HM8350A View Datasheet(PDF) - Unspecified
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HM8350A Datasheet PDF : 3 Pages
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pointing device causes two sets of direc-
tion and speed information to be con-
veyed to the computer. One set corre-
sponds to movement left and right (X
axis), the other to movement back and
forth (Y axis). For radio tuning applica-
tions, only one set of information is
needed, corresponding to clockwise or
counterclockwise dial rotation.
It is possible to perform the function
of the D flip-flop in software running
on a microprocessor. Signals A and B
are fed to two discrete inputs of the
processor and code is written to com-
pare their states. Problems usually
arise, though, when the rotation rate
exceeds the processor’s ability to keep
up. Backward steps and other strange
things can occur when the processor is
unable to read each and every transi-
tion of both signals. For that reason,
mouse makers and embedded-control
system designers have stayed away
from decoding in software and have
relied on hardware to do the job.
For a computer-controlled trans-
ceiver, a mouse chip seems an excel-
lent choice. It does the decoding and
delivers serial data commands that
correspond to shaft rotation. Power
consumption is so low that power
may be derived from the host com-
puter. I chose the HM8350A from
Hualon Microelectronics.1
To get quadrature signals, a slotted
wheel is generally used. Optical emit-
ters and sensors are mounted on
opposite sides of the wheel to generate
signals A and B. A single infrared emit-
ter is common, accompanied by a
pair of phototransistors (see Fig 3).
The distance between the phototran-
sistors is chosen to be the product of
an odd integer and one-quarter of the
distance between transparent slots in
the wheel so that light intensity var-
ies in quadrature at the sensors. When
the signals from the phototransistors
are squared (hard-limited) inside the
mouse chip, they resemble signals A
and B as shown in Fig 1.
HM8350A chips, slotted wheels and
photo-electronic devices may be sal-
vaged from discarded mice that failed
because of mechanical reasons. One
alternate way of making a slotted
wheel is described below.
not be too hard to create a pattern of
radial segments that were alternately
light and dark. Armed with a laser
printer and some laser-compatible
transparency film, I made my own
photo-interrupter disk, shown in Fig 4.
I made the disk about 1 inch in diam-
eter and with pairs of light and dark
segments. I punched a hole through the
center of the disk so that I could attach
it to the end of the shaft using a regular
fastener. The end of the shaft was
drilled and tapped for #4-40 hardware
to accept the disk (see Fig 5). A little
touch-up with a fine permanent marker
made the disk usable.
The shaft bearing is a critical part of
tuning-knob design because any play,
either back-and-forth or side-to-side, is
quite undesirable. The shaft-to-bearing
fit must be quite close and it was not
easy finding the right shaft material.
Fig 3—The general arrangement for
generating signals A and B.
Fortunately, rod stock is available in
very fine gradations of diameter. A gen-
erous blob of silicone grease helped
eliminate any remaining play.
The shaft bearing I used has a
3/8-inch threaded bushing for panel
mounting. A lever is used to provide
variable friction on the shaft. The
housing was made for this accessory
from a track-lighting enclosure (see
Fig 6). A heavy base was necessary to
prevent the assembly from traveling
across the desk when in use. The base
is made from a roughly circular slug
that was cut from thick steel plate at
Fig 4—An opto-interrupter disk made from
laser-printer transparency film.
The Slotted Wheel
and Shaft Bearing
I wanted to use a 1/4-inch shaft for my
tuning knob and the slotted wheels from
mice were too small. I reasoned that
with modern CAD software, it should
1Notes appear on page 00.
Fig 5—The disk attached to the shaft.
2 Mar/Apr 2002
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