Embedded Systems – Homework 9

ECE471: Embedded Systems – Homework 9
Temperature Display

1. You will need the i2c display from Homework 5 as well as either the MCP3008/TMP36 from Homework 7 or the DS18B20 sensor from Homework 8 (your choice).
Upon completing this assignment you can turn back in the parts you have signed out.
Be sure to check for errors and to comment your code!
2. Make your temperature code modular (2pts)
Take one of your temperature reading homeworks as a basis for this project. Copy your code into
read_temp.c
The interface needed, as described in read_temp.h is
double read_temp(void);
So have your code read the temperature, and return it as a double value in degrees C.
If there is an error, report a temperature of less than -1000 degrees.
The provided test_temp program uses this interface, so once your code is ready, running make
should build test_temp and it should print the temperature when you run it.
3. Make your display code modular (3pts)
Take your i2c display code and put it into the write_display.c file.
The interface it should have as described in write_display.h is:
int init_display(void);
int write_display(int fd,double value);
int shutdown_display(int fd);
The init_display() function should init the display (including turning it on, brightness, etc) and
return the file descriptor.
The write_display() function should take the file descriptor and the temperature value and put
that onto the display. (See below for more details).
The shutdown_display() function should take the file descriptor and close it, as well as any
other cleanup that needs to be done (which might be none, depending on how your code works).
The write_display() function should print the provided floating point temperature value (in F
or C, your choice).
Your code should handle four cases:
(a) Temperatures from 0 to 99.9 degrees, inclusive. 0.0 ≤ temp ≤ 99.9 These should be displayed
as two digits, a decimal point, another digit, and then a degree symbol (which is just a crude circle
made of the top 4 segments on the display). Leading zeros should be suppressed (i.e. display
"1.2" not "01.2", 0 should be "0.0")
(b) Temperatures between -99.9 and 0 degrees. −99.9 ≤ temp < 0 These should display a minus
sign and then two digits of temperature, then the degree symbol.
(c) Temperatures between 100 and 999 degrees. 100 ≤ temp ≤ 999 should print three digits of
temperature, then the degree symbol.
(d) Invalid temperatures that won’t fit the display (and errors reading the thermometer) should be
reported (via the display) in a method that isn’t a valid temperature. It is your choice how to
indicate this.
Once you have the code working, you can use the provided test_display program to test that it
is printing things properly. It takes one command line argument, which is the floating point value to
print.
4. Testing the display output (1pt)
Use the test_display program to test various inputs and be sure they meet the standards as described.
In the README, list 5 test values you used, and write a brief note for why you chose those values /
why you think they cover the functionality of the interface.
5. Temperature Display (1pt)
Now modify display_temp.c to be a program that uses the interface above to reads the temperature once a second and writes the value to the display.
6. Something Cool
No something cool for this homework. Put any coolness to use in your final project.
7. Questions (1pt)
Edit the README file to have your name and answer the following questions.
(a) Name one example of poorly written embedded code that had disastrous results.
(b) Why might it be good to always try to write correct, documented, well tested code even if you
think it’s not going to ever be used in anything important?
8. Linux Fun (2pts)
Do the following on a raspberry pi. If for whatever reason you do the exercise on some other sort of
machine, describe what kind you used.
When a file is created or modified on Linux various timestamps are updated. atime (last access time)
mtime (last modified time) and ctime (last attribute update).
The ls -lt (that’s a lowercase L) will show all files and their last modified time.
The Linux touch command will update the timestamps on a file to the current time (and create the
file if it doesn’t exist). You can also specify the time. You can do things like
touch --date "1983-10-16 14:40" blah
which will update the timestamp on the file blah to the specified date.
You can also do fun things like
touch --date "next Thursday" blah
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(a) Use touch to change the file modification time of the "fakedate" file (included with the test code)
with a date from some other year (not 2020).
(b) What happens if you try to create a date in the year 2044? Why?
(c) Sometimes students turn in homework late, but suggest “check the file timestamp, it shows I
finished it before the deadline”. Why might this not be the most convincing argument?
9. Submitting your work
• Run make submit which will create a hw9_submit.tar.gz file containing Makefile,
README, display_temp.c, and fakedate. You can verify the contents with tar -tzvf
hw9_submit.tar.gz
• e-mail the hw9_submit.tar.gz file to me by the homework deadline. Be sure to send the
proper file!
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