Transcript Digital Temperature Gauge

Project Proposal
ECE 4330
Patricia Kaminski
Digital Temperature Gauge
System Overview
Software Design
Hardware Design
Testing
System Overview
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System senses temperature and reports
back to the user through the terminal.
 It demonstrates the programming and use
of a complex external device with the
Atmel AVR (Dallas DS1821 Programmable
Digital Thermostat and Thermometer).
 Added an extra feature to the original
design, a cooling system to actively drop
the temperature of an overheated part.
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Hardware Design
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Uses the Dallas DS1821
Digital Thermometer
connected to PA1.
Temperature reporting by
RS232 to terminal.
Extended design added a
cooling fan connected in
the same manner as Lab
6/7/8 that turns on at 25C
and increases in speed with
increasing temperature.
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Hardware Design
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The programmable thermometer is a highly master-slave
device (AVR is master). Every access of the thermometer
requires a starting signal, even for every bit sent/received.
Every command first requires an initialization (reset) of
pulling the data line low for 480us, the direction of the port is
then set to input (allowing the pull-up resistor to pull the data
line high). 15-60us later the DS1821 pulls the data line low for
60-240us, signaling that the DS1821 is ready for a command.
Each command is 8 bits, with each bit written with a 60us
time slot. “One” values are written by pulling the line low for
up to 15us and then releasing the line (changing the port to
input) for the rest of the 60us. “Zero” values are written by
pulling the line low for the full 60us.
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Hardware Design
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Commands that return data (8 bits) require the line to be set
low for 1us and then the line is released. During the 15us
after forcing the data line low and then released, the DS1821
will then force the line either low or high to send the bit.
Difficulties:
– Because of the difficulty of verifying correct operation, all timing
values had to be verified by oscilloscope.
– The documentation did not specify the direction of bits being
sent/received. Experimenting showed they are sent/received in
a low to high order.
– A surface mount chip was accidentally sent, requiring the
soldering of wires to connect it to the board.
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Software Design
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Each temperature read (and actions) from the
DS1821 requires this instruction sequence:
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Initialization (Reset)
Start Convert Command (0xEE)
Initialization (Reset)
Read Temperature Command (0xAA)
Reading the data bits from the DS1821
Adjusting the PWM value (OCR2) to turn on or speed up
the fan (depending on temperature)
– Outputting the Centigrade and the calculated Fahrenheit
values to the terminal
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Testing
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Testing was accomplished by comparing reported
values from the terminal to an external thermometer.
Temperature changes were made by a hair dryer and
an ice pack.
Results: Successful temperature readings and fan
operation to cool the chip. Steady state temp was
room temp, 22C. Using hair dryer caused reported
temp to rise to 62C. Fan operation brought the temp
back down to room temp quickly. The ice pack
caused temp to drop to 13C. After removal of the
pack, temp was sensed at room temp again.
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