Embedded Systems – Homework 7

ECE471: Embedded Systems – Homework 7
SPI, A/D and Temperature Probe

1. Use your Raspberry Pi for this homework.
You will need an MCP3008 SPI A/D converter as well as a TMP36 temperature sensor (looks like a
transistor) that were in the parts packet.
You can view the datasheet for the MCP3008:
http://web.eece.maine.edu/~vweaver/classes/ece471_2014f/datasheets/MCP3008.pdf
You can view the datasheet for the TMP36:
http://web.eece.maine.edu/~vweaver/classes/ece471_2014f/datasheets/TMP35_36_37.pdf
Camera
Pin1 Pin2
Composite
Audio
HDMI
Power
Pin25 Pin26
Ethernet
USB
Audio/Video
Pin1 Pin2
Ethernet
USB USB
Power
HDMI
Audio/Video
Pin1 Pin2
Power
HDMI
HDMI
USB USB Ethernet
Figure 1: Location of header on Raspberry Pi Model B and B+/2/3, 4B
2. Get the MCP3008 providing values over SPI (3 points)
First wire up the SPI device to the Raspberry Pi. You can use Figure 1, Figure 2 and Table 1 for
guidance.
(a) Put the MCP3008 on a breadboard and connect to your Pi:
i. Connect 3.3V on the Pi to VDD (pin16) on the MCP3008.
Also connect 3.3V to VREF (pin15) on the MCP3008.
ii. Connect GND on the Pi to AGND (pin14) on the MCP3008.
Also connect GND to DGND (pin9) on the MCP3008.
iii. Connect SCLK on the Pi to CLK (pin13) on the MCP3008.
iv. Connect MOSI on the Pi to DIN (pin11) on the MCP3008.
v. Connect MISO on the Pi to DOUT (pin12) on the MCP3008.
vi. Connect CE0 on the Pi to CS (pin10) on the MCP3008.
Table 1: Raspberry Pi Header Pinout
3.3V 1 2 5V
GPIO2 (SDA) 3 4 5V
GPIO3 (SCL) 5 6 GND
GPIO4 (1-wire) 7 8 GPIO14 (UART_TXD)
GND 9 10 GPIO15 (UART_RXD)
GPIO17 11 12 GPIO18 (PCM_CLK)
GPIO27 13 14 GND
GPIO22 15 16 GPIO23
3.3V 17 18 GPIO24
GPIO10 (MOSI) 19 20 GND
GPIO9 (MISO) 21 22 GPIO25
GPIO11 (SCLK) 23 24 GPIO8 (CE0)
GND 25 26 GPIO7 (CE1)
ID_SD (EEPROM) 27 28 ID_SC (EEPROM)
GPIO5 29 30 GND
GPIO6 31 32 GPIO12
GPIO13 33 34 GND
GPIO19 35 36 GPIO16
GPIO26 37 38 GPIO20
GND 39 40 GPIO21
(b) For this first part, we will measure 0V on CH0 (pin1) and 3.3V on input CH1 (pin2). Hook up
power and ground to those pins on the MCP3008.
(c) Enable SPI support in Linux running on your Pi. This is a bit confusing as they have changed the
directions for this at least three times over the years.
• The current way to do this is to run the sudo raspi-config command, select “advanced options”, select “spi” and then say “yes” that you want this enabled and “yes” you
want to load the module.
• You can look at dmesg | grep spi after reboot to see if SPI support was found and
enabled.
• If you have created your own user account, you will want to do a sudo addgroup
YOURUSER spi
(d) Modify the test_spi.c file to read values from the MCP3008 and print them to the screen.
See the classnotes for more details.
• Open the /dev/spidev0.0 file for read/write access.
• Use ioctl to set the mode to SPI_MODE_0
• In an infinite loop, read the value of CH0 and CH1 once a second and print the voltages to
the screen.
– As described in class use ioctl to transmit 3 bytes:
* Be sure to clear out your transmit buffer with zeroes first (using memset or similar)
or you may get errors!
* Transmit three bytes:
2
i. the first byte contains just the start bit,
ii. the second byte has the single-ended mode bit, followed by 3 bits indicating the
channel to read, followed by zeros
iii. the third byte should be all zeros.
* Receive 3 bytes. The first byte can be ignored, the bottom 2 bits of the second byte
are bits 9 and 8, and the third byte is the bottom 8 bits of the result.
– Put the 10 bits you receive into an integer, then use VIN =
value×VREF
1024 to convert to a
floating-point voltage.
