Determining magnitude of damping resistor for CMOS image sensor

Question

I'm designing a camera module using Sony's IMX224 image sensor. This module would be connected to STM32F4 discovery board. At this moment I want to make a simple camera module so that I can write program for controlling the image sensor through the STM32F4. After that I'd optimise its circuits and then make a new module

But I'm wondering if I really need 'damping resistors' for data output pins of the IMX224.

1. Do you need damping resistors? The IMX224 has 12 pins for CMOS parallel data output and they would be directly connected to the I/O pins of STM32F4 board. According to data sheet, people need to put a damping resistor for each data pin. But I'm not sure if you really need them. because some camera modules don't have such resistors, like the following link.
   http://www.elecfreaks.com/store/ov7670-camera-module-p-705.html It has only 2 resistors. But it seems to work fine.

2. If you really need such resistors for impedance matching. How large resistance would you need? 10Ω? 33Ω? The data pins of the sensor have output impedance of 50Ω. I guess the magnitude of the damping resistors should depend on data rate of image sensor. But the problem is...you can change data rate after by program when you actually use the camera and then such fixed value damping resistor would make problem, or?

I'm sorry for the stupid questions. But any help would be appreciated. If I have assumed something wrong, please feel free to let me know.

Answer 1

From the product information page, I can see that there are multiple interfaces available. The following is probably what is going on.

The resistors are highly likely to be transmission line series terminators for the parallel interface and whether you need them or not depends on the distance between the source (camera) and destination (processor board) compared to the rise / fall time of the signal.

The parallel interface can operate at 74.25 Mpixels/sec and this is probably one pixel per clock (one parallel word per pixel, very probably), so there is a cycle time of about 13.5 nanoseconds.

The usual rule of thumb is that if a rise / fall time is less than 1/10 of the electrical distance between the two points, you should use them.

In cabling, the electrical distance is about 200 picoseconds per inch, and for most FR4 materials it is about 170 picoseconds per inch for inner layers.

Without a datasheet to show me the rise / fall time of those signals, I cannot tell definitively whether you need them or not. Typical values for series terminators (Note 1) are between 22 and 33 ohms.

I can estimate the rise and fall times as (probably) no more than 1/10 of a bit time (for a given bit), yielding 1.35 nanoseconds. This is about 7.6 inches on FR4, or slightly longer (about 9 inches of cable). If your interface is less than about 1 inch (ball park of about 1/10 of the signal electrical length), you can probably get away without them.

If you were using the LVDS interface, you would need a 100 ohm differential interface.

Note 1. The actual value depends on the impedance of the cable / PCB wiring and the output impedance of the driver, such that Z(driver) + R(series) ~ Z (wiring).