WM8255 datasheet(32/49 Pages) CIRRUS

Part #	WM8255

Description	Single Channel 16-bit CIS/CCD AFE with RGB LED Current Drive

PDF	49 Pages

Scroll/Zoom	100%

Manufacturer	CIRRUS [Cirrus Logic]

Direct Link	http://www.cirrus.com

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32 / 49 page

Production Data

WM8255

PD, Rev 4.7, August 2013

Figure 24 An Example of How LED Current Limiting Can Be Operated

LED CONTROL WORKED EXAMPLE

As an example of configuring the LED current drive control, consider the scan of a US Letter page

size with the AFE configured to sample at 12 MHz MCLK and 3:1 MCLK:VSMP ratio.

The aim is to calculate LEDPWMPER for the given MCLK frequency to give a PWM frequency of

typically 2.5kHz. Then the number of PWM cycles per line scan is calculated to check that there is

sufficient imaging time and coarse trim range.

Assumptions

MCLK:VSMP ratio

= 3:1

MCLK frequency

= 12MHz

VSMP frequency

= 4MHz

LED Enable counter range

= 2^7 (maximum) PWM clock periods

LED PWM counter range

= 2^12 (maximum) PWM clock periods

Colour Scan Mode

Target blanking period

= 25uSec

Desired PWM frequency

= 2.5kHz

Colour sequence

= red, green, blue

Sensor scan width

= 9 inches

Scan resolution

= 2400 dpi

WM8255 Datasheet(PDF) 32 Page - Cirrus Logic