WM8255 datasheet(27/49 Pages) CIRRUS

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WM8255

Production Data

PD, Rev 4.7, August 2013

ILIMITEN [1:0]

DESCRIPTION

At a state transition, one blanking period will be used before the LED can be

enabled.

LED DAC current will be changed and next LED in sequence shall be selected.

No Safe Operating Area test will be performed.

At a state transition, two blanking periods will be used before the LED can be

enabled.

During the first blanking period, should the LED DAC current exceed

LEDIMAX, ILIMITFLAG is set and the LED DAC current is reduced by the

percentage set by ILIMITDEC.

During the second blanking period, should the LED current exceed LEDIMAX,

ILIMITFLAG is set and the LED DAC current will be shutdown.

At a state transition, one blanking period will be used before the LED can be

enabled.

Should the LED current exceed LEDIMAX, ILIMITFLAG is set. The LED

current will not be reduced automatically. In this situation, the user should take

measures to protect the LED by reducing the LED current.

Not a valid setting

Table 4 Modes of Operation of ILIMITEN Register

SETTING THE INITBLANK REGISTER

With reference to Figure 19, the period tBLANK is the initial blank period with no illumination. The initial

blank period time must be controlled by setting a value for a 9-bit register INITBLANK. Setting this

microseconds. The last three LSBs are fixed to zero and only the 6 MSBs are adjustable.

The value needed for the register INITBLANK is calculated by :-

(tBLANK *0.8)/ MCLK period = INITBLANKdec.

This number should be rounded up to an integral

decimal number.

The binary equivalent of INITBLANKdec should be calculated and, making sure the last 3 LSBs are

zero, should be set in the register.

For example:-

MCLK = 24 Mhz => 41.6ns (tPER)

(tBLANK * 0.8) / tPER = (25s * 0.8) / 41.6ns = 480.77

The nearest integral number where the binary equivalent has zero values in the last 3 LSBs is 480.

480 = 111100000 bin. Therefore the 6 MSBs to be set to the register INITBLANK are:- 111100

By default INITBLANK is set to zero which sets the initial blank period to equal the PWM period/0.8

set by LEDPWMPER.

If ILIMITEN is set to ‘01’ then this blanking period will be doubled. Note that the INITBLANK register

value does not require a new value in this situation.

WM8255 Datasheet(PDF) 27 Page - Cirrus Logic

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