LM9832 datasheet(23/42 Pages) NSC

23 / 42 page

www.national.com

3.8 Line Buffer

The line buffer uses the external DRAM as a FIFO line buffer to

store the pixel data (which is generated at a fixed rate, synchro-

nous to the CCD clocks) and send it back to the PC at an asyn-

chronous, unpredictable, and non-constant rate.

The LM9832 supports 2 sizes of DRAM, 256k x 16bit and 1M x

16bit. 216kbytes (108kwords) of the capacity of the DRAM is con-

sumed by the offset and shading coefficients and the gamma

tables. That leaves 296kbytes of memory available for line buffer

when using a 256k x 16 bit DRAM, or 1832kbytes of memory

when using a 1M x 16 bit DRAM.

The line buffer is tightly coupled to the stepper motor (4.0 Step-

per 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% of its capacity, the scan must be paused before it starts

acquiring a line which will overflow the buffer. This Pause Thresh-

old limit (register 4E) is programmable in 2 kbyte (256k x 16 bit

DRAM) or 8kbyte (1M x 16 bit DRAM) increments between 0 and

255.

To maximize scanner performance and minimize pausing due to

buffer full conditions, the pause threshold should be set using this

formula:

Pause Threshold (kB) = Available_Memory - (Line_Length + 1)

where Available_Memory = 296kbytes (256k x 16b DRAM) or

1832kbytes (1M x 16 bit DRAM),

Line_Length = (Bytes/Line)/1024

Where C = 1 for “1 Channel Grayscale”, 3 for all other modes,

Data_Pixels = Data Pixels End (registers 24, 25) - Data Pixels

Start (registers 22, 23)

HDPI_Divider = Horizontal DPI divider = 1, 1.5, 2, 3, 4, 6, 8, or 12

B = Bits per Pixel = 16 (14 bit mode), 8, 4, 2, or 1

or Pause Threshold (kB)/8 (1M x 16 DRAM)

When the Pause Threshold is reached the buffer sends a com-

mand to the stepper motor controller to stop scanning. The

remainder of the line being processed will continue being pro-

cessed and be sent to the buffer. If the Lines To Process After

Pause Scan Signal register (register 54) is greater than 0, then

room for these additional lines needs to be added into the Pause

Threshold value calculation.

Note that the scanner software on the host PC must set a Pause

Threshold value low enough to ensure that any data that comes

after a pause request (the rest of the current line and any subse-

quent lines if register 54 bits 0-2 are greater than 0) will fit into the

DRAM buffer. If the Pause Threshold is set too high, the Line

Buffer may overflow, creating discontinuities in the scanned

image.

After a pause, the buffer will continue to transmit data to the PC

until it hits the Resume Threshold limit (register 4F), which is also

programmable in 2 kbyte (256k x 16 bit DRAM) or 8kbyte (1M x

16 bit DRAM) increments between 0 and 255. When the Resume

Threshold is reached, the Line Buffer sends the motor controller a

command to resume.

4.0 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 8), or a staircase approximation of a sine wave (in

Microstep mode, Figures 10 and 11).

The LM9832 always counts stepper motor steps in units of

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

LM9832 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 (= fMCLK/8). 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 FSPI = 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), StepSize is the number of pixel peri-

ods/microstep, and X = 3 for line rate and 1 for pixel rate modes.

Whenever the stepper motor has been moving and then comes to

a stop, the LM9832 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.

Bytes/Line

INT

⋅

INT

Data Pixels

HDPI_Divider

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



 CB

⋅

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







Figure 8: Stepper Motor Waveform - Full Stepping

1 full step = 4

microsteps

LPI

4FSPI

StepSize

pixels/line

⋅

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

Applications Information (Continued)

LM9832 датащи(PDF) 23 Page - National Semiconductor (TI)

LM9832 датащи(HTML) 23 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

датащи скачать

ссылки URL