Freescale Semiconductor
Application Note
Document Number: MPC5604B
Rev. 0, 01/2011
Using a RCA camera for line
detection
Example code based on MPC5604B MCU
by:
Francisco Ramirez Fuentes, Marco Trujillo, Cuauhtli Padilla, Rodrigo
Mendoza
Contents
1 Introduction
This application note provides details on how a RCA camera
works and how it can be used for the specific application of
following a line (for the Smart Car Race competition). It
includes tips and refers to example code on how to process the
signal from the camera on the MPC5604B Freescale
microcontroller (MCU); the example code can be downloaded
as AN4245SW from http://www.freescale.com.
In this application note, the 1vpp@75ohms output of the RCA
camera and the composite video signal output from the camera
is explained. The RCA camera used for this application note
(CM-26N/P C-MOS – color camera; Figure 2) consist of a
pixel matrix array, and a mounted lens of 7.9 mm that
provides a field of view equal to subject distance. Some of the
advantages of using this camera are:
• No control signals
• Only one output signal
• Complete image view
• Changeable lens
• Auto gain
• Auto contrast
• Auto focused
Besides the above benefits, the only disadvantage is the output
signal of the camera is completely analog, which means the
user has to be creative to process this signal in order to make it
“understandable”. Additionally, this application note provides
an example on how to process this signal. Another advantage
© 2010 Freescale Semiconductor, Inc.
1
Introduction.................................................................1
2
Camera Signal Interpretation......................................2
3
Signal Processing........................................................3
4
External components required for the RCA camera.....
4
5
Software/Driver design and description......................5
6
Conclusion..................................................................7
Camera Signal Interpretation
of this RCA camera is that it is really small and weightless; it is also user friendly since it only needs 3 cables to make it
work, supply, ground, and output. These type of cameras are easy to be found through Internet, this particular one was
gathered at http://www.sparkfun.com.
NOTE
Rev 0 of this application note provides only the drivers as explained in the example code. A complete
example using these drivers will be provided in the next revision of the application note.
2 Camera Signal Interpretation
How is light interpreted? As mentioned before, the camera is a combination of an image sensor and a lens. The light that
bounces from the environment enters through the lens, then the lens deflects the light into the sensor. The sensor consists of a
microscopic array of capacitors that gain charge depending on light intensity, therefore all pixel charge at the same time and
the sensor releases each pixel value in one output signal one after the other until all pixel charges are released. The following
image illustrates the process.
Figure 1. Imaging process in the Lens
The output of a RCA camera is compliant with NTSC standard (not linear), this means the user will be receiving a 60 half
images in a second – 30 frames per second (fps). The user needs to consider that the reference code written for this document,
is based on a line detection; so, for a complete imaging treatment, extra implementation will be done. The important things to
be considered while using this camera are the speed and ease of processing.
The line scan cameras only give the information of a line, RCA cameras show a complete panorama view, with this more
complete information users can also consider the behavior of the “line” in the near future, meaning that curves can be early
detected. As mentioned before, the user need not worry about lighting conditions as auto gain, compensation and auto
contrast are enabled on the RCA camera used for this example. To begin understanding the form of the output of this camera,
an easy practice is to connect the output to an oscilloscope (Figure 2).
Using a RCA camera for line detection, Rev. 0, 01/2011
2
Freescale Semiconductor, Inc.
Signal Processing
Figure 2. RCA camera and NTSC output signal
3 Signal Processing
RCA cameras have a general output that almost all standard cameras have; its output is characterized by the -1V to 1V range.
The output video signal is heavily composed as shown in Figure 3:
• there is a sync tip used to synchronize the TV with the horizontal video transmission
• after 4.7 µs a color burst is received (this tells the TV which pixels will be filled depending on the color selected by the
camera)
• after 4.7 µs, the composed information of the whole pixels
• then the vertical synchronization comes (Figure 3b)
Figure 3. NTSC signal description
This application note shows how to process this data with the ADC module. Generally, the faster the better, so the resolution
required for your application needs to be considered. For example; if conversions are made in 10 µs, it may not be fast
enough to have a good resolution for this application, since the video part of the signal last 52.6 µs, it means this data
conversion will be 5 pixel resolution.
Using a RCA camera for line detection, Rev. 0, 01/2011
Freescale Semiconductor, Inc.
