LM9830 datasheet(25/45 Pages) NSC

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the least significant bit) of the original 8 bit pixel data, and pn indi-

cates 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 byte for transmission, that final byte is not

sent.

The gamma table in 2.5 Gamma Correction Tables allows the

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

reduced pixel depth modes.

2.7 Line Buffer

The line buffer uses the external SRAM to store the pixel data at

the fixed rate and send it back to the PC at an asynchronous,

unpredictable, and non-constant rate.

This buffer is tightly coupled to the stepper motor (3.0 Stepper

Motor Controller), and is responsible for stopping the motor

before the buffer overflows and starting the motor again as the

buffer nears empty.

If the scanner is generating pixel data faster than the PC can

acquire it, the line buffer will start to fill up. As the buffer nears

100% full, the scan must be paused before it starts acquiring a

line it cannot store because of lack of RAM. This Pause Thresh-

old limit (register 4E) is programmable in 1 kbyte increments

between 0 and 255 kbytes but should be no higher than 100% of

the buffer RAM size minus 1 line of data (for single output CCDs

and CIS) or 3 lines of data (for triple output CCDs and CIS).

When this point is reached the buffer sends a command to the

stepper motor controller to stop scanning. The remainder of the

line being processed will continue being processed and be sent

to the buffer. If the Lines To Process After Pause Scan Signal reg-

ister (register 54) is greater than 0, then room for these additional

lines need to be added into the Pause Threshold value calcula-

tion.

After a pause, the buffer will now transmit data to the PC until it

hits the Resume Threshold limit (register 4F), which is also pro-

grammable in 1 kbyte increments between 0 and 256kbytes.

When the Resume Threshold is reached, the Line Buffer sends

the motor controller a command to resume.

Note that the scanner software on the host PC is responsible for

ensuring that the Pause Threshold value is low enough to ensure

that any data that comes after a pause request (the rest of the

current line and any subsequent lines if register 54 bits 0-2 are

greater than 0) will fit into the SRAM buffer size, which is equal to

SRAM size - COEFFICIENT size.

The pause condition is reached when the number of bytes in the

buffer is equal to the value in register 4E * 1024. The scan will

resume when the number of bytes in the buffer is equal to (the

value in register 4F * 1024 + 1023).

Since the external SRAM also contains the pixel gain and offset

data (see 2.2 Pixel Rate Offset Correction Block and 2.3 Pixel

Rate Gain Correction Block), the buffer is as large as the SRAM

size minus the coefficient storage. Supported SRAM sizes are

64kbyte, 128kbyte, and 256kbyte. Coefficient data always takes

up a total of 16kbytes for 300dpi sensors and 32kbytes for 600dpi

sensors.

3.0 Stepper Motor Controller

The stepper motor controller sends a series of pulses to the step-

per motor to move the paper past the sensor (sheetfed) or the

sensor past the paper (flatbed). The speed at which the paper

moves relative to the sensor, combined with the integration time

of the image sensor, determines the effective vertical resolution

(Lines Per Inch, or LPI).

The stepper motor is moved forwards and backwards by two sig-

nals, A and B, 90° out of phase with each other. The phase for the

forward direction is set in Configuration Register 45.

The A and B signals are either squarewaves (in Full Step Mode,

Figure 16), or a staircase approximation of a sine wave (in

Microstep mode, Figures 18 and 19).

The LM9830 always counts stepper motor steps in units of

microsteps. A full step is equal to four microsteps. Even when the

LM9830 is in Full Step Mode, it is counting in microsteps, and will

increment the stepper motor (generating a full step) every four

microsteps.

The microstep Step Size is defined in units of time. These units of

time are pixel periods, as defined in the horizontal pixel counter.

In the 3 channel pixel rate input mode, the pixel period is the

fADC/3 (= fMCLK/24). In the 3 channel line rate and 1 channel

modes, the pixel period is equal to fADC/3 (= fMCLK/24). The Step

Size is stored in the Scanning Step Size configuration register

as a 14 bit value. During normal operation, the stepper motor is

advanced 1 microstep every Step Size pixel periods. The LPI can

be calculated as follows:

Where C = the number of full steps required to move the image

one inch, pixels/line is the number of pixel periods it takes to scan

one horizontal line (equivalent to the value stored in the Line End

registers), and StepSize is the number of pixel periods/microstep

Whenever the stepper motor has been moving and then comes to

a stop, the LM9830 waits for the time specified in the Hold Cur-

rent Timeout register and then de-asserts the A, B, A, and B out-

puts to cut power to the motor. When the stepper motor is not

scanning or fast-feeding (Command = 00), A, B, A, and B are de-

asserted in all stepper modes.

There are two modes of stepper motor operation: fullstepping and

microstepping.

3.1 Full Step Mode

In Full Step Mode the output is a pulse stream, as shown in Fig-

ure 16. The amplitude of the pulses is controlled by the output of

Figure 16: Stepper Motor Waveform - Full Stepping

1 full step = 4

microsteps

LPI

StepSize

pixels/line

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