lm9831 National Semiconductor Corporation, lm9831 Datasheet - Page 23

• Manufacturer Part Number: lm9831
• Description: 42-bit Color, 1200dpi Usb Image Scanner
• Manufacturer: National Semiconductor Corporation

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
Register 4E value = Pause Threshold (kB)/2 (256k x 16 DRAM)
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
Bytes/Line
=
2 INT
INT
----------------------------------------------------------------------- -
----------------------------------- -
HDPI_Divider
Data Pixels
16
C B
23
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 LM9831 always counts stepper motor steps in units of
microsteps. A full step is equal to four microsteps. Even when the
LM9831 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
f
modes, the pixel period is equal to f
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 LM9831 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.
4.1 Full Step Mode
In Full Step Mode the output is a pulse stream, as shown in Fig-
ure 8. The amplitude of the pulses is controlled by the output of
ADC
/3 (= f
Figure 8: Stepper Motor Waveform - Full Stepping
A
A
B
B
MCLK
/24). In the 3 Channel Line Rate and 1 channel
LPI
=
4FSPI
----------------------------------- -
pixels/line X
1 full step = 4
microsteps
StepSize
ADC
(= f
MCLK
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/8). The Step