Intel MCS48 User Manual page 45

SINGLE
COMPONENT
SYSTEM
PRESCALER
XTAL^
15
*
32
CLEARED ON START
TIMER
T1
EDGE
DETECTOR
LOAD OR READ
8
BIT TIMER/
EVENT COUNTER
STOPT
NOT CLEARED ON
RESET
ENABLE
JUMP
ON
TIMER FLAG
OVERFLOW
FLAG
^Q
-»-INT
TIMER/EVENT
COUNTER
pending
timer
interrupt
is
reset
by
the
Call
to Jocation
7
or
may
be
removed
by
executing
a
DIS
TCNTI
instruction.
As
an Event Counter
Execution
of
a
START CNT
instruction
con-
nects the T1
input pin to the
counter
input
and
enables
the counter.
Subsequent
high
to
low
transitions
on
T1
will
cause
the counter
to
increment.
The
maximum
rate at
which
the
counter
may
be incremented
is
once
per
three
instruction
cycles (every 7.5/^sec
when
using a
6MHz
crystal)
—
there
is
no
minimum
frequency. T1 input
must
remain
high
for at
least
500ns
(at
6MHz)
after
each
transition.
As
a Timer
Execution
of
a
START T
instruction
connects
an
internal
clock
to
the counter
input
and
enables
the
counter.
The
internal
clock
is
derived by passing the basic
400
KHz
machine
cycle clock
ALE
through a
^
32
prescaler.
The
prescaler
is
reset during the
START
T
instruction.
The
resulting 12.5
KHz
clock
increments
the counter every 80
/xsec
(assuming
6
MHz
XTAL). Various delays
between 80
/usee
and 20 msec
(256
counts)
can be
obtained by
presetting the counter
and
detecting overflow.
Times
longer than 20
msec may
be achieved by accumulating mul-
tiple
overflows
in
a
register
under
software
control.
For
time
resolution less
than
80
//.sec
an
external clock
can be
applied
to
the T1
input
and
the
counter operated
in
the event
counter
mode.
ALE
divided by 3
or
more
can
serve as
this
external
clock.
Very small delays
or "fine
tuning"
of larger
delays
can be
easily
accomplished by
software delay
loops.
2.1.11
Clock
and
Timing
Circuits
Timing generation
for
the
8048
is
completely
self-contained with the exception
of
a
fre-
quency
reference
which can be XTAL,
inductor, or external clock source.
The
Clock and Timing
circuitry
can be
divided
into
the following functional blocks:
Oscillator
The
on-board
oscillator
is
a high gain series
resonant
circuit
with
a frequency range
of
1
to
6MHz. The
X1
external pin
is
the input
to the
amplifier
stage
while
X2
is
the output.
A
crystal
or inductor
connected between
X1
and X2
provides the
feedback and phase
shift
required
for oscillation.
A
5.9904
MHz
crystal
provides
for
easy
derivation
of
all
standard communications
frequencies.
If
an
accurate frequency reference and
maximum
processor
speed
are not required, an
induc-
2-9