Hardware Reference
In-Depth Information
The shafts are connected to encoders that have slits through which light can pass.
As the mouse moves, the shafts rotate, and light pulses strike the detectors when-
ever a slit comes between an LED and its detector. The number of pulses detected
is proportional to the amount of motion.
Although mice can be set up in various ways, a common arrangement is to
have the mouse send a sequence of 3 bytes to the computer every time the mouse
moves a certain minimum distance (e.g., 0.01 inch), sometimes called a
mickey
.
Usually, these characters come in on a serial line, one bit at time. The first byte
contains a signed integer telling how many units the mouse has moved in the
x
-di-
rection since the last time. The second byte gives the same information for
y
motion. The third byte contains the current state of the mouse buttons. Sometimes
2 bytes are used for each coordinate.
Low-level software in the computer accepts this information as it comes in and
converts the relative movements sent by the mouse to an absolute position. It then
displays an arrow on the screen at the position corresponding to where the mouse
is. When the arrow points at the proper item, the user clicks a mouse button, and
the computer can then figure out which item has been selected from its knowledge
of where the arrow is on the screen.
Video games typically have heavy user I/O demands, and in the video console
market specialized input devices have been developed. In this section we look at
two recent developments in video game controllers, the Nintendo Wiimote and the
Microsoft Kinect.
Wiimote Controller
First released in 2006 with the Nintendo Wii game console, the Wiimote con-
troller contains traditional gamepad buttons plus a dual motion-sensing capability.
All interactions with the Wiimote are sent in real time to the game console using an
internal Bluetooth radio. The motion sensors in the Wiimote allow it to sense its
own movement in three dimensions, plus when pointed at the television it provides
a fine-grained pointing capability.
Figure 2-35 illustrates how the Wiimote implements this motion-sensing func-
tion. Tracking of the Wiimote's movement in three dimensions is accomplished
with an internal 3-axis accelerometer. This device contains three small masses,
each of which can move in the
x
,
y
, and
z
axis (with respect to the accelerometer
chip). These masses move in proportion to the degree of acceleration in their par-
ticular axis, which changes the capacitance of the mass with respect to a fixed
metal wall. By measuring these three changing capacitances, it becomes possible
to sense acceleration in three dimensions. Using this technology and some classic
calculus, the Wii console can track the Wiimote's movement in space. As you
