LM9833 datasheet(32/42 Pages) NSC

32 / 42 page

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7.0 The USB Interface

The LM9833 uses the USB (Universal Serial Bus) interface.

Refer to the LM9833 software package for details on USB com-

munication.

7.1 The USB Pins

Data is received and transmitted through the D+ and D- pins.

These are 3V differential signals. Figure 44 shows the recom-

mended circuitry between the LM9833’s D+ and D- pins and the

scanner’s USB connector.

8.0 Scanning

The following sections describe the typical steps taken to scan an

image.

8.1 Start Scanning - Initiating an Image Scan

An image scan is initiated by writing a Scan command to Register

07. The LM9833 will move the sensor forward the number of full-

steps specified in registers 4A/4B and begin scanning. Scanning

ends when the host writes a new command to the command reg-

ister (Idle, High Speed Forward or High Speed Reverse) or when

PAPER SENSE 1 or PAPER SENSE 2 changes state (if pro-

grammed to do so).

The line buffer is reset when the Scanning bit is SET, not when it

is cleared. The host can continue to read stored data out of the

line buffer after a scan has stopped.

Pixel data is read from configuration register address 00. Regis-

ters at other addresses can be read during a scan (to read the

LM9833’s status registers, abort the scan, etc.).

If for some reason you want to pause the scan for some length of

time and resume later, do NOT stop the scan (return to Idle). Sim-

ply stop reading pixel data. When the buffer fills up, the LM9833

will automatically stop scanning and turn off power to the stepper

motor (when the delay goes beyond the time specified in the Hold

Current Timeout register).

The last 2 bytes of every line is a status word indicating how

much data is in the image buffer at the time the status word was

written. This information is in the 8 LSBs of the status word, and

has the same format as Register 01.

8.2 Reconstructing 8 bit Image Data Received By the PC

When reconstructing an image from the stream of data received

from the LM9833, it is useful to know the format of the data. The

LM9833 does not perform deinterleaving on the pixel data, it

comes out exactly as the sensor sends it. Deinterleaving must be

performed on the host PC.

For a single output CCD/CIS that outputs one line of data with

colors alternating at the line rate, the output format is:

R1, R2, R3, R4,..., Rn-2, Rn-1, Rn (line m)

G1, G2, G3, G4,..., Gn-2, Gn-1, Gn (line m + 1)

B1, B2, B3, B4,..., Bn-2, Bn-1, Bn (line m + 2)

For a triple output CCD/CIS that outputs 3 lines of data (each x

pixels apart in the vertical direction) with colors alternating at the

pixel rate, the output would be:

R1, G1, B1, R2, G2, B2,..., Rn-1, Gn-1, Bn-1, Rn, Gn, Bn

with the Red data representing line m+x, the Green data repre-

senting line m, and the Blue data representing line m-x. “x” is the

separation between lines, which depends on the physical dis-

tance between the R, G, and B sensors and the rate at which the

sensor is moving over the image.

The length of a line of image data sent to the PC depends on sev-

eral factors:

• The range of pixels to be scanned (Data Pixels): Data Pixels =

(Data Pixels End - Data Pixels Start),

• The horizontal resolution set in the configuration register

(HDPI_Divider)

• The number of bits per pixel (1, 2, 4, or 8, called B), and

• The color mode: pixel rate (C=3) or line rate (C=1).

The scanner software on the host must strip the 2 byte status

word from the end of each line before reconstructing the image.

8.2.1 Reconstructing 16 bit Image Data Received By the PC

In the 16 bit Data Mode the Gamma Correction and Pixel Packing

stages are bypassed. Each pixel comes out as 2 bytes instead of

1, doubling the amount of memory needed to store one line. The

data format is shown in Figure 45. This mode is otherwise identi-

cal to the 8 bit mode. The number of bytes per line in 16 bit mode

is given in this equation:

The 16 bit mode is used to acquire 16 bit data for accurate gain

and offset calibration.

8.3 High Speed Forward

When register 07 is set to a 1, the LM9833 moves the motor for-

ward at maximum speed (determined by the fast feed stepsize,

registers 48 and 49) until a 0 is written to register 07 or either one

Figure 44: Recommended USB Component Values

LM9833 D-

(pin 83)

1.5k

Ω

LM9833 VREGULATOR

(pin82)

LM9833 D+

(pin 84)

Ω

D+ USB

Connector

Ω∗

D- USB

Connector

∗

Optional - forces LM9833 into

suspend mode if USB cable is

not attached to scanner.

10pF 10pF

7654

Type

b15 b14 b13 b12 b11 b10

First Byte

Second Byte

Figure 45: 16 bit Data Format

Bytes/Line

INT

⋅

INT

Data Pixels

HDPI_Divider

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





⋅

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







Bytes/Line

INT

Data Pixels

HDPI_Divider

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

() C

⋅⋅

Applications Information (Continued)

LM9833 Datasheet(PDF) 32 Page - National Semiconductor (TI)

LM9833 Datasheet(HTML) 32 Page - National Semiconductor (TI)

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