WM8170 datasheet(25/41 Pages) WOLFSON

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WM8170

Product Preview Rev 1.0

WOLFSON MICROELECTRONICS LTD

PP Rev 1.0 March 2000

CONTROLLING THE BLACK LEVEL CLAMP CIRCUITRY

There are three Management Interface control bits that are associated with the INTBLCENB pulse

generator, STPANAL, STPIIR, and PGAUPD. Their function is shown in Tables 6, 7 and 8:

STPANAL

FUNCTION

Allows the INTBLCENB signal to be steered to the Analogue DC

correction Loop.

Disables the INTBLCENB signal from being steered to the Analogue

DC correction Loop. The INTBLCENB pulses that would have been

steered to the Analogue Enable are steered to the IIR Filter enable

instead.

Table 6 STPANAL control bit operation

STPIIR

FUNCTION

Allows the INTBLCENB signal to be steered to the IIR Filter

Disables the INTBLCENB signal from being steered to the IIR Filter

Table 7 STPIIR control bit operation

PGAUPD

FUNCTION

The INTBLCENB signal will be steered to the Analogue DC

correction loop as programmed on every frame.

The INTBLCENB signal will only be steered to the Analogue DC

correction loop as programmed if the PGA gain register has been

written to.

Table 8 PGAUPD control bit operation

The PGAUPD control bit enables the internal WM8170 circuitry to steer the INTBLCENB signals to

the Analogue Control Loop for one frame only following a write to the PGA Gain register. The

INTBLCENB signals will be steered to the IIR Filter on all other frames. This implies that there will

only be ANLINE lines subject to line noise, only for a single video frame, and only if the PGA gain

signals will be steered to the Analogue Control Circuitry on every frame.

SETTING THE IIR FILTER COEFFICIENT

Writing to the COEFF register controls the IIR filter response. Table 9 shows the coefficients

available on the WM8170, and the effective attenuation of the RMS noise during the optically black

pixels that will be present on the calculated error.

COEFF

VALUE

IIR

FILTER

COEFF

RMS Noise

Attenuation

(dB)

RMS Noise

Attenuation

(Times)

10%

1.0%

0.1%

000

18.0

7.9

334

2307

001

21.0

11.3

146

673

4651

010

24.1

16.0

293

1352

9339

011

27.1

22.6

588

2709

18714

100

30.1

32.0

1177

5423

37466

101

33.1

45.2

2356

10853

74969

110

36.1

64.0

4714

21711

149976

111

39.1

90.5

9430

43427

299988

Table 9: IIR Filter Coefficients

The 10%, 1.0% and 0.1% columns give the number of pixels that the IIR filter has to be enabled for

to achieve the percentage settling, i.e. if the input has a 100LSB step then the number of pixels

required for the filter to settle to 1LSB is equal to the number in the 1% column. For example, if the

filter coefficient is set to 7, and the input has a 100LSB step, it will require 1352 enabled pixels for the

filter to settle to 1LSB of the final error value. There is a trade-off between the number of pixels

required for the filter to settle, and the RMS noise attenuation achieved on the calculated error value.

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