The Image Sensor Interface (ISI) supports direct connection to the ITU-R BT. 601/656 8-bit mode compliant sensors and up to 12-bit grayscale sensors. It receives the image data stream from the image sensor on the 12-bit data bus.
This module receives up to 12 bits for data, the horizontal and vertical synchronizations and the pixel clock. The reduced pin count alternative for synchronization is supported for sensors that embed SAV (start of active video) and EAV (end of active video) delimiters in the data stream.
The Image Sensor Interface interrupt line is connected to the Advanced Interrupt Controller and can trigger an interrupt at the beginning of each frame and at the end of a DMA frame transfer. If the SAV/EAV synchronization is used, an interrupt can be triggered on each delimiter event.
For 8-bit color sensors, the data stream received can be in several possible formats: YCbCr 4:2:2, RGB 8:8:8, RGB 5:6:5 and may be processed before the storage in memory. When the preview DMA channel is configured and enabled, the preview path is activated and an ‘RGB frame’ is moved to memory. The preview path frame rate is configured with the FRATE field of the ISI_CFG1 register. When the codec DMA channel is configured and enabled, the codec path is activated and a ‘YCbCr 4:2:2 frame’ is captured as soon as the ISI_CDC bit of the ISI Control Register (ISI_CR) is set.
When the FULL bit of the ISI_CFG1 register is set, both preview DMA channel and codec DMA channel can operate simultaneously. When a zero is written to the FULL bit of the ISI_CFG1 register, a hardware scheduler checks the FRATE field. If its value is zero, a preview frame is skipped and a codec frame is moved to memory instead. If its value is other than zero, at least one free frame slot is available. The scheduler postpones the codec frame to that free available frame slot.
The data stream may be sent on both preview path and codec path if the value of bit ISI_CDC in the ISI_CR is one. To optimize the bandwidth, the codec path should be enabled only when a capture is required.
In Grayscale mode, the input data stream is stored in memory without any processing. The 12-bit data, which represent the grayscale level for the pixel, is stored in memory one or two pixels per word, depending on the GS_MODE bit in the ISI_CFG2 register. The codec preview datapath is not available when grayscale image is selected.
A frame rate counter allows users to capture all frames or 1 out of every 2 to 8 frames.