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Converter translates Bayer raw data to RGB format

CMOS image sensors include the color filters of an RGB (red/ green/blue) Bayer array, which lets the sensor detect colors. The image data, the output from the image sensor, is Bayer raw data (Figure 1). Unfortunately, most consumer-grade imagedisplaying devices require an RGBimage- data format with red, green, and blue in each pixel’s data. Therefore, you often need a Bayer-raw-datato- RGB converter between an image sensor and a displaying device. This Design Idea describes such a converter in Verilog HDL (hardware-description language). You can implement the code, available from the online version of this Design Idea at www.edn-europe.com/article.asp?articleid=3838, into a CPLD or an FPGA.

To make the design easy to understand, the RGB data is only 24 bits deep. A 320x240-pixel test-bench pattern verifies the design (Figure 2). The image data for red, green, and blue are 88h, 66h, and 22h, respectively. Figure 3 shows the timing of the converter, and Figure 4 shows the flow chart. In Figure 3, the PCLK (processor-clock) signal’s rising edge latches the 24-bit RGB data. The LVAL (line-valid) cap signal is the synchronized line-valid signal for reading the data. The process begins when the FVAL (frame-valid)- data signal goes high. When that action occurs, sensor data begins writing to FIFO₁ (first-in/first-out), FIFO₂, and the data register. After saving the data, FIFO₁ reads it out. FIFO₂ and the data register read their data at the same time. Those readouts occur one clock cycle after FIFO₁ starts reading. The next 24-bit RGB data remains the same in the data register, and it combines the data from FIFO₁ and FIFO₂, which read out at the same rising edge of the clock. The line-count signal shows whether the data is even or odd, which influences the combinational sequences of the data that reads out from FIFO₁, FIFO₂, and the data register.

Figure 5 is the ModelSim simulation waveform of the converter. The 24-bit RGB output-data values 88h, 66h, and 22h are red, green, and blue data, respectively. The figure shows the 24-bit RGB data as having red, green, and blue values of 88h, 66h, and 22h, respectively, during every line-data period. The line-data period matches the default pixel value in the image data’s test-bench pattern.