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This equation also applies to the divide by 1.5 function.

The output of this stage is sent through the gamma and pixel

packing stages, resulting in output data formatted as shown in

Figure 15.

2.4.2 Pixel Processing 10/30 Bit Mode

Scanning in 10 bit mode (30 bit color) supports the Horizontal

DPI Divider function as well as the pixel rate shading and offset

functions. The output data is formatted as shown in Figure 12

(X=mask out in software).

The software on the host PC must perform any gamma correction

desired.

There are two variations on the 10 and 12 bit output modes: Full

Duplex and Half Duplex, determined by bit 5 of Configuration

tions per pixel: offset data read, gain data read, pixel MSB write,

pixel LSB write, pixel MSB read, pixel LSB read). Since there are

8 MCLKs per pixel, the writes take 2 MCLK periods and the reads

take 1 MCLK period. This mode is preferred because it permits

faster scanning, but it requires fast SRAM access.

The Half Duplex mode accommodates slower SRAM. In Half

Duplex mode, the data in the SRAM can not be read by the host

PC until the buffer is full. Therefore there are two phases to scan-

ning data in the Half Duplex mode. The first is writing pixel data to

SRAM using 4 operations/pixel (offset data read, gain data read,

pixel MSB write, pixel LSB write). In this mode the read and write

cycles will all be 2 MCLKs long. The second phase is reading the

contents of the SRAM and sending them to the host PC. This

read operation uses 2 MCLK read cycles/byte.

2.4.3 Pixel Processing: 12/36 Bit Mode

Scanning in 12 bit mode (36 bit color) can only be done at the

optical resolution of the sensor, and the shading and offset func-

tions can not be used. The Horizontal DPI Divider function must

be set to 1 (register 09, bits 0-2 = 0) to get 12 bit data. The shad-

ing and offset functions must also be disabled (registers 3E, 3F,

40, and 41 all set to 0). The 10 bit pixel output from the multiplier

is combined with the 2 12 bit LSBs from the ADC to recreate the

12 bit pixel. The output data is formatted as shown in Figure 13

(X=mask out in software).

The software on the host PC must perform all offset and shading

correction, horizontal resolution adjustment, and gamma correc-

tion.

The 12 bit mode uses the same Full or Half Duplex options

described in 2.4.2 Pixel Processing 10/30 Bit Mode

2.5 Gamma Correction Tables

There are 3 gamma lookup tables for R, G, and B. The input to

the table is the 10 bit pixel data coming from the previous stage

(2.4 Pixel Processing Block). The output is the 8 bit gamma cor-

rected pixel data. The tables are therefore 1K bytes x 8 bits in

size. Each gamma table (red, green, and blue) can be loaded with

any arbitrary user-defined transfer curve.

The gamma tables are loaded through the dataport (see 5.1 The

DataPort: Reading and Writing to Gamma, Offset, and Gain

Memory). In most LM9830 modes, the DataPort selects which

color (Red, Green or Blue) gamma table will be read from or writ-

ten to. In 1 Channel Mode A, the only gamma table that can be

accessed is the gamma table for the 1 Channel Mode A color

selected by bits 3 and 4 of register 26.

2.6 Pixel Packing/Thresholding Block

Some scans require only one bit per pixel (“line art” mode), others

may need only 2 or 4 bits/pixel. To increase scanning speed for

lower pixel depths, the LM9830 packs the desired MSBs of multi-

ple pixels together, increasing the transmission speed to the host

by a factor of 2, 4, or 8. Figure 15 shows how the pixels are

packed together for 8, 4, 2, and 1 bit pixel depths. In Figure 15,

“b” indicates the bit position (b7 = the most significant and b0 =

Divide

DPI

(600 DPI sensor)

DPI

(300 DPI sensor)

600

300

1.5

400

200

300

150

200

100

150

100

875

37.5

Figure 11: Decreasing Horizontal Resolution

76543210

Order

9876

First Byte

43210

Second Byte

Figure 12: 10 Bit Mode Pixel Data Format

543210

Order

XXXX

First Byte

765

43210

Second Byte

Figure 13: 12 Bit Mode Pixel Data Format

Pixel

Depth

765

432

8b7 p0 b6 p0 b5 p0 b4 p0 b3 p0 b2 p0 b1 p0 b0 p0

4b7 p0 b6 p0 b5 p0 b4 p0 b7 p1 b6 p1 b5 p1 b4 p1

2b7 p0 b6 p0 b7 p1 b6 p1 b7 p2 b6 p2 b7 p3 b6 p3

1b7 p0 b7 p1 b7 p2 b7 p3 b7 p4 b7 p5 b7 p6 b7 p7

Figure 15: Packing Multiple Pixels Into One Byte

10MSBs of 12 bit Output

255

1023

Figure 14: Gamma Table

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