3
External components required for the RCA camera
Generally, it will be useful to have 20 pixels resolution or more, for some applications 10 pixels will be enough (low
resolution applications), this means that the MCU will be able to sample at less than 3 ms to have a good image resolution.
Other important things to be considered is the ADC resolution. The 10-bit ADC of the MPC5604B MCU helps to make a 40
pixel resolution image.
It is important to understand that the following have been considered to read the video signal:
• If the ADC is fast enough to read the 4.7 µs sync signal then it can process all of it with the ADC module
• User should be able to recognize the synchronization pulses to build the pixel matrix, and know when to begin the next
capture of the image
Since in this application note, the idea is to recognize a “black line” on a white background, user does not need to worry on
decoding the color signal since every color will send the same information as black has no light interpretation and white is
composed of all colors. After this, the main process is to wait 4.7 µs after the horizontal sync to trigger the ADC capture.
Every time the MCU sees the vertical sync (Figure 5a) it must be able to recognize it with the ADC, so that the next
incoming video signals are the beginning of a new image, and it should start at beginning of the matrix. Every time it sees a
horizontal sync it means one video line has finished and it must start capturing the next video line.
The time it has to process the gathered matrix is in the vertical sync period, at this time there is no live video streaming so the
MCU can do all the required process in this space.
Figure 4. Video signal and vertical synchronization signal
4 External components required for the RCA camera
The voltage levels required for the MPC5604B are between 3.3 and 5.0 volts. So having the video signal running between
these levels is needed, and it is simply achieved by injecting a voltage to the signal. Figure 4 presents a simple schematic to
make the signal rise up between the desired voltage levels; user can select a static regulator or a variable one. A variable
voltage regulator is recommended as it will manipulate the output more easily. In this example the variable regulator is
injecting 1.3 volts with a strong pull-up (10KΩ) into the output, and this assures the incoming signal goes from 0 V to 2.0 V.
So the strong pull-up assures there is less consumed current and protects it from a short circuit. After setting this and assuring
it works, user should be able to see NTSC signal (Figure 3a).
Using a RCA camera for line detection, Rev. 0, 01/2011
4
Freescale Semiconductor, Inc.
Software/Driver design and description
Figure 5. Suggested schematic to handle NTSC signal
5 Software/Driver design and description
Interfacing MPC5604B with a RCA camera
Configurations set to interface MPC5604B to the RCA:
• core running @64 MHz
• ADC module in scan mode – 1MHz speed for one conversion every 1µs
• Interrupts with the end of conversion on the ADC module
• ADC watchdog to increase performance
For capturing pixel values and developing useful data, the code has been developed to capture an entire image (40x250
pixels) at a rate of 60 fps; there is a space between each taken frame so the user can process the gathered image data.
Figure 6. APIs implementation
Using a RCA camera for line detection, Rev. 0, 01/2011
Freescale Semiconductor, Inc.
5
Software/Driver design and description
According to Figure 6, user simply can decide how frames data will be processed. As the frame was already gathered, the
u32picturematrix[X] variable is a matrix of 1x246, it contains the position of the line from 5 to 51, of every line of pixels,
this means that 51-5 = 46 pixels. This means we have 46 pixels for every line, and we have 246 lines available which gives us
a 246x46 pixel resolution.
For example (Figure 5a), if u32picturematrix[123]=28 , then this means that right in the center of the frame is being captured
a black color (black line centered; 246/2 = 123, 28 = black color).
This matrix (Table 1) provides us all the information of the picture we need, if we receive a 0 in the function it means, it has
acquired an invalid value; which will happen when there was no line to detect or the camera is observing just a white
background with no line to follow.
To obtain data you should go and process directly the u32picturematrix since the entire image taking has been already done.
In the example code, you can find many lines are taken so the user can detect the position of the black line against the
position of the camera.
Table 1. Low level functions
Variables involved
u32picturematrix
Global array used to save the picture
taken; it gives you information of the po‐
sition of the line in 246 lines.
u32pixeloverflow
Global variable used for detecting which
is actual pixel
u32startpulse
Global variable used to know when and
where are the sync pulses.
u8done
Global variable used to know when it
can process the taken image.
u32imageline
Global variable used for example code.
u32index
Global variable used as an index to
know in which is the actual pixel line.
u32debuggingpurposevariable
Global variable used for debugging pur‐
poses.
u32min
Global variable used to know min posi‐
tion value of the line
u32posmin
Global variable used to know position of
the line.
u32difpicture
Global variable used to validate taken
data.
u32sum
Global variable for the example.
u32cont
Global variable used to know if image
overflows.
