LM9833 datasheet(36/42 Pages) NSC

36 / 42 page

www.national.com

12.16 Paper Sensor #2 Doesn’t Stop High Speed Reverse

In the LM9831, a false-to-true transition on Paper Sensor #2

would stop a high speed reverse (usually used for homing in flat-

bed scanners). Due to the changes made to accommodate the

FullStep Timeout Function function, the ability to stop a high

speed reverse function was removed.

12.17 Turbo and Preview Modes

These modes actually existed in the LM9831, but were not docu-

mented.

The Turbo and Preview modes allow additional pixel averaging

(horizontal resolution reduction) to be done in the analog domain.

This can be useful, for example, when you have a 1200 dpi scan-

ner and wish to scan at 75 or 50dpi. The HDPI divider function’s

lowest resolution is divide-by-12. With the HDPI divider set to

divide-by-8 and turbo or preview mode set to x2, the horizontal

resolution will be 1200/16 = 75dpi. With the HDPI divider set to

divide-by-12 and turbo or preview mode set to x2, the horizontal

resolution will be 1200/24 = 50dpi. The HDPI divider and

Turbo/Preview modes can be used in any combination.

For a Preview factor of xN, the Preview Mode operates by

increasing the pixel clocks to the CCD by a factor of N, while sup-

pressing (N-1) reset pulses out of every N pixels. This is only use-

ful for CCDs (or CIS sensors made with CCD technology).

In the Turbo Mode, the entire analog front end is run N times

faster, and every N pixels are averaged together before they are

converted to digital by the ADC. When using Turbo Mode, the

range of registers 0F to 18 is reduced by the Turbo Mode factor,

according to the following table:

13.0 PORTING SOFTWARE FOR LM9830 TO LM9833

The LM9833 is similar in architecture to the LM9830. Porting a

TWAIN driver from the LM9830 to the LM9833 is relatively

straightforward if consideration is given to the following issues.

The LM9833 includes almost all the features of the LM9830, plus

several new ones. The first step is to change the LM9830 Twain

driver so that it works with the LM9833. The second step is to

take advantage of the new features of the LM9833 that will allow

you to obtain even better, faster scans than you obtained with the

LM9830.

13.1 Porting Step 1

13.1.1 Adjust for Register Changes

While more than 50% of the registers in the LM9833 are in the

same location and perform the same function as they did in the

LM9830, many other registers have changed. Sometimes the

address of a register changed, sometimes the location of the bits

inside a register were moved, some register settings were com-

bined or deleted, and the size of some registers was changed.

Please compare the register listings for the LM9830 and LM9833

carefully. This is a list of registers that have changed:

Registers 1, 2, 3, 4, 7, 9, B, 19, 1A, 1B, 3E-41, 42, 43-44, 4E-4F,

51-53, 54, 5A, 5B, 5E.

13.1.2 Choosing the MCLK Divider (Register 0x08)

The datarate coming out of the Horizontal DPI Divider must be

1.1MHz or less. If it is faster than this, the LM9833 will not oper-

ate correctly. Since the maximum USB datarate is about 1MHz,

this does not impact the performance of the scanner in any way.

This is the Clock Divider Rule:

(MCLK_divider)(HDPI_divider)(ITA) >= 6

The ITA (Integration Time Adjust) refers to register 19, and will be

discussed in a later section. If register 19 = 0, then the value of

ITA = 1 for the purposes of this formula.

If register 19 = 0, this formula means that if the HDPI_divider = 1,

the MCLK_divider must be set to divide-by-6 (reg 08 = 10 [deci-

mal]) or higher. If the HDPI_divider = 4, the MCLK_divider must

be set to divide-by-2 (reg 08 = 2) or higher. If the HDPI_divider is

6 or larger, then the MCLK_divider can be set to divide-by-1

(reg08 = 0).

See 13.2.2 Integration Time Adjustment Function for addi-

tional information.

13.1.3 Calibration

In the LM9830, calibration was always performed at the optical

resolution of the scanner. For example, if the optical resolution of

the scanner was 600dpi, then calibration was performed at

600dpi even if the scan was going to be at 300dpi or 150dpi.

To keep the speed of the LM9833 high while using slower DRAM

(instead of SRAM), the architecture of the LM9833 was changed

so that the Horizontal DPI adjust function is performed before the

pixel rate offset and shading correction, instead of after (as in the

LM9830).

This means that the calibration routine needs to be changed so

that register 9 is set to the desired scan resolution before calibra-

tion.

13.1.4 Pixel Rate Offset Correction

The LM9833 uses 16 bits for the offset correction of each pixel.

Mode

Pixel Rate

Registers

0F to 18

Range

3 Channel,

Turbo off

MCLK/24

0 - 23

3 Channel,

Turbo x2

MCLK/12

0 - 11

3 Channel,

Turbo x3

MCLK/8

0 - 7

3 Channel,

Turbo x4

MCLK/6

0 - 5

3 Channel,

Turbo x6

MCLK/4

0 - 3

1 Channel,

Turbo off

MCLK/8

0 - 7

1 Channel,

Turbo x2

MCLK/4

0 - 3

Applications Information (Continued)

LM9833 Datasheet(PDF) 36 Page - National Semiconductor (TI)

LM9833 Datasheet(HTML) 36 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Datasheet Download

Link URL