WM8255 datasheet(19/49 Pages) CIRRUS

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WM8255

Production Data

PD, Rev 4.7, August 2013

LED CURRENT DRIVE CONTROL

The WM8255 allows the user to control:



the sequence of illumination



the period of illumination



the intensity of illumination

of the red, blue and green LEDs used to illuminate the image during a scan.

A sequence state machine is used to progress the sequence and control the duration of the LED

selection. The progression of the sequence state machine is dependent on whether the WM8255 is in

colour mode or monochromatic mode. The intensity of illumination is controlled on either control of the

LED drive current or pulsing of the LED driving current.

LED CURRENT DRIVE SEQUENCE CONTROL

With reference to Figure 12, the WM8255 uses a LED sequence state machine to control the

sequence and duration of the illumination of the red, green and blue LEDs.

The LED sequence state machine can progress through up to 4 states each sequence. Each of the

sequence states (STATE_0 to STATE_3) may be set to one of four values to determine on whether a

red, green or blue LED is illuminated or all LEDs are off. The register STATERST will determine the

number of states the sequence state machine can progress through. Once the sequence state

machine has reached the reset state, the LED state machine will remain in this state until again

initiated.

LED CURRENT DRIVE SEQUENCE DEFINITION

Figure 13 illustrates the functionality of the sequence state mapping. The current state of the

sequence machine (SEQ_STATE) is an internal register used to select one of the four register state

mappings.

This register increments until the value specified by STATERST at which point the

sequence will hold. Figure 14 shows two examples when STATERST is set to “11bin” for a 4 state

sequence, and to “01bin” for a 2 state sequence.

LEDEnable

Fine Counter

(7-bit)

LEDOn

LEDOff

PWM Counter

(12-bit)

0->LEDPWMPER

PWMCtr

PWMEn

LED Target

Current

LEDEnStart

LEDEnStop

LEDTarget

Ena

LEDEnRBG

LEDEn

LEDPWMDutyCyc

LEDPWMDCRed

LEDPWMDCBlue

LEDPWMDCGrn

LEDPWMPER

LEDENSTART

LEDENSTOPRed

LEDENSTOPBlue

LEDENSTOPGrn

LEDIDACRed

LEDIDACBlue

LEDIDACGrn

State0

State1

State2

State3

StateRst

Mono_ColourMode

PWMMaxCount

Blanking

Counter

MCLK

Divider

LEDPWMPer

INITBLANK

CLKDIV

Figure 12 Block Diagram of PWM LED Control

WM8255 Datasheet(PDF) 19 Page - Cirrus Logic

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