4255 查看數據表（PDF） - Allegro MicroSystems

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4255

8-Bit microcontroller

Allegro MicroSystems 

4255

8-BIT

MICROCONTROLLER

FUNCTIONAL DESCRIPTION (cont’d)

Direction control outputs

Two of the microcontroller outputs are devoted to

direction control. The direction signals follow the se-

quencing to provide the 1/8th-step ‘sinusoidal’ wave-

forms. Direction outputs are parallel, and connected

directly to the power IC inputs (A3955 in the schematic).

Obviously, both direction signals must coincide with the

8-bit serial-to-parallel conversion signals that control the

phase current ratios (D2, D1, and D0).

PFD control outputs

Another pair of microcontroller outputs constitute

parallel, direct control of the recirculation paths during

each PWM current cycle. The percentage of fast-decay

(or four-quadrant) to slow-decay (or two-quadrant)

recirculation determines how effectively the output current

tracks a sinusoidal waveform. Mixed-mode operation was

utilized for those portions of each microstepping cycle in

which the current is decreasing. The application of

mixed-mode decay allows the PWM current to properly

decay during each fixed-off-time interval.

The typical application shown includes a pair of

49.9 kΩ variables and allows setting the ratios of fast- and

slow-decay to match the individual characteristics of the

step motor, step frequency, supply voltage, etc. As the

stepping frequency is increased, waveform anomalies

become more pronounced, and the benefit of mixed mode

(or fast decay) also becomes very noticeable.

A comprehensive discussion of mixed-mode operation

is included in Allegro Technical Paper STP 97-5.

Oscillator input/clock input

This device connection serves as one terminal for a

parallel-cut crystal (oscillator 2 is the other). In the

typical application, two 20 pF capacitors are recommend-

ed, and a series resistor from oscillator out may be re-

quired if AT-cut crystals are utilized.

Also, this terminal is used if an external clock (from a

system computer, etc.) is connected here in lieu of a

crystal or ceramic resonator. If so, the oscillator out

connection is left open. The 20 MHz frequency limit

applies to using any clock source.

Oscillator output/clock output

This is the other device connection used for the

internal crystal oscillator (or ceramic resonator) that

supplies the microcontroller with an oscillator/time base

(≤20 MHz).

Direction input

This input controls the sequencing of the winding

currents. Reversing direction may necessitate deceleration

(perhaps full stop) to avoid problems associated with the

load inertia causing the motor to overshoot the proposed

stop/reverse position.

A more extensive discussion of this microcontoller

and microstepping is included in Allegro Technical Paper

STP 99-11.

CONTROL OF PHASE CURRENTS

The power-driver ICs regulate the winding currents,

and the user establishes the design value via a stable

voltage reference and a current-sensing resistor for each

winding (R8 and R19).

115 Northeast Cutoff, Box 15036

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

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