WM8255 datasheet(26/49 Pages) CIRRUS

26 / 49 page

Production Data

WM8255

PD, Rev 4.7, August 2013

There is an operational difference in monochrome mode in that LEDENSTART is only used in the first

state. In subsequent states the LED enable is always activated after the appropriate number of

blanking periods (LEDENSTOP is used in the same method as colour mode operation).

NON PWM MODE

The PWM functionality can be disabled and just the current DAC can be used. The duty cycle

LEDPWMDC register for each colour should be set to equal the duty cycle period register,

LEDPWMPER. The value of both these registers should be set to an appropriate value.

LED CURRENT DRIVE CURRENT BLANKING PERIOD

At the start of a LED sequence state transition the LED controller performs a blanking period. The

blanking period is a period of time reserved to switch to the next LED drive current state in the

sequence. In addition, during the blanking period the absolute LED drive current will be updated, a

Safe Operating Area (SOA) test may be performed and finally the next LED in the sequence will be

selected then driven. At completion of the blanking period, the PWM controller is enabled.

Figure 18 shows a basic blanking period during a state transition of switching the Red LED off, then

Green LED on.

The blanking period may be split into a sequence of five operations:

The Red RGB switch will be switched off and any current forced to flow in the shunt current

path.

The LED current DAC will be disabled. During this period, the current flow in the auxiliary

current path will tend to zero.

The LED current DAC value will be updated to the value necessary to drive the Green LED.

During this time, the current flow in the auxiliary current path will tend to the updated LED

current DAC current.

A SOA test will be performed on the updated LED current DAC current.

The Green RGB switch will be switched on.

The transition from one state to the next takes the finite time defined by the period tblank.

With reference to Table 4, the basic blanking period may be optimised dependant on the mode of

operation.

Figure 18 LED Current DAC Current during a Blanking Period

WM8255 Datasheet(PDF) 26 Page - Cirrus Logic