LM9832 datasheet(22/42 Pages) NSC

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the external DRAM and accessed at the pixel rate. A digital sub-

tractor subtracts the 16 bit offset word (corresponding to that

pixel’s offset error) from each pixel.

The subtractor saturates at 0, i.e. if the coefficient to be sub-

tracted is greater than the ADC output code, the result is an out-

put of 0.

The offset words stored in DRAM are typically calculated by

scanning a black calibration strip at 14 bits, and storing the

results in the DRAM using the DataPort.

The offset subtractor only uses the upper 14 bits of the 16 bit

word. When scanning in 14 bit mode, a pixel is transmitted as a

16 bit word, with the upper 14 bits containing the image data. The

2 LSBs of the image data and the offset correction word should

be 00.

The offset correction equation is:

3.4 Pixel Rate Gain Correction Block

This is a digital multiplier that multiplies the output word from the

subtractor by a 16 bit digital correction coefficient corresponding

to that pixel’s gain error. The coefficients are stored in the exter-

nal RAM and accessed at the pixel rate.

The multiplier saturates at 16383, i.e. if the result of the multipli-

cation is greater than 16383, the multiplier output is 16383.

The gain equation is:

Note that a coefficient of 0 represents a gain of 0. On the LM9830

and previous parts, a coefficient of 0 represented a gain of 1. To

achieve a gain of 1, the coefficient should be set to 16384.

3.5 Gamma Correction Tables

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

the table is the 12 MSBs (most significant bits) of the 14 bit pixel

data coming from the previous stage (3.4 Pixel Rate Gain Cor-

rection Block). The output is the 8 bit gamma corrected pixel

data. The tables consume 12k words (4K bytes x 16 bits, only the

8 LSBs of each word is used) of the external DRAM. Each

gamma table (red, green, and blue) can be loaded with any arbi-

trary user-defined transfer curve.

The gamma tables are loaded through the dataport (see 6.1 The

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

Memory). The DataPort selects which color (Red, Green or Blue)

gamma table will be read from or written to.

3.6 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 LM9832 packs the desired MSBs of multi-

ple pixels together into 1 16 bit word, increasing the transmission

speed to the host by a factor of 2, 4, 8, or 16. Figure 6 shows how

the pixels are packed together for 8, 4, 2, and 1 bit pixel depths.

In Figure 6, “b” indicates the bit position (b7 = the most significant

and b0 = the least significant bit) of the original 8 bit pixel data,

and pn indicates the original pixel sequence, i.e p0, p1, p2, p3...

If there are not enough unpacked pixels at the end of a line to

complete the packed word for transmission, that final word is not

sent. For example, doing an 8 bit pixel rate scan with a HDPI

divider of 1 and an odd number of pixels will truncate the blue

component of the last pixel.

The gamma table in 3.5 Gamma Correction Tables allows the

user to set the threshold of each transition for various line art or

reduced pixel depth modes.

3.7 14 Bit Output Mode

The LM9832 also supports a 14 bit out mode. This can be used to

get very accurate data for calibration or to scan a 14 gray/42 bit

color image. This mode is set through register 9, bit 5. In the 14

bit output mode, the gamma and pixel packing stages are

bypassed, and the 14 bit data from the ADC is stored in DRAM,

formatted as shown in Figure 7.

The 14 bit data is stored as a 16 bit word, with the 2 least signifi-

cant bits of the 16 bit word set to 0.

The memory reserved for the gamma table is used to store image

data in the 14 bit mode. After scanning in 14 bit mode, the

gamma table must be reloaded for operation in 8, 4, 2, or 1 bit

mode.

Pixel

OUT

Pixel

coefficient

–

Pixel

OUT

Pixel

coefficient

16384

---------------------------

⋅

12MSBs of 14 bit Output

255

4095

Figure 5: Gamma Table

Pixel

Depth

bit

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

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

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

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

bit

b7 p1 b6 p1 b5 p1 b4 p1 b3 p1 b2 p1 b1 p1 b0 p1

b7 p2 b6 p2 b5 p2 b4 p2 b7 p3 b6 p3 b5 p3 b4 p3

b7 p4 b6 p4 b7 p5 b6 p5 b7 p6 b6 p6 b7 p7 b6 p7

b7 p8 b7 p9 b7 p10b7 p11b7 p12b7 p13b7 p14b7 p15

Figure 6: Packing Multiple Pixels Into One Word

MSB

1413121110

b13

b12

b11

b10

LSB

6543

210

Figure 7: 14 Bit Output Mode Data Format

Applications Information (Continued)

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