CS4122

http://onsemi.com

5

APPLICATIONS INFORMATION

THEORY OF OPERATION

The CS4122 is for interfacing between a microcontroller

or microprocessor and air−core meters commonly used in

automotive vehicles for speedometers, tachometers and

auxiliary gauges. These meters are built using 2 coils placed

at 90

° orientation to each other. A magnetized disc floats in

the middle of the coils and responds to the magnetic field

generated by each coil. The disc has a shaft attached to it that

protrudes out of the assembly. A pointer indicator is attached

to this shaft and in conjunction with a separate printed scale

displays the vehicle’s speed, engine’s speed or other

information such as fuel quantity or battery voltage.

The disc (and pointer) respond to the vector sum of the

voltages applied to the coils. Ideally, this relationship

follows a sine/cosine equation. Since this is a transcendental

and non−linear function, devices of this type use an

approximation for this relationship. The CS4122 uses a

tangential algorithm as shown in Figure 2 for the major

(360

°) gauge. Only one output varies in any 45° range.

Note: The actual slopes are segmented but are shown here

as straight lines for simplicity.

SIN+

Output

Max(128)

SIN−

Output

COS+

Output

COS−

Output

Max(128)

Max(128)

Max(128)

Min(0)

Min(0)

Min(0)

Min(0)

0

°

45

°

90

°

135

°

180

°

225

°

270

°

315

° 360°

000

001

010

011

100

101

110

111

000

MUX bits (D9−D7)

Degrees of Rotation

Figure 2. Major Gauge Outputs

Quadrant I

q + Tan−1

(VSIN

)) * (VSIN*)

(VCOS

)) * (VCOS*)

For

q + 0.176°to 44.824° :

VSIN + Tanq

0.748

VBB

VCOS + 0.748

VBB

For

q + 45.176°to 89.824° :

VSIN + 0.748

VBB

VCOS + Tan(90° * q)

0.748

VBB

Quadrant II

q + 180° * Tan−1

(VSIN

)) * (VSIN*)

(VCOS

)) * (VCOS*)

For

q + 90.176°to 134.824° :

VSIN + 0.748

VBB

VCOS +*Tan (q * 90°)

0.748

VBB

For

q + 135.176°to 179.824° :

VSIN + Tan(180° * q)

0.748

VBB

VCOS +*0.748

VBB

Quadrant III

q + 180° ) Tan−1

(VSIN

)) * (VSIN*)

(VCOS

)) * (VCOS*)

For

q + 180.176°to 224.824° :

VSIN +*Tan (q * 180°)

0.748

VBB

VCOS +*0.748

VBB

For

q + 225.176°to 269.824° :

VSIN +*0.748

VBB

VCOS +*Tan (270° * q)

0.748

VBB