Intel MCS48 User Manual page 57

SINGLE
COMPONENT SYSTEM
2.7 Oscillator
and Clock
The
8021 contains
its
own
onboard
oscilla-
tor
and
clock
circuit,
requiring
only
an
external timing control
element. This
con-
trol
element can be
a
crystal,
inductor, or
clock
in.
The
capacitor normally required
in
inductor timing control operation
is
inte-
grated
onto
the
8021
.
All
internal
time
slots
are derived
from the external element,
and
all
outputs are a function
of
the
oscillator
frequency.
PinsXTAL.1 and
XTAL2
are
used
to
input the particular control
element.
An
instruction
cycle consists
of
10
states,
and
each
state
is
a time
slot
of
3
oscillator
periods.
(See Figure
2)
Therefore,
to
obtain
a 10
/usee instruction cycle,
a
3MHz
crystal
should be
used.
An
oscillator
frequency
of
approximately
3MHz may
also
be obtained
by connecting
a
470
/uH
inductor
between
XTAL1 and XTAL2. Note
that
the required
inductance
may
vary
and
should be
adjust-
ed
as necessary.
The
8021
utilizes
dynamic
RAM
and
certain
other
dynamic
logic.
Due
to
the clocking
required with
dynamic
circuits,
the
oscil-
lator
frequency must be
equal
to
or greater
than
600K
Hz, or
improper
operation
may
occur.
2.8
Timer/Event Counter
The
8021 has
internal
timer/event counter
circuits that
can monitor elapsed time
or
count external events
that
occur during
program
execution.
The
circuit
has an
8-bit
binary
up-counter
that
is
presettable
and
readable
with
two
MOV
instructions.
These
instructions transfer the
contents
of
the
accumulator
to
the
counter and
vice-versa.
The
counter content
is
not affected by
Reset,
and
is
initialized
solely
by
the
MOV
T,A
instruction.
The
counter
is
stopped by
a
RESET
or
STOP TCNT
instruction
and
remains stopped
until
started as
a timer
by
a
STRT T
instruction or as
an event counter
by
a
STRT
CNT
instruction.
Once
started,
1,2,3,
4,
5,6,7,
8, 9,
10
11,2.3,
4. 5,
6.7, 8.9.
10
j
^j
: i
I 1 I
I I !
I
1_L_
^\/
DATA
\V
A
*
a
X
ALL PORTS FOR
IN
AND OUTL
INSTRUCTIONS
FIGURE
2.
8021
TIMING
DIAGRAM
2-21