Dynamic, Self-Calibrating, Threshold-Detecting, Differential

Speed and Direction Hall-Effect Gear Tooth Sensor IC

ATS657

7

Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Functional Description

Data Protocol Description

When a target passes in front of the branded face of the pack-

age, each tooth of the target generates a pulse at the output of the

IC. Each pulse provides target speed and direction data: speed

is provided by the pulse rate, while direction of target rotation is

provided by the pulse width.

The ATS657 can sense target movement in both the forward and

reverse directions. The maximum allowable target rotational

speed is limited by the width of the output pulse and the shortest

low-state duration the system controller can resolve.

Forward Rotation

(see panel A in figure 1) When the target is

rotating such that a tooth near the package passes from pin 4 to

pin 1, this is referred to as forward rotation. Forward rotation is

Reverse Rotation

(see panel B in figure 1) When the target is

rotating such that a tooth passes from pin 1 to pin 4, it is referred

to as reverse rotation. Reverse rotation is indicated on the output

generated by forward rotation.

Non-Direction Output

In situations where the IC is not able to

discern direction of target rotation, as occurs during initial cali-

Timing

As shown in figure 2, the pulse appears at the output

slightly before the sensed magnetic edge traverses the branded

face. For targets in forward rotation, this shift, Δfwd, results in

the pulse corresponding to the valley with the sensed mechanical

edge, and for targets in reverse rotation, the shift, Δrev, results in

the pulse corresponding to the tooth with the sensed edge. The

sensed mechanical edge that stimulates output pulses is kept the

same for both forward and reverse rotation by using only channel

1 for switching.

The overall range between the forward and reverse pulse occur-

rences is determined by the 1.5 mm spacing between the Hall

elements of the corresponding differential channel. In either

direction, the pulses appear close to the sensed mechanical edge.

The size of the target features, however, can slightly bias the

occurrence of the pulses.

(A) Forward Rotation

(B) Reverse Rotation

Rotating Target

Branded Face

of Package

Pin 1

Pin 4

Pin 1

Pin 4

Rotating Target

Branded Face

of Package

Figure 1. Target rotation

Figure 2. Output pulse timing

∆rev

Reverse Rotation

Forward Rotation

Output Pulse

(Forward Rotation)

Output Pulse

(Reverse Rotation)

Tooth

Valley

∆fwd

t

t