CS4122_05 Datasheet, PDF (5/9 Pages) ON Semiconductor – Triple Air−Core Gauge Driver with Serial Input Bus

CS4122
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.
0°
Max(128)
SIN+
Output
Min(0)
Max(128)
SIN−
Output
Min(0)
Degrees of Rotation
45° 90° 135° 180° 225° 270° 315° 360°
Max(128)
COS+
Output
Min(0)
Max(128)
COS−
Output
Min(0)
000 001 010 011 100 101 110 111 000
MUX bits (D9−D7)
Quadrant I
ƪ ƫ q + Tan−1
(VSIN₎)
(VCOS₎)
*
*
(VSIN_*)
(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
ƪ ƫ q + 180° * Tan−1
(VSIN₎)
(VCOS₎)
*
*
(VSIN_*)
(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
VCOS + *0.748 VBB
VBB
Quadrant III
ƪ ƫ q + 180° ) Tan−1
(VSIN₎)
(VCOS₎)
*
*
(VSIN_*)
(VCOS_*)
For q + 180.176°to 224.824° :
VSIN + *Tan (q * 180°) 0.748
VCOS + *0.748 VBB
VBB
For q + 225.176°to 269.824° :
VSIN + *0.748 VBB
VCOS + *Tan (270° * q) 0.748
VBB
Figure 2. Major Gauge Outputs
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