SAM-III Construction Manual
See last page for document and copyright information, File: SAM-III Magnetometer Construction.doc, Page 69
XVII.
Temperature Sensor
Description
Note: The temperature sensor option is no longer supported. The only information available for it is
described below.
If the SAM-III is loaded with the optional SAM3T_ENG.HEX operating system firmware, it has built-in
capability to measure temperature using the Dallas Semiconductor/Maxim DS18B20 digital
thermometer. The SAM3_HEX operating system firmware does not have this capability. The
SAM3T_ENG.HEX firmware can be loaded using pic_downloader as described in sect. XII.
The DS18B20 uses a 1-wire bus protocol and measures temperature from -
55
°
C to +125
°
C. Between -10
°
C and +85
°
C it has an absolute accuracy of
±
0.5
°
C. Maximum resolution is 0.0625
°
C (12-bit).
Note
: Because the 1-wire bus protocol was not designed for distance, the temperature sensor cannot be
located more than approximately 2 m from the SAM-III processor module. This precludes collocating the
temperature sensor with the magnetometer sensors where the magnetometer sensors are remotely
located (which will be the case in almost all applications).
The SAM-III hardware and application software do not use or display the temperature data; the SAM-III
reads the temperature from the DS18B20 and sends it along with magnetometer sensor induction
values in the data stream for later access by the user. The temperature sensor generally would be
located with the magnetometer sensors to obtain temperature data for downstream correlation of
magnetometer sensor drift with temperature. When the SAM-III is powered up, the operating system
software initializes the magnetic sensors (~10 seconds) and then addresses the DS18B20.
The temperature is not displayed on the LCD because there is no room for it. The temperature data is
included in the SAM-III data stream only if a digital thermometer is detected. If a digital thermometer is
not detected, the data stream will not include temperature data.
The temperature sensor is connected as shown below. The schematic shows two filter capacitors, a
high-value tantalum capacitor across the Vdd input and a small multilayer ceramic capacitor across the
DQ data output. These capacitors normally are needed only when there is interference to the
temperature sensor input/output due to a long sensor cable.