MLX90333
Triaxis 3D-Joystick Position Sensor
3901090333
Page 11 of 38
Data Sheet
Rev. Preliminary
May 07
The conditioned analog signals are converted through an ADC (configurable
− 14 or 15 bits) and provided
to a DSP block for further processing. The DSP stage is based on a 16 bit RISC micro-controller whose
primary function is the extraction of the two (2) angular information from the three (3) raw signals (after so-
called front-end compensation steps) through the following operations:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
Z
Z
X
V
k
V
ATAN
α
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
Z
Z
Y
V
k
V
ATAN
β
where kZ is a programmable parameter.
In a joystick based on a “ball & socket” joint as shown on Figure 3 (right), the magnet (axial
magnetization) moves on a hemisphere centered at the pivot point. The flux density is described through
slightly more complex equations but the MLX90333 offers an alternate algorithm to extract both angular
informations:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
=
2
2
)
(
)
(
Y
t
Z
Z
X
V
k
V
k
V
ATAN
α
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
=
2
2
)
(
)
(
X
t
Z
Z
Y
V
k
V
k
V
ATAN
β
where kZ and kt are programmable parameters.
The DSP functionality is governed by the micro-code (firmware
− F/W) of the micro-controller which is
stored into the ROM (mask programmable). In addition to the
″ATAN″ function, the F/W controls the whole
analog chain, the output transfer characteristic, the output protocol, the programming/calibration and also
the self-diagnostic modes.
In the MLX90333, the
″ATAN″ function is computed via a look-up table (i.e. it is not obtained through a
CoRDiC algorithm).
Due to the fact that the
″ATAN″ operation is performed on the ratios ″V
X/VZ″ and ″VY/VZ″, the angular
information are intrinsically self-compensated vs. flux density variations (due to airgap change, thermal or
ageing effects) affecting the magnetic signal. This feature allows therefore an improved thermal accuracy
vs. joystick based on conventional linear Hall sensors.
Once the angular information is computed (over 360 degrees), it is further conditioned (mapped) vs. the
target transfer characteristic and it is provided at the output(s) as:
•
an analog output level through a 12 bit DAC followed by a buffer
•
a digital PWM signal with 12 bit depth (programmable frequency 100 Hz
… 1 kHz)
•
a digital Serial Protocol (SP
− 14 bits computed angular information available)
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