OV5642 datasheet(39/208 Pages) OMNIVISION

39 / 208 page

01.08.2009

PRELIMINARY SPECIFICATION

proprietary to OmniVision Technologies

4-5

4.5.2 average-based algorithm

The average-based AEC controls image luminance using registers AEC CTRL0F (0x3A0F), AEC CTRL10 (0x3A10),

AEC CTRL1B (0x3A1B), and AEC CTRL1E (0x3A1E). In average-based mode, the value of register AEC CTRL0F

(0x3A0F) indicates the high threshold value, and the value of register AEC CTRL10 (0x3A10) indicates the low threshold

value. The value of register AEC CTRL1B (0x3A1B) indicates the high threshold value for image change from stable

state to unstable state and the value of register AEC CTRL1E (0x3A1E) indicates the low threshold value for image

change from stable state to unstable state. When the target image luminance average value AVG R10 (0x5690) is within

the range specified by registers AEC CTRL1B (0x3A1B) and AEC CTRL1E (0x3A1E), the AEC keeps the image

exposure and gain. When register AVG R10 (0x5690) is greater than the value in register AEC CTRL1B (0x3A1B), the

AEC will decrease the image exposure and gain until it falls into the range of {0x3A10, 0x3A0F}. When register AVG R10

(0x5690) is less than the value in register AEC CTRL1B (0x3A1E), the AEC will increase the image exposure and gain

until it falls into the range of {0x3A10, 0x3A0F}. Accordingly, the value in register AEC CTRL0F (0x3A0F) should be

greater than the value in register AEC CTRL10 (0x3A10). The gap between the values of registers AEC CTRL1B

(0x3A1B) and AEC CTRL1E (0x3A1E) controls the image stability.

The AEC function supports both manual and auto speed selections in order to bring the image exposure into the range

set by the values in registers AEC CTRL0F (0x3A0F) and AEC CTRL10 (0x3A10). For manual mode, the speed

supports both normal and fast speed selection. AEC set to normal mode will allow for the slowest step increment or

decrement in the image exposure to maintain the specified range. AEC set to fast mode will provide for an approximate

ten-step increment or decrement in the image exposure to maintain the specified range. For auto mode, the speed step

will automatically be adjusted according to the difference between the target and present values. The auto ratio of steps

can be set by register bits AVG R10[6:2] (0x5690); thus, the AEC speed can be adjusted automatically by the image

average value or controlled manually.

selection made. If the target image AVG R10 (0x5690) is greater than AEC CTRL11 (0x3A11), AEC will decrease by

half. If register AVG R10 (0x5690) is less than AEC CTRL1F (0x3A1F), AEC will double.

As shown in figure 4-4, the AEC/AGC convergence uses two regions, the inner stable operating region and the outer

control zone, which defines the convergence step size of fast and slow conditions.

figure 4-4

desired convergence

control zone

stable operating region

desired convergence

5642_DS_4_4

OV5642 Datasheet(PDF) 39 Page - OmniVision Technologies, Inc.