WM8255 datasheet(33/49 Pages) CIRRUS

33 / 49 page

WM8255

Production Data

PD, Rev 4.7, August 2013

The number of MCLK cycles per PWM period is given by:

LEDPWMPER = 12 MHz / 2.5kHz = 4800.

This exceeds the maximum 4095 so CLKDIV must be set to ‘0001’.

Therefore 12Mhz / 2 = 6MHz

Therefore LEDPWMPER = 6MHz / 2.5kHz = 2400

The number of MCLK cycles necessary for blanking period is given by:

(tBLANK *0.8)*MCLK = INITBLANKdec.

(25uSec * 0.8)*12MHz = 240

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

zero, should be set in the register.

240 = 0 1111 0000bin. Therefore the 6 MSBs to be set to the register INITBLANK are :- 011110bin

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

The number of MCLK cycles per line scan is given by:

MCLK cycles per line scan = 9inch * 2400dpi * 3 = 64800

The number of MCLK cycles available for imaging is given by:

MCLK cycles per line scan - MCLK cycles per line scan = 64800 – 240 = 64560

The Number of PWM cycles per line scan is given by:

MCLK cycles per PWM period LEDPWMPER = 2400

Number of PWM cycles per line scan = 64560 / 2400 = 26.9

The nearest integral number of PWM cycles per line scan then 26.

The illumination period should be checked. Assuming 26 PWM cycles per line, with the first

PWM

cycle reserved for the current DAC setup and transition, and an 80% duty cycle

Period of illumination

= 26 * 0.8 / 2.5kHz = 8.32 msec

Period of illumination coarse trim LSB

= 1 / 2.5kHz

= 400 usec

Period of illumination fine trim LSB

= 1 / 6MHz

= 166.66 nsec

If the period of illumination per line scan is too short, the imaging period is limiting the scan period.

The period between TG pulses should be increased and step 2 should be repeated to calculate the

number of MCLK cycles per line scan.

Set values for LEDPWMDCR / G / B as appropriate.

Using the number of PWM cycles per line scan for reference, set values for LEDENSTART and

LEDENSTOPR / G / B as appropriate.

Set values for LEDIDACR / G / B as appropriate.

A red, green, blue, red, green, blue colour transition is required, so set STATE0 = 00 for red,

STATE1 = 01 for green and STATE2 = 10 for blue. Set STATERST = 10

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