LM628, LM629

SNVS781C – JUNE 1999 – REVISED MARCH 2013

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Table 6. Trajectory Control Word Bit Allocation (continued)

Bit Position

Function

Bit 1

Position Will Be Loaded

Bit 0

Position Data Is Relative

Bit 12 determines the motor direction when in the velocity mode. A logic one indicates forward direction. This bit

has no effect when in position mode.

Bit 11 determines whether the LM628 operates in velocity mode (Bit 11 logic one) or position mode (Bit 11 logic

zero).

Bits 8 through 10 are used to select the method of manually stopping the motor. These bits are not provided for

one to merely specify the desired mode of stopping, in position mode operations, normal stopping is always

smooth and occurs automatically at the end of the specified trajectory. Under exceptional circumstances it may

be desired to manually intervene with the trajectory generation process to affect a premature stop. In velocity

mode operations, however, the normal means of stopping is via bits 8 through 10 (usually bit 10). Bit 8 is set to

logic one to stop the motor by turning off motor drive output (outputting the appropriate offset-binary code to

apply zero drive to the motor); bit 9 is set to one to stop the motor abruptly (at maximum available acceleration,

by setting the target position equal to the current position); and bit 10 is set to one to stop the motor smoothly by

using the current user-programmed acceleration value. Bits 8 through 10 are to be used exclusively; only one bit

should be a logic one at any time.

Bits 0 through 5 inform the LM628 as to whether any or all of the trajectory controlling parameters are about to

be written, and whether the data should be interpreted as absolute or relative. The user may choose to update

any or all (or none) of the trajectory parameters. Those chosen for updating are so indicated by logic one(s) in

the corresponding bit position(s). Any parameter may be changed while the motor is in motion; however, if

acceleration is changed then the next STT command must not be issued until the LM628 has completed the

current move or has been manually stopped.

The data bytes specified by and immediately following the trajectory control word are written in pairs which

comprise 16-bit words. Each data item (parameter) requires two 16-bit words; the word and byte order is most-to-

least significant. The order of sending the parameters to the LM628 corresponds to the descending order shown

in the above description of the trajectory control word; i.e., beginning with acceleration, then velocity, and finally

position.

−1 (3FFFFFFF

hex). The bottom 16 bits of both acceleration and velocity are scaled as fractional data; therefore, the least-

significant integer data bit for these parameters is bit 16 (where the bits are numbered 0 through 31). To

determine the coding for a given velocity, for example, one multiplies the desired velocity (in counts per sample

interval) times 65,536 and converts the result to binary. The units of acceleration are counts per sample per

sample. The value loaded for acceleration must not exceed the value loaded for velocity. Position is a signed,

32-bit integer, but ranges only from

The required data is written to the primary buffers of a double-buffered scheme by the above described

operations; it is not transferred to the secondary (working) registers until the STT command is executed. This fact

can be used advantageously; the user can input numerous data ahead of their actual use. This simple pipeline

effect

can

relieve

potential

host

computer

data

communications

bottlenecks,

and

facilitates

easier

synchronization of multiple-axis controls.

STT COMMAND: START MOTION CONTROL

Command Code: 01 Hex

Data Bytes: None

Executable During Motion: Yes, if acceleration has not been changed

20

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