4255

8-BIT

MICROCONTROLLER

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

Table 2 — Microcontroller terminal descriptions

A4255CA

A4255CLN

DIP

SSOP

Function

Description

Comments, connections, etc.

1

1

Input

Mode select 1

Static and/or dynamic control of stepping mode

2

2

Input

Mode select 2

Static and/or dynamic control of stepping mode

3

3

Input

Step in

Governs full-step rate (

÷8 for A3955; ÷16 for A3957)

4

4

Input

Reset

Resets DIR., MS2, MS1, and MS0 to 0000 (detent)

5

5, 6

Power

Ground

Logic power return

6

7

Output

Monitor out

Signals full-step rotor alignment (active low)

7

8

I/O

Shift clock

Pull up for A3955; pull down for A3957

8

9

Output

Serial data out

Shifts 8-bit serial data to 74HC595 serial input

9

10

Output

Strobe out

Latches 8-bit data into 74HC595 (latch clock in)

10

11

Output

Phase B

Controls current direction in phase B

11

12

Output

PFD B

Phase B recirculation control

12

13

Output

Phase A

Controls current direction in phase A

13

14

Output

PFD A

Phase A recirculation control

14

15, 16

Power

Supply

15

17

Output

Osc/clock out

Crystal oscilator connection

16

18

Input

Osc/clock in

Crystal oscillator connection/external clock input

17

19

Input

Direction control

Determines direction of step motor rotation

18

20

Input

Mode select 0

Static and/or dynamic control of stepping mode

FUNCTIONAL DESCRIPTION (cont’d)

Mode-select inputs

These three inputs (MS2, MS1, and MS0) determine

the stepping format, disable/enable motor power, and reset

the controller/sequencer. In conjunction with the direction

input, the mode inputs control the sixteen operating states

listed. Deactivating stepper power in any position except

‘detent’ (i.e., a single phase activated) results in the motor

rotor advancing, or retracting, from its intermediate

position and alignment with a natural (i.e., minimum-

reluctance flux field) orientation. The absolute position

may be affected by inertia, load, fractional position,

ringing, etc. and cannot be determined without feedback.

Phase currents must be maintained to immobilize the

rotor/load in any intermediate position.

Step (clock) input

The sequencer stepping-clock frequency is a multiple

of the actual stepping rate. The A3955 requires a step-

ping-clock input frequency equal to eight times the

intended stepping rate for the motor; the A3957 requires a

stepping clock rate that is sixteen times the actual stepping

rate of the motor. However, neither design necessitates

that the step frequency be varied should the operating

mode(s) be switched during operation. Shifting from one

stepping format does not mandate a simultaneous (and

equivalent) change in the clock frequency.

Using a 20 MHz crystal (maximum limit for the

A4255) allows a 50 kHz stepping clock for the A3955,

and this equates to 6 250 full steps per second (50 kHz/8).

For the A3957 this 50 kHz stepping clock translates to