AN2676 Freescale Semiconductor / Motorola, AN2676 Datasheet - Page 5

• Manufacturer Part Number: AN2676
• Description: Image Capture with i.MX21
• Manufacturer: Freescale Semiconductor / Motorola

Upon the arrival of SOF, an interrupt triggers, which forces the software to run in the ISR register of the
CSI module. DMA is then enabled.
To avoid loss of data, DMA must be enabled within the latency shown in Figure 3. The latency is
determined by the timing of the sensor, usually from a few tens of MCLK to a few thousands of MCLK.
For non-real time operating systems for example, Linux, the latency of the interrupt service routine is not
guaranteed. There may be an uncertain delay inserted before DMA is enabled. This results in data loss. The
problem becomes more serious as the system load increases.
The loss sync problem is localized to one frame and does not propagate to the subsequent frames. Every
frame is synchronized with its own SOF. Each has an equal chance of loss sync.
5.2 CCIR Progressive Mode
Most smart sensors support CCIR656 encoding. Markers indicating the start of active video (SAV) and
end of active video (EAV) are embedded in the data stream. Only PIXCLK and data bus are used. VSYNC
and HSYNC are no longer required. The CCIR decoder should be able to reconstruct the VSYNC and
HSYNC from the embedded timing.
Figure 4 illustrates the general timing of a single line:
However, the CCIR656 standard was originally designed for interlaced video. For the progressive mode,
which is the subject of concern with CMOS sensors, there are no strict definitions for start of frame (SOF)
and end of frame (EOF). Different manufacturers can adapt the spec to their own, so embedded VSYNC
markers behave slightly differently from each other. To accommodate different sensors, the CSI module is
designed to work with both embedded (or internal) VSYNC and external VSYNC. The latter case is
suitable only for those sensors that provide a VSYNC signal.
In most cases, external VSYNC mode is more useful.
CCIR progressive mode is enabled through CSI Control Register 1(CR1).
6 Bayer Statistics
Bayer statistics provide the reference for camera control and basic image pre-processing. In the CSI
module, a hardware statistics block is included to help software with exposure control, focus control, and
white balance control.
The hardware accepts only Bayer data format, which is the one output from a raw image sensor. Smart
sensors that output RGB or YUV are not supported. Indeed, smart sensors do camera control and image
pre-processing with their own hardware before data is sent out.
All configurations on the Bayer statistics block are done through CSI Control Register 2.
MOTOROLA
Freescale Semiconductor, Inc.
Figure 4. General Timing of a Single Line
For More Information On This Product,
MC9328MX21 Application Note
Go to: www.freescale.com
Bayer Statistics
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