Specifications

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4.12. FLOW CONTROL
Flow Control enables to capture or playback still images or moving pictures by
using a small memory efficiently. In the existing method, as data is not readable
until all of the captured JPEG images are saved in the memory, it is unavailable to
capture the image of which capacity exceeds the memory capacity. But with Flow
Control, the host can simultaneously read data from the memory as writing
captured JPEG data in the memory. Therefore, Flow Control can process any JPEG
images regardless of the size and capacity of images (However, there is a
limitation in I/O read/write speed of the host). Also, as there is no limitation on
the internal memory when taking the moving pictures, it is available to take
moving pictures without time and capacity limitation.
4.12.1. Detailed Description on Flow Control Architecture
The architecture of Flow Control is similar to Circular Queue; the beginning of a
memory is connected to the end. Therefore there is no limitation on the size of
JPEG data.
To use Flow Control, it is required to decide how much memory is used. The 16bit
FIFO size register shows the size of memory to be used. For example, when the
value of the register is 0x5000, that means 0x5000=20480 (Word). In this case,
40Kbyte is usable. If the size of data is larger than the FIFO size, the exceeding
data is automatically taken by Register 0. Therefore the host does not request
additional process or devices.
In case of encoding that captures images, if the host reads the encoded data as
soon as data is encoded in the memory, the amount of readable data in the
memory is not enough. Then the process of read and halt should be repeated.
JPEG Threshold size setting solves this problem; the data processing is not started
until a specified size of data is saved in the memory and the specified size is JPEG
Threshold size. For example, set JPEG Threshold size at 0x2000 (16Kbyte) and
send JPEG Capture command, and the encoding data is saved in the memory.
When the saved data size reaches to the JPEG Threshold size which has been set
in advance, JPEG Threshold interrupt is generated. Then the interrupt is notified
to the host and the host starts to read the memory. The JPEG Threshold interrupt
is generated when the JPEG Threshold size is same with the interval size between
the point where data is written and the point where the host starts to read in the
memory. Therefore, this interrupt can be generated even while the processing is
going on as well as the first reaching.
When using this Flow Control, a critical problem may occur if one process (Write
process or Read process) is quicker than the other process. In other words, new
data can be overwritten on the existing data before the host reads the existing data
when Write process is quicker than Read process during Image Capture process.
Reversely, the read data can be read again when the host reads data quicker than
Write process. To solve these problems, two margins are made;
Underflow/Overflow Margin. The Underflow margin is generated when the Read
speed is quicker than the Write speed and the interval between the write point
and the read point is within a certain range. The host should halt the Read process