WM8170 datasheet(21/41 Pages) WOLFSON

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WM8170

Product Preview Rev 1.0

WOLFSON MICROELECTRONICS LTD

PP Rev 1.0 March 2000

The total period (P) of the combined Analogue Correction Loop and Digital Correction Circuitry,

measured in Pixel Periods, is given by the following formula:

P = ( [ANDUR] x 4 + 2

[DIGDUR] )

Pixel Periods

Where:

[ANDUR] is the contents of the Analogue Duration register ANDUR[6:0].

[DIGDUR] is the contents of the Digital Duration register DIGDUR[2:0].

The selection of ANDUR and DIGDUR values is at the discretion of the user, but the following

considerations should be made. The internal up/down counter is 8-bit, which covers 256 steps. The

counter is incremented/decremented ANDUR times per BLCENB falling edge, which implies that the

minimum number of falling edges on BLCENB to guarantee that the analogue loop has reached its

final value is 256/ANDUR. In actual use however, the change in DC conditions through the WM8170

will be relatively small, which will mean that the analogue loop will settle to the new value quicker.

The value chosen for DIGDUR depends on the number of black pixels available, but a larger value

means that any black pixel noise is averaged over a greater number, which will result in a more

accurate error value being stored.

The analogue DC correction loop error voltage within the WM8170 is not subject to drift because it is

derived from a digitally controlled DAC, which implies that once the DC correction circuitry has

settled, the correction circuitry can be turned off. This can be achieved by setting the control bit

STOPDC via the Management Interface. The correction circuitry should be re-enabled after any write

to the PGA register so that DC change due to the different gain is accommodated for.

There are two main ways that the DC correction circuitry can be used. The BLCENB can be activated

either during the optically black lines at the beginning or end of the CCD output field, or during the

optically black pixels at the beginning or end of each video line. Using the first option results in the

DC conditions being established before any active video is present, and since completely optically

black lines are present large values can be programmed to ANDUR and DIGDUR respectfully. With

the second option, smaller values for ANDUR and DIGDUR would have to be used, as the number of

black pixels in each line is limited. With both options, it is recommended that the correction circuitry

be enabled for one frame of video, and then turned off again.

Although the WM8170 only requires a falling edge on BLCENB to initiate the correction circuitry

users may input BLCENB signals which are low for the complete duration of the optically black pixels

if preferred (this signal format is generally available from CCD timing generator devices). In this case

the WM8170 can be programmed to output an error flag on the PTDO pin if the BLCENB input

returns to a high state before both the Analogue and Digital Enables are complete. This allows

checking that the ANDUR and DIGDUR values programmed to the device will not cause a potential

error in the correction circuitry because active video has been included in the internal calculations.

See the Programmable Threshold Detect Output section for details of all error flags available.

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