WM8255 datasheet(13/49 Pages) CIRRUS

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WM8255

Production Data

PD, Rev 4.7, August 2013

DEVICE DESCRIPTION

INTRODUCTION

A block diagram of the device showing the signal path is presented on Page 1.

The WM8255 processes the sampled video signal on VINP with respect to the video-reset level or an

internally/externally generated reference level through the analogue-processing channel.

This processing channel consists of an Input Sampling block with optional Reset Level Clamping

(RLC) and Correlated Double Sampling (CDS), an 8-bit programmable offset DAC and an 8-bit

Programmable Gain Amplifier (PGA).

The ADC then converts each resulting analogue signal to a 16-bit digital word. The digital output from

the ADC is presented on a 4-bit wide bus.

On-chip control registers determine the configuration of the device, including the offsets and gains

applied to each channel. These registers are programmable via a serial interface.

The device can control the brightness and timing of the Red, Green and Blue LEDs used in a CIS

sensor. This is controlled via the serial control interface and external timing pins.

INPUT SAMPLING

The WM8255 has a single analogue processing channel and ADC, which can be used in a flexible

manner to process both monochrome and line-by-line colour inputs.

Monochrome: The selected input (VINP) is sampled, processed by the analogue channel, and

converted by the ADC. The same offset DAC and PGA register values are always applied.

Colour Line-by-Line: VINP is sampled and processed by the analogue channel before being

converted by the ADC. The gains and offset register values applied to the PGA and offset DAC can

be switched between the independent Red, Green and Blue digital registers (e.g. Red

 Green

Blue

 Red…) at the start of each line in order to facilitate line-by-line colour operation. The

INTM[1:0] bits determine which register contents are applied (see Table 1) to the PGA and offset

DAC. By using the INTM[1:0] bits to select the desired register values only one register write is

required at the start of each new colour line.

RESET LEVEL CLAMPING (RLC)

To ensure that the signal applied to the WM8255 VINP pin lies within the valid input range (0V to

AVDD) the CCD output signal is usually level shifted by coupling through a capacitor, CIN. When

active, the RLC circuit clamps the WM8255 side of this capacitor to a suitable voltage during the CCD

reset period. The RLCINT register bit controls is used to activate the Reset Level Clamp circuit.

A typical input configuration is shown in Figure 5. The Timing Control Block generates a clamp pulse,

CL, from MCLK and VSMP (when RLCINT is high). When CL is active the voltage on the WM8255

side of CIN, at VINP, is forced to the VRLC/VBIAS voltage (VVRLC) by switch 1. When the CL pulse

turns off, the voltage at VINP initially remains at VVRLC but any subsequent variation in sensor voltage

(from reset to video level) will couple through CIN to VINP.

RLC is compatible with both CDS and non-CDS operating modes, as selected by switch 2. Refer to

the CDS/non-CDS Processing section.

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