WM8255 datasheet(21/49 Pages) CIRRUS

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WM8255

Production Data

PD, Rev 4.7, August 2013

MONOCHROME (COMPOSITE) MODE

In Monochrome Mode, the progression between sequence states is triggered by the completion of the

previous sequence state. This allows a complete LED sequence change for each line scan.

With both TG and LEDSTART high, the sequence state machine is synchronously set to the

STATE_0 by MCLK. When the STATE_0 has reached the end of its enable period, the sequence

state machine is progressed to the next state by MCLK. This is repeated until the maximum number

of states determined by STATERST has been reached. At this point the LED drive current will be

switched away from the selected LEDs. The sequence state machine will be held in this state until

restarted by LEDSTART and TG.

If at any time both TG and LEDSTART are high, the sequence is synchronously set back to

STATE_0 by MCLK.

LED CURRENT DRIVE INTENSITY CONTROL

The LED current driver is programmable to allow the LED light intensity to be adjusted independent of

the LED light wavelength. Two methods are available for this:



The absolute LED current drive may be set using a programmable 8-bit current DAC.

The current DAC range for each of the LEDs may be adjusted to one of four ranges

using the register LEDIRNG.



The LED current drive may be pulsed using a pulse width modulated. The pulse width

modulated period is set using the register LEDPWMPER and on time using the

registers LEDPWMDCR, LEDPWMDCB and LEDPWMDCG.

Control of the absolute LED current drive and PWM modulation are independently programmable for

each of the red, green and blue LEDs using the register map. The signals LEDSTART, TG and MCLK

determine timing.

LED CURRENT DRIVE STATE TRANSITION AND PWM SWITCHING

The WM8255 is the combination of a LED current switching matrix and an AFE. With reference to

Figure 15, during a typical line scan the video signal of the previous line scan is digitised by the AFE

while the image of the current line scan is illuminated. To suppress any switching noise of the LED

switching matrix coupling into the AFE, care is taken while switching the current.

Two types of current switching are available in the WM8255, state transition switching and PWM

switching. State transition switching occurs when either a new LED is to be selected or the LED

current DAC has to be updated. PWM transition switching occurs when the illumination intensity is

controlled by pulsing the LED drive current. In colour mode, state transition switching should occur at

the start of a line scan. In mono mode, state transition switching can occur during the line scan. In

either colour mode or mono mode, PWM transition switching can occur during the line scan.

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