0000 1910h
+10.125 0000 1010 0010 0000 0A20h
+0.5 0000 0000 1000 0000 0080h
0 0000 0000 0000 0000 0000h
-0.5 1111 1111 1000 0000 FF80h
-10.125 1111 0101 1110 0000 F5E0h
-25.0625 1110 0110 1111 0000 E6F0h
-55 1100 1001 0000 0000 C900h