Specifications

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4.12.2. Modem CPU Requirement
Encoding/Decoding in use of Flow Control is performed in real-time. So, if the
memory access time of Modem CPU is late, Overflow or Underflow can occur.
Therefore, Modem CPU should quickly write or read data in the memory to
prevent Overflow or Underflow.
In case of Encoding, following factors can have the influence upon Flow Control:
FIFO size, I/O 1 cycle time, and JPEG data size. It is hard to predict JPEG data size
exactly because the complexity of the image received from the sensor or JPEG
quality factor have the influence upon JPEG data size. But in general, for VGA
(640x480), JPEG data size is 120-140Kbytes in the worst case. So if the encoding
test is performed with the worst case, normal images will have no problem. FIFO
size and I/O 1 cycle time (TCLK) can be set in Modem CPU and the values will be
got from several tests.
Following table shows the test result in use of VGA sensor.
● Hynix VGA Sensor (640x480), 32fps.
FIFO Size 40KB 40KB 64KB 64KB
I/O TCLK 420ns 680ns 420ns 680ns
Max JPEG
Size
140KB 107KB over 140KB 130KB
Table 4-5. Examples of Preview Rotation
This result can be influenced by the CPU and other factors but will not be
changed much. Refer to the test result to set FIFO Size and TCLK.
Encoding/Decoding in use of Flow Control is implemented by polling. So data
may be not read/written in time when interrupts occur from Modem CPU in
operation. Therefore, it is recommended to block interrupts before starting
encoding/decoding.