Using a RCA camera for line detection, Rev. 0, 01/2011
6
Freescale Semiconductor, Inc.
Conclusion
6 Conclusion
Table 2. Advantages and disadvantages of this method
Advantages
Disadvantages
• No external analog processing needed
• Never faces problems for calibrating
• Can “see” in dark places
• Cannot accelerate frame rate
• You have less time to process the image since almost
all the time is taken to gather it.
This method is comfortable as external processing/filtering is not necessary and there is no calibration problem. The only
problem can be the excessive amount of information gathered that needs to be processed by the ADC. So the architecture of
the application needs to consider this amount of throughput.
Using a RCA camera for line detection, Rev. 0, 01/2011
Freescale Semiconductor, Inc.
7
How to Reach Us:
Home Page:
www.freescale.com
Web Support:
http://www.freescale.com/support
USA/Europe or Locations Not Listed:
Freescale Semiconductor
Technical Information Center, EL516
2100 East Elliot Road
Tempe, Arizona 85284
+1-800-521-6274 or +1-480-768-2130
www.freescale.com/support
Europe, Middle East, and Africa:
Freescale Halbleiter Deutschland GmbH
Technical Information Center
Schatzbogen 7
81829 Muenchen, Germany
+44 1296 380 456 (English)
+46 8 52200080 (English)
+49 89 92103 559 (German)
+33 1 69 35 48 48 (French)
www.freescale.com/support
Japan:
Freescale Semiconductor Japan Ltd.
Headquarters
ARCO Tower 15F
1-8-1, Shimo-Meguro, Meguro-ku,
Tokyo 153-0064
Japan
0120 191014 or +81 3 5437 9125
[email protected]
Asia/Pacific:
Freescale Semiconductor China Ltd.
Exchange Building 23F
No. 118 Jianguo Road
Chaoyang District
Beijing 100022
China
+86 10 5879 8000
[email protected]
For Literature Requests Only:
Freescale Semiconductor Literature Distribution Center
1-800-441-2447 or +1-303-675-2140
Fax: +1-303-675-2150
[email protected]
Document Number: MPC5604B
Rev. 0, 01/2011
Information in this document is provided solely to enable system and sofware
implementers to use Freescale Semiconductors products. There are no express or implied
copyright licenses granted hereunder to design or fabricate any integrated circuits or
integrated circuits based on the information in this document.
Freescale Semiconductor reserves the right to make changes without further notice to any
products herein. Freescale Semiconductor makes no warranty, representation, or
guarantee regarding the suitability of its products for any particular purpose, nor does
Freescale Semiconductor assume any liability arising out of the application or use of any
product or circuit, and specifically disclaims any liability, including without limitation
consequential or incidental damages. "Typical" parameters that may be provided in
Freescale Semiconductor data sheets and/or specifications can and do vary in different
applications and actual performance may vary over time. All operating parameters,
including "Typicals", must be validated for each customer application by customer's
technical experts. Freescale Semiconductor does not convey any license under its patent
rights nor the rights of others. Freescale Semiconductor products are not designed,
intended, or authorized for use as components in systems intended for surgical implant
into the body, or other applications intended to support or sustain life, or for any other
application in which failure of the Freescale Semiconductor product could create a
situation where personal injury or death may occur. Should Buyer purchase or use
Freescale Semiconductor products for any such unintended or unauthorized application,
Buyer shall indemnify Freescale Semiconductor and its officers, employees, subsidiaries,
affiliates, and distributors harmless against all claims, costs, damages, and expenses, and
reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury
or death associated with such unintended or unauthorized use, even if such claims alleges
that Freescale Semiconductor was negligent regarding the design or manufacture of
the part.
RoHS-compliant and/or Pb-free versions of Freescale products have the functionality and
electrical characteristics as their non-RoHS-complaint and/or non-Pb-free counterparts.
For further information, see http://www.freescale.com or contact your Freescale
sales representative.
For information on Freescale's Environmental Products program, go to
http://www.freescale.com/epp.
Freescale™ and the Freescale logo are trademarks of Freescale Semiconductor, Inc.
All other product or service names are the property of their respective owners.
© 2011 Freescale Semiconductor, Inc.
">
