4255

8-BIT

MICROCONTROLLER

115 Northeast Cutoff, Box 15036

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

Copyright © 2000, Allegro MicroSystems, Inc.

A4255CLN

0.65 mm (0.026”) pitch

RECOMMENDED OPERATING CONDITIONS

over operating temperature range

Dwg. PP-071-1

MONITOR OUT

SHIFT CLOCK

SERIAL DATA OUT

STROBE OUT

DIRECTION

CONTROL IN

OSC/CLOCK IN

OSC/CLOCK OUT

SUPPLY

GROUND

STEP IN

RESET

19

15

6

1

2

3

4

5

10

14

13

12

11

20

18

17

16

9

8

79

SUPPLY

GROUND

To ease and simplify the design effort, the user only

provides the following signals: (a) direction, (b) stepping

clock (8x the full-step frequency), (c) mode logic (three

inputs determine the operation for full, half, quarter, or

eighth stepping), (d) reset input (initiates a ‘detent’

position), and (e) recirculation control (this allows estab-

lishing the percent of fast- vs slow-decay in the phase

winding). The microcontroller program providess auto-

matic recirculation control. This eliminates the need for

evaluating the impact of stepping rate vs the sinusoidal

current profile.

Although recirculation control can provide slight

improvements (i.e., lower current ripple, reduced motor

heating [a few degrees], and diminish audible noise levels

[minimal differences]), this entails an evaluation of the

motor (and step frequencies) to determine the proper ratio

of fast- and slow-decay. The benefits of tuning the

recirculation ratios are small, and the time and effort

required can be considerable. Hence, the uninitiated user

should opt for the automatic recirculation control, and

avoid the essentially unnecessary activity.

FUNCTIONAL DESCRIPTION

MICROCONTROLLER OPERATION

Although ‘hardware’ control of the microstepping ICs

is feasible, without a specific (ASIC), monolithic IC

controller the prime solution becomes a ‘software’ option.

From the user’s perspective, a ‘preprogrammed’ micro-

controller appears little, or no, different than a ‘dedicated’

controller and sequencer IC expressly created for mi-

crostepping applications of the power-driver ICs. Further,

the flexibility of a software-based drive is certainly a basic

benefit (high-volume production of 8-bit microcontrollers

transposes to low-cost circuitry).

As an indicator of the logic signals needed to control

the power ICs, Table 1 lists the required A3955 inputs to

the 3-bit DAC for eighth-step operation (the similar

A3957 uses a 4-bit DAC for sixteenth-step operation).

These I/O signals are serial data from the microcontroller,

then converted to a parallel mode by a 74HC595 as the

‘interface’ between the microcontroller and the two

microstepping power ICs.

The versatility offered by software control allows the

operating modes listed in Table 1. This table itemizes the

various logic inputs that determine direction, stepping