This article introduces the pixel clock calculation of Image Sensor.
In the process of configuring the Image Sensor register, the pixel clock is very important, and its correct setting is related to the frame rate and the correctness of the exposure parameters. We have learned from relevant materials that there are many methods for pixel clock, which one should we choose? This article discusses common pixel clock calculations and makes a choice according to the actual situation.
1. Pixel clock calculation
1) According to the image sensor internal PLL frequency multiplication, frequency division output calculation
Usually, the Image Sensor specification describes its clock tree, which describes the meaning of each clock output. The picture shows the clock tree of the OV Image Sensor, and its pixel clock is SCLK. The setting of frame rate and exposure parameters are all based on this clock. To design the pixel clock, the following formula can be used:
Pixel_Rate=HTS*VTS*FPS
Note: The FPS here is theoretical FPS (design FPS)
2) Calculated based on frame size and frame rate
This is our most common one. Note that the FPS here is the measured frame rate , not the theoretical frame rate. Of course, it is another matter if you use this formula to design the PLL pixel clock in 1).
Pixel_Rate=HTS*VTS*FPS
3) Calculated based on MIPI CSI transmission rate
We know that there is a certain relationship between MIPI CSI data and pixel clock, which was introduced in the previous article ( MIPI CSI related calculations) , and will not be detailed here.
Pixel_Rate=Data_Rate_Per_Lane*Lanes/Color_Depth
2. Pixel Clock Calculation Selection
1) If the pixel clock can be calculated by internal PLL frequency multiplication and frequency division, this method is preferred, because it is the pixel clock actually used inside the hardware and is the most accurate.
2) Calculate the pixel clock according to the frame size and frame rate. In fact, this method is based on the posterior method. The FPS here is actually the frame rate of your actual test output. We speculate the actual pixel clock based on this.
3) Calculate the pixel clock according to the MIPI CSI transmission rate. This method needs to consider that MIPI CSI is based on the protocol. The actual MIPI CSI rate is larger than the theoretical rate. In this case, the error in calculating the pixel clock is relatively large.
Summary: This article introduces 3 methods of pixel clock, and analyzes how to make a choice. In short, pixel clock is very important, it will affect your frame rate and exposure time calculation.