Theory of Operation
The
heart
of
the
OpenTracker
circuit
is
a
Freescale
MC908KX8
microcontroller
unit
(MCU).
The
MCU
contains
192
bytes
of
RAM
and
roughly
8
kilobytes
of
Flash
program
memory.
It
runs
at
a
clock
speed
of
7.3728
MHz.
U2
is
a
linear
voltage
regulator
that
provides
regulated
5
VDC
power
to
the
circuit,
and
optionally
to
an
external
device
connected
to
V
ext
.
C1
and
C2
are
the
input
and
output
filter
capacitors,
respectively.
Y1,
C6,
C7,
and
R10
form
the
clock
oscillator
circuit.
The
oscillator
frequency
of
29.4912
MHz
is
divided
by
four
in
the
MCU’s
clock
module
to
produce
the
bus
clock
signal.
Audio
output
originates
at
pin
13
of
the
MCU.
This
pin
is
configured
as
a
timer
channel
output
and
generates
a
pulse
‐
width
modulated
signal
between
0
and
5
volts.
Audio
tones
are
generated
in
software
using
a
sine
wave
lookup
table.
R2
and
R8
limit
the
audio
output
level,
and
C2
couples
the
AC
component
of
the
signal
to
the
audio
output
at
X1
pin
1.
Pin
15
of
the
MCU
produces
the
PTT
output
signal.
It
switches
Q3,
pulling
the
audio
output
down
through
R3
for
handheld
PTT,
and
pulling
X1
pin
3
to
ground
for
other
radios.
An
externally
applied
PTT
signal
on
X1
pin
8
will
pull
the
PTT
output
low
through
D2,
and
will
pull
pin
14
of
the
MCU
low
(it
is
normally
held
high
by
an
internal
pull
‐
up
resistor)
through
D3.
The
audio
input
from
the
radio
is
AC
‐
coupled
by
C3
and
biased
by
R9.
R9
sets
the
carrier
detect
threshold.
X1
pin
2
provides
a
DC
‐
coupled
input
for
radios
with
a
squelch
or
carrier
operated
relay
output.
R5,
R1,
and
Q4
form
an
inverter/buffer
circuit
for
the
RS
‐
232
input.
The
RS
‐
232
output
polarity
is
controlled
in
software.
The
output
level
swings
between
0
and
5
volts,
and
may
not
be
compatible
with
all
RS
‐
232
devices.
U3
is
a
temperature
sensor
with
an
output
of
10
mV
per
Kelvin.
R6
limits
its
input
current,
and
its
output
drives
one
of
the
analog
inputs
on
the
MCU.
R7
and
R11
form
a
voltage
divider,
the
output
of
which
is
one
third
of
the
supply
voltage.
This
voltage
drives
another
analog
input
on
the
MCU.
The
LED
is
driven
by
a
high
‐
current
output
line
from
the
MCU,
through
current
limiting
resistor
R4.
Lowering
the
value
of
R4
will
increase
the
brightness
of
the
LED.
OpenTracker Manual - Version 1.05 – 6/25/06
page
18
of
23
