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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

LM9833 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 LM9833 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.

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

Size register) until the Pause Scanning signal is received.

• 50% speed forward for z full steps (1 microstep = 2* #pixels in

Fast Feed Step Size register)

• 25% speed forward for y full steps (1 microstep = 4*#pixels in

Fast Feed Step Size register)

• Stopped for x full steps (1 microstep = #pixels in Fast Feed

Step Size register).

• 25% speed backward for y full steps (1 microstep = 4*#pixels in

Fast Feed Step Size register)

• 50% speed backward for z full steps (1 microstep = 2* #pixels in

Fast Feed Step Size register)

• Full speed backward (1 microstep = #pixels in Fast Feed Step

Size register) for number of microsteps in the Steps to

Reverse register

• 50% speed backward for z full steps (1 microstep = 2* #pixels in

Fast Feed Step Size register)

• 25% speed backward for y full steps (1 microstep = 4*#pixels in

Value

Additional Lines to Store in DRAM

00(a only)

1 (a and b)

2 (a, b and c)

...

Figure 14: Lines to Process after Pause Scan Signal Register

Value

First Line to Transmit After Pause

...

Figure 15: Lines to Discard After Resume Scan Signal

Figure 16: Stepper Motor Resuming

Microstep

Pulse

Resume

Scanning

Signal

abc

Speed

DAC output

Stopped

(x = 0 to 3)

x = number of full step clocks to wait

before reversing motor.

25%

(y = 0 to 3)

y = number of full steps at 25% of final

speed. Full step period = 4 full step

clocks.

50%

(z = 0 to 3)

z = number of full steps at 50% of final

speed. Full step period = 2 full step

clocks.

Figure 18: Acceleration Profile Settings

Figure 17: Reversing - The Goal

Microstep

Pulse

Pause

Scanning

Signal

Microstep Pulse

(if motor had not

paused)

Applications Information (Continued)

LM9833 Scheda tecnica(PDF) 25 Page - National Semiconductor (TI)

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