Intel MCS48 User Manual page 305

8253/8253-5
8253
READ/WRITE
PROCEDURE
Write Operations
The
systems software
must program each
counter
of
the
8253 with the
mode
and
quantity desired.
The
program-
mer must
write out to
the 8253
a
MODE
control
word and
the
programmed number
of
count
register
bytes
(1
or
2)
prior to
actually
using the selected counter.
The
actual
order of the
programming
is
quite
flexible.
Writing out of
the
MODE
control
word
can be
in
any
sequence
of
counter
selection,
e.g.,
counter #0 does
not
have
to
be
first
or
counter #2
last.
Each
counter's
MODE
control
word
register
has a
separate address so
that
its
loading
is
completely sequence independent. (SCO,
SC1)
The loading
of the
Count
Register with the actual
count
value,
however,
must be done
in
exactly the
sequence
programmed
in
the
MODE
control
word
(RLO, RL1).
This
loading of
the counter's
count
register
is still
sequence
independent
like
the
MODE
control
word
loading, but
when
a selected count
register
is
to
be loaded
it
must be
loaded with the
number
of
bytes
programmed
in
the
MODE
control
word
(RLO, RL1).
The one
or
two
bytes
to
be loaded
in
the
count
register
do not have
to
follow the
associated
MODE
control word.
They can be
programmed
at
any
time following the
MODE
control
word
loading as
long as the correct
number
of bytes
is
loaded
in
order.
All
counters are
down
counters.
Thus, the value loaded
into the
count
register
will
actually
be decremented.
Loading
all
zeroes into a
count
register
wiH
result
in
the
maximum
count
(2
16
for
Binary
or
10
4
for
BCD).
In
MODE
the
new
count
will
not restart
until
the load has
been
completed.
It
will
accept
one
of
two
bytes
depending on
how
the
MODE
control
words
(RLO, RL1) are
program-
med.
Then
proceed
with the
restart
operation.
MODE
Control
Word
Counter n
LSB
Count
Register
byte
Counter n
MSB
Count
Register byte
Counter n
Note:
Format shown
is
a
simple
example
of loading the
8253
and
does not imply
that
it is
the only format that
can be
used.
Figure
6.
Programming Format
No.
1
No. 2
No. 3
No.
4
No. 5
No. 6
No. 7
No. 8
No.
9
A1
A0
MODE
Control
Word
Counter
1
MODE
Control
Word
Counter
1
1
MODE
Control
Word
Counter 2
1
Count
Register
Byte
Counter
1
Count
Register Byte
MSB
Counter
1
Count
Register Byte
LSB
_
_
Counter 2
1
.,__
Count
Register
Byte
MSB
_
a
.
y
Counter
2.
1
Count
Register Byte
Counter
..„„
Count
Register
Byte
MSB
_
Counter
Note:
The
exclusive addresses of each counter's
count
register
make
the
task of
programming the
8253
a
very simple matter,
and
maximum
effective use of the device
will
result
if
this feature
is
fully utilized.
Figure
7.
Alternate
Programming Formats
8-15
00745A