lm9831 National Semiconductor Corporation, lm9831 Datasheet - Page 25

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

When the Resume Scan signal is received, the stepper motor
controller waits the appropriate number of pixel periods after the
next TR pulse and then starts stepping again at the normal rate.
The first new line transmitted is determined by the Lines to Dis-
card After Resume Scan Signal register. The discard value
must be the same as the value in the Lines to Process After
Pause Scan Signal register.
4.4 Pause Behavior - Reversing Mode
If the Full Steps to Reverse When Buffer is Full register is >0,
then the Reversing Mode is enabled.
The Reversing Mode eliminates spatial distortion due to the
pausing of a scan. When the Pause Scan signal is received, the
line currently being processed is completed and stored in RAM
(line “b” in Figure 17). When the scan resumes, ideally the
LM9831 would send out lines “c” and after under the exact same
speed and positional conditions the scanner was in before the
scan paused (as indicated by the dotted line in Figure 17).
When the Pause Scan signal is received, the LM9831 processes
the remainder of the line currently being read from the CCD (line
b), and stores the offset (in pixel periods) between the last TR
pulse and the last step. It then stops, reverses, stops, and waits
for the Resume Scan signal. Once Resume Scan is asserted, the
motor controller waits for the previously stored number of pixels
periods, then starts moving forward again, maintaining the same
phase relationship between the TR pulse and the stepper motor
control signals. The result is as if the stepper motor had never
paused.
Figure 14: Lines to Process after Pause Scan Signal Register
Figure 15: Lines to Discard After Resume Scan Signal
Microstep
0
1
2
...
7
0
1
2
...
7
TR
Pulse
Value
Value
Figure 16: Stepper Motor Resuming
0(a only)
1 (a and b)
2 (a, b and c)
...
7
b
c
d
...
i
Additional Lines to Store in DRAM
First Line to Transmit After Pause
Scanning
Resume
Signal
Register
a
b
c
d
25
Stopping, reversing, and resuming forward motion all follow the
curve programmed in the Acceleration Profile configuration reg-
ister. There are 3 segments (Stopped, 25%, and 50%), and the
number in each register indicates the number of full steps to stay
at that acceleration. A value of 0 indicates that that segment is to
be skipped. For example, a value of 0 in all three registers would
mean that the motor would instantly reverse when the buffer is
full, then instantly stop after going back the specified number of
lines.
This acceleration profile is used any time the motor is started,
stopped, or reversed.
The acceleration profile for stopping, reversing, stopping, and
going forward again is this:
• Full speed forward (1 microstep = #pixels in Scanning Step
• 50% speed forward for z full steps (1 microstep = 2* #pixels in
• 25% speed forward for y full steps (1 microstep = 4*#pixels in
• Stopped for x full steps (1 microstep = #pixels in Scanning
• 25% speed backward for y full steps (1 microstep = 4*#pixels in
• 50% speed backward for z full steps (1 microstep = 2* #pixels in
• Full speed backward (1 microstep = #pixels in Scanning Step
• 50% speed backward for z full steps (1 microstep = 2* #pixels in
• 25% speed backward for y full steps (1 microstep = 4*#pixels in
Size register) until the Pause Scanning signal is received.
Scanning Step Size register)
Scanning Step Size register)
Step Size register).
Scanning Step Size register)
Scanning Step Size register)
Size register) for number of microsteps in the Steps to
Reverse register
Scanning Step Size register)
(if motor had not
Microstep Pulse
Stopped
(x = 0 to 3)
25%
(y = 0 to 3)
50%
(z = 0 to 3)
Microstep
TR
Register
paused)
Speed
Pulse
Figure 18: Acceleration Profile Settings
Figure 17: Reversing - The Goal
a
x = number of full step clocks to wait
before reversing motor.
y = number of full steps at 25% of final
speed. Full step period = 4 full step
clocks.
z = number of full steps at 50% of final
speed. Full step period = 2 full step
clocks.
Scanning
Pause
Signal
b
DAC output
c
www.national.com
d
e