LM628 datasheet(8/24 Pages) NSC

8 / 24 page

Pinout Description (Continued)

Immediately after releasing the reset pin from the LM628,

the status port should read “00”. If the reset is successfully

completed, the status word will change to hex “84” or “C4”

within 1.5 ms. If the status word has not changed from hex

“00” to “84” or “C4” within 1.5 ms, perform another reset and

repeat the above steps. To be certain that the reset was

properly performed, execute a RSTI command. If the chip

has reset properly, the status byte will change from hex “84”

or “C4” to hex “80” or “C0”. If this does not occur, perform an-

other reset and repeat the above steps.

Pin 28 (16), Supply Voltage (V

DD): Power supply voltage

(+5V).

Theory of Operation

INTRODUCTION

The typical system block diagram (See

Figure 1) illustrates a

servo system built using the LM628. The host processor

communicates with the LM628 through an I/O port to facili-

tate programming a trapezoidal velocity profile and a digital

compensation filter. The DAC output interfaces to an exter-

nal digital-to-analog converter to produce the signal that is

power amplified and applied to the motor. An incremental en-

coder provides feedback for closing the position servo loop.

The trapezoidal velocity profile generator calculates the re-

quired trajectory for either position or velocity mode of opera-

tion. In operation, the LM628 subtracts the actual position

(feedback position) from the desired position (profile genera-

tor position), and the resulting position error is processed by

the digital filter to drive the motor to the desired position.

Table 1 provides a brief summary of specifications offered by

the LM628/LM629:

POSITION FEEDBACK INTERFACE

The LM628 interfaces to a motor via an incremental encoder.

Three inputs are provided: two quadrature signal inputs, and

an index pulse input. The quadrature signals are used to

keep track of the absolute position of the motor. Each time a

logic transition occurs at one of the quadrature inputs, the

LM628 internal position register is incremented or decre-

mented accordingly. This provides four times the resolution

over the number of lines provided by the encoder. See

Fig-

ure 9. Each of the encoder signal inputs is synchronized with

the LM628 clock.

The optional index pulse output provided by some encoders

assumes the logic-low state once per revolution. If the

LM628 is so programmed by the user, it will record the abso-

lute motor position in a dedicated register (the index register)

at the time when all three encoder inputs are logic low.

If the encoder does not provide an index output, the LM628

index input can also be used to record the home position of

the motor. In this case, typically, the motor will close a switch

which is arranged to cause a logic-low level at the index in-

put, and the LM628 will record motor position in the index

grounding the index input will cause the LM628 to malfunc-

tion.

TABLE 1. System Specifications Summary

Position Range

−1,073,741,824 to 1,073,741,823 counts

Velocity Range

0 to 1,073,741,823/2

16 counts/sample; ie, 0 to 16,383 counts/sample, with a resolution of 1/216

counts/sample

Acceleration Range

0 to 1,073,741,823/2

16 counts/sample/sample; ie, 0 to 16,383 counts/sample/sample, with a

resolution of 1/2

16 counts/sample/sample

Motor Drive Output

LM628: 8-bit parallel output to DAC, or 12-bit multiplexed output to DAC

LM629: 8-bit PWM sign/magnitude signals

Operating Modes

Position and Velocity

DS009219-10

FIGURE 8. 12-Bit Multiplexed Output Timing

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