WM8224 datasheet(20/50 Pages) WOLFSON

20 / 50 page

WM8224

Production Data

PD, Rev 4.1, June 2012

In situations where the input video signal does not have a stable reference level it may be necessary

to clamp only during those pixels which have a known state (e.g. the dummy, or “black” pixels at the

start or end of a line on most image sensors). This is known as line-clamping and relies on the input

capacitor to hold the DC level between clamp intervals. In non-CDS mode (CDS=0) this can be done

directly by controlling the RSMP input pin to go high during the black pixels only. Note that internal

input voltage buffers should be enabled using the FOL_EN register bit when using this mode of

operation.

Alternatively it is possible to use RSMP to identify the black pixels and enable the clamp at the same

time as the input is being sampled (i.e. when VSMP is high and RSMP is high). This mode is enabled

by setting CLMPCTRL=1 and the operation is shown in Figure 15.

MCLK

VSMP

RSMP

RLC switch control,

"CLMP"

(RLCEN=1,CLMPCTRL=1)

RLC switch closed when RSMP=1 &&

VSMP=1 (during "black" pixels)

Video and reference sample

taken on fallling edge of VSMP

INPUT VIDEO

SIGNAL

unstable

reference level

dummy or

"black" pixel

video level

Figure 15 Reset Level Clamp Operation (CLMPCTRL=1), non-CDS mode only

RLCEN

CLAMPCTRL

OUTCOME

USE

RLC is not enabled. RLC switch is always open.

When input is DC coupled and within supply

rails.

RLC switch is controlled directly from RSMP input

pin:

RSMP=0: switch is open

RMSP=1: switch is closed

When ASIC explicitly provides a reset sample

signal and the input video waveform has a

suitable reset level.

VSMP applied as normal, RSMP is used to

indicate the location of black pixels

RLC switch is controlled by logical combination of

RSMP and VSMP:

RSMP && VSMP = 1: switch is closed

Switch is re-opened when:

VSMP=0 (non-CDS mode)

VSMP=0 and RSMP=0 (CDS mode)

When clamping during the video period of black

pixels or there is no stable per-pixel reference

level.

This method of operation is generally only

sensible in non-CDS mode.

Table 2 Reset Level Clamp Control Summary

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