4255

8-BIT

MICROCONTROLLER

www.allegromicro.com

format, reset, 1/8th vs 1/16th sub-steps, etc. Note that

during power up, shift clock (SCLK) is sampled for a pull-

up or pull-down resistor to establish the fractional step

limit. A pull up sets up a 1/8th-step format (for the

A3955) and pull down sets up 1/16th-operation (for the

A3957).

Table 2 lists the microcontroller terminal descriptions

and provides the essence of the circuit operation (a

schematic illustrating a typical stepper design follows). A

brief description of the microcontroller I/O should clarify

the connections of the various elements of the drive

electronics.

FUNCTIONAL DESCRIPTION (cont’d)

Table 1 — Controller/sequencer IC operational logic

Binary inputs

Operating mode

Comments

DIR

MS2

MS1

MS0

(Command executed on L

→ H of CLK)

(Applicable power ICs)

0000

CW, Full step (single-phase)

A3955/57

0001

CW, Half step (constant torque)

A3955/57

0010

CW, 1/4 step (constant torque)

A3955/57

0011

CW, 1/8th step (constant torque)

A3955/57

0100

CW, 1/16th step (constant torque)

A3957 only

0101

Disable A3955/57 holding torque

At present position

0110

Enable A3955/57 holding torque

From present position

0111

Reset A4255 sequencer IC

A3955/57

1000

CCW, full step (single-phase)

A3955/57

1001

CCW, half step (constant torque)

A3955/57

1010

CCW, 1/4 step (constant torque)

A3955/57

1011

CCW, 1/8th step (constant torque)

A3955/57

1100

CCW, 1/16th step (constant torque)

A3957 only

1101

Disable A3955/57 holding torque

At present position

1110

Enable A3955/57 holding torque

From present position

1111

Reset A4255 sequencer IC

A3955/57