Daniel M. Bartolini | NYU Interactive Telecommunications Program | Sensor Workshop | Tom Igoe | Fall 2005

Monday, September 26, 2005

Hamamatsu CMOS Image Sensors

A note before beginning: I do not have any code or examples of this sensor in use yet, as I do not yet have my sensor. Also, as this is an image sensor, it will ultimately require a processor faster than a PIC chip to be able to do any sort of image processing.

S8377/S8378 Series

CMOS image sensors (complimentary metal oxide semiconductors) are image sensors that are in fact large photodiode arrays. They act as massive logic chips, in this case turning their pixels on and off as the value of light changes. CMOS sensors, because of their logic capabilities, consume less power than many comparable ICs, making them ideal for many manufacturers who can sacrifice speed for affordability.

CMOS is actually a major class of ICs, beyond being an image sensor. For a much more thorough explanation of CMOS ICs and their uses, check out Wikipedia: http://en.wikipedia.org/wiki/CMOS God bless that site.

My interest is in these ICs as image sensors specifically. CMOS sensors are finding their way currently back into higher end optical devices and cameras, which have traditionally used CCD (charge-coupled devices) for image capture. The reason is that CCDs, due to advanced fabrication processes, allow more light to hit each of the photodiodes in each pixel. CMOS sensors have the same arrangement of pixels as CCDs, but, due to cheaper construction, have an array of transistors next to each diode, which amplifies the signal. The advantage is that each pixel can be read independently if it is so desired. The disadvantage, however, is the transistors can block light hitting the pixels. For a more detailed comparison of the two, check out the following:

For a high end example of where CMOS imaging technology is being implemented, check out Sony’s very small HD camcorder, the HDR-HC1: http://sonystyle.com

For a lower level example of the use of CMOS image processors, the CMUcam and vision board, used for motor control and robot vision, is a pretty cool instrument, one which also interfaces with lower level microprcessors like the PIC: http://www.seattlerobotics.com/cmucam.htm

Electrical Characteristics:
The Hamamatsu S8377/S8378 image sensors operate on +5V DC supply voltage, and can be powered on a breadboard. There is, however, a driver circuit for this sensor which simplifies the amount of input signals, and amplifies and simplifies the output signals. The datasheet for that circuit can be found at http://www.datasheetcatalog.com

Much like the Q-Prox capacitance touch sensors we often use, these sensors have a gain selection which impacts the sensitivity of the sensor. The sensors are typically set to “High Gain” when they have an input on the Gain Terminal of 0V. When you give that pin +5V, the sensor is actually set “Low.”

The sensors have input pins for the following:

Vdd (supply voltage)
: +5V

Vg (gain selection voltage)
: +5V = “Low” and 0v = “High”
CLK (clock pulse): varies- the oscillator-in pin accepts pulses which operate the sensor’s internal Shift Register. The shift register is responsible for outputting all logic data from the chip, and the data rate is equal to the speed of the clock.
-as a note, the maximum input frequency is 500khz.

ST (start pulse)
: this is responsible for starting the storage of signals in the shift register. The ST acts as a gate for the data in the shift register. It seems that when this pin receives a pulse of +5V it starts the register operation. Another pulse tells the data to be output. The frequency of the pulse on this pin determines the signal storage time.


Blogger Derek Rydall said...

I was out looking for sites on screenplays, and came across your blog. It's not what I was looking for, but it was interesting stuff. I think I'll add it to my links if that's cool. Thanks!

11:27 PM

Blogger Daniel M. said...

Thanks for your comments. Please, link away.

8:07 PM


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