– CH0 should be roughly 0V and CH1 should be 3.3V.
(e) Be sure to comment your code and check for errors!
TMP36
3.3V Vout GND
1
2
3
4
5
6
7
8 9
10
11
12
13
14
15
16
MCP3008
Figure 2: TMP36 and MCP3008 Pinouts
3. Hook up the TMP36 to the SPI device (3 points)
(a) Copy your test_spi.c file to display_temp.c
(b) Connect the TMP36 temperature probe to CH2.
Connect pin1 (3.3V) of the TMP36 to 3.3V
Connect pin2 (Vout) of the TMP36 to CH2 on the MCP3008
Connect pin3 (GND) of the TMP36 to ground
WARNING! the datasheet shows the pins from the bottom not the top. If you reverse the
power/ground settings on the chip it will quickly heat up to 100+ degrees and will possibly
be ruined! Follow the diagram in Figure 2 and you will be OK.
(c) Modify the code to print the current temperature as read by the probe, once a second. You can
print degrees C or F as per your preference, but make sure the units are displayed.
(d) The temperature can be determined with the following equation:
deg_C = (100 × voltage) − 50
(e) Also the following might be useful:
deg_F = (deg_C ×
9
5
) + 32
Beware of C floating point/integer conversion rules!
(f) Be sure to comment your code!
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4. Something Cool (1 point)
Copy your code to temp_cool.c and modify it to do something cool. Describe what you did in the
README file.
• Monitor the temperature, and after a while print the high/low temperatures recorded.
• Monitor the temperature and print a message if a temp is exceeded (for example, print a message
if someone touches the probe long enough to raise the temperature).
• Hook up the analog discovery board and plot one SPI transaction on the MOSI/MISO/CS0/CLK
lines. Send the plot in with your assignment.
• Note: Homework 9 involves displaying the temperature on the LED display so while it would be
cool to do that, you might as well wait a few weeks.
5. Questions (2 points)
Answer the following in the README file:
(a) You are designing an embedded system for a car that controls the anti-lock brakes. The specification says that to work properly the brakes needed to start pulsing within 10ms. Would this be
a hard, firm, or soft real-time task? Why?
(b) You are designing another part of the car. The specification says that if you push the “tune”
button on the stereo that it should switch stations within 1s. Would this be a hard, firm, or soft
real-time task? Why?
(c) You are working on the “info-tainment” system for the car, and it has a movie player for the
backseat. The specification calls for the video decoder to be able to maintain a framerate of
60Hz. Is this a hard, firm, or soft real-time task? Why?
(d) What is one disadvantage of SPI compared to i2c?
(e) What is one advantage SPI has over i2c?
(f) You want to connect a second temperature probe to the MCP3008 from this homework, but with
the TMP36 at the end of a 150-foot cable. Would this work? Why or why not?
(g) The MCP3008 datasheet says:
“... the time between the end of the sample period and the time that all 10 data bits have been
clocked out must not exceed 1.2 ms (effective clock frequency of 10 kHz). Failure to meet this
criterion may introduce linearity errors into the conversion outside the rated specifications.”
Your boss asks you to talk an MCP3008 by bitbanging SPI via GPIO from regular Linux. How
might this be a problem? Can you guarantee this timing if controlling the pins from userspace?
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6. Linux Fun (1 point)
Linux shells have what’s known as job control. This isn’t a question, but you can try it out and see
how it works. To suspend something, press Control-Z. So if you are editing code in nano, try pressing
Control-Z. It should bring you back to the prompt. To get back to nano again type fg. You can use
this to run make while editing and going back without losing your place. You can also use bg to
put programs running in the background, but be careful (you don’t want that to happen to nano for
example).
Linux has very useful devices under /dev besides the i2c and spi nodes.
(a) The first is /dev/null. What happens if you pipe a command to it?
For example ls > /dev/null?
(b) The next is /dev/full. What happens if you pipe a command to it?
For example ls > /dev/full?
(c) /dev/zero contains nothing but zeros. Why might that be useful?
(d) What do you think /dev/random contains?
7. Submitting your work
• Run make submit which will create a hw7_submit.tar.gz file containing Makefile,
README, test_temp.c, temp_cool.c and display_temp.c.
You can verify the contents with tar -tzvf hw7_submit.tar.gz
• e-mail the hw7_submit.tar.gz file to me by the homework deadline. Be sure to send the
proper file!
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