LM628, LM629

SNVS781C – JUNE 1999 – REVISED MARCH 2013

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The derivative-term sampling interval is also programmable via this command. After writing the command code,

the first two data bytes that are written specify the derivative-term sampling interval and which of the four filter

parameters is/are to be written via any forthcoming data bytes. The first byte written is the more significant. Thus

the two data bytes constitute a filter control word that informs the LM628 as to the nature and number of any

following data bytes. See Table 4.

Table 4. Filter Control word Bit Allocation

Bit Position

Function

Bit 15

Derivative Sampling Interval Bit 7

Bit 14

Derivative Sampling Interval Bit 6

Bit 13

Derivative Sampling Interval Bit 5

Bit 12

Derivative Sampling Interval Bit 4

Bit 11

Derivative Sampling Interval Bit 3

Bit 10

Derivative Sampling Interval Bit 2

Bit 9

Derivative Sampling Interval Bit 1

Bit 8

Derivative Sampling Interval Bit 0

Bit 7

Not Used

Bit 6

Not Used

Bit 5

Not Used

Bit 4

Not Used

Bit 3

Loading kp Data

Bit 2

Loading ki Data

Bit 1

Loading kd Data

Bit 0

Loading il Data

Bits 8 through 15 select the derivative-term sampling interval. See Table 5. The user must locally save and

restore these bits during successive writes of the filter control word.

Bits 4 through 7 of the filter control word are not used.

Bits 0 to 3 inform the LM628 as to whether any or all of the filter parameters are about to be written. The user

may choose to update any or all (or none) of the filter parameters. Those chosen for updating are so indicated by

logic one(s) in the corresponding bit position(s) of the filter control word.

The data bytes specified by and immediately following the filter control word are written in pairs to comprise 16-

bit words. The order of sending the data words to the LM628 corresponds to the descending order shown in the

above description of the filter control word; i.e., beginning with kp, then ki, kd and il. The first byte of each word is

the more-significant byte. Prior to writing a word (byte pair) it is necessary to check the busy bit in the status byte

for readiness. 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 UDF 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.

UDF COMMAND: UPDATE FILTER

Command Code: 04 Hex

Data Bytes: None

Executable During Motion: Yes

The UDF command is used to update the filter parameters, the specifics of which have been programmed via the

LFIL command. Any or all parameters (derivative-term sampling interval, kp, ki, kd, and/or il) may be changed by

the appropriate command(s), but command UDF must be executed to affect the change in filter tuning. Filter

updating is synchronized with the calculations to eliminate erratic or spurious behavior.

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