Microcontroller Advanced Kit -
Temperature Sensor Project
This tutorial shows how to
use a microcontroller to interface to a Dallas
1620 temperature sensor IC. This value can be
used by the microcontroller directly or, as in
this tutorial, sent to the PC. It uses the
AT89C2051 microcontroller to collect data
and send it to the PC. A MAX232CPE chip is used
to convert the signals from and to RS232 levels
for sending and receiving through the serial
port. For more details on some aspects of the
hardware, refer to the
Data Collection tutorial.
The
Temperature Sensor
To get a temperature
reading we use the Dallas DS1620 integrated
circuit. It is an 8 pin chip that has a built in
system that measures the temperature and
converts the reading into a 9 bit binary value.
It has an accuracy of 0.5 degrees C and a range
of -55 to 125 C. The temperature reading is
updated about once per second. A digital
interface is included in the chip that allows us
to connect a microcontroller to the chip and
send it commands and receive the temperature
data back from the chip.
The temperature is
received in the microcontroller as 2 bytes. The
second byte only contains a sign bit to signify
whether the temperature is above or below 0
degrees Celsius. For this project we are
ignoring the sign bit and just using the first
byte. We will assume the temperature is above 0
degrees C (32 F). The value in the first byte is
the number of 0.5 degree increments. For
example, if we get a 1 then the temperature is
0.5 degrees C. If we get a 10 then the
temperature is 5 degrees C. The range of
possible values is 0 to 250 which is 0 to 125
degrees C. (The DS1620 can also measure down to
-55 degrees C).
For this project we are only using the serial
interface pins, 1, 2, and 3. The pins 5, 6, and
7 have other functions that are used in
thermostats. They change from 0 to 1 when a
certain temperature is reached (for example, to
turn a heater on and off).
For more details about this and the details of
the DS1620,
Refer to the diagram
below to build the circuit. The
Data Collection Tutorial has more detailed
instructions on using the MAX232 chip.
The Software
The basic process of
compiling an assembly language program and
loading it into the microcontroller was covered
in the
first microcontroller project. The
2051 assembly language program for this project
is temp.asm. It is included on the software CD
that comes with the kit. You will need a device
programmer such as the
PG302 to download the program into the 2051.
The temp.asm program demonstrates a serial
interface with another chip. This is a fairly
common situation. The serial interface can be
created with only 2 or 3 pins. There is usually
a clock line and a data line. In this case there
is also a control line called Reset that acts as
a control signal, signaling the begin and end of
each communication sequence. The clock is
generated by the microcontroller, giving the
microcontroller complete control over the bit by
bit transmission. In this case the
microcontroller is responsible for initiating
each exchange of data over the serial link.
Another possible configuration (for example, 2
microcontrollers linked together) could allow
the device on either end to initiate an
exchange.
