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Philips PM2519 - Page 34

Philips PM2519
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2-18
2.4.3.
12¢
interface
The
|2C
bus
differs
considerably
from
the
conventional
bus
structures
in
that
data-transfer
is
effected
in
a
bit-
serial,
rather
than
in
byte-paratiel
format.
tn
a
conventional
microcomputer
such
as
the
8048
for
instance,
12
address,
8
data
and
4
control
lines
are
necessary
for
parallel
data
transfer.
The
12c
8440
microcomputer,
on
the
other
hand
requires
only
2
lines
to
transfer
serially
the
same
amount
of
data.
Chips
used
for
ADC,
RAM
and
LCD
drivers
are
12¢
compatible
and
use
also
the
same
two
lines.
These
two
lines
are
respectively
the
SDA
(serial
data
line)
and
SCL
(serial
clock
ine)
the
function
of
which
is
to
synchronise
data-transfer
between
the
appropriate
12¢
devices.
Almost
any
number
of
devices
can
be
connected
to
the
12¢
bus.
Each
device
is
allocated
its
own
specific
7-bit
address,
which
enables
any
two
of
these
devices
to
communicate
with
each
other
upon
receipt
of
a
message
prefixed
with
the
appropriate
7-bit
address.
This
specific
7-bit
address
usually
comprises
a
fixed
address
part
(4
bits),
a
user
definable
part
(3
bits).
The
latter
being
assignable
by
tying
‘Define
Address”
pins
to
high
or
low
levels.
Address
recognition
is
effected
in
the
12¢
interface
hardware
of
each
device,
and
this
eliminates
the
need
for
decoding
logic.
The
use
of
an
automatic-invoked
arbitration
procedure,
which
prevents
two
or
more
devices
from
transmitting
simultaneously,
makes
12¢
technology
eminently
suitable
for
a
multiprocessor
system.
“For
an
appraisal
of
the
12¢
data-transfer
process,
consider
the
operation
of
the
PCD
8571,
1k-bit
CMOS
RAM,
in
conjunction
with
the
8440
microcomputer.
When
connected
to
the
12C
bus
this
8-pin
RAM
serves
as
a
ts
slave
transceiver
to
the
master
processor.
To
transmit
data
to
the
RAM,
the
processor
first
transmits
the
specific
7-bit
address,
plus
a
Write
Action
Identifier
bit.
The
master
processor
then
defines
the
specific
location
it
wants
to
address,
and
starts
to
transmit
its
data.
Correct
synchronisation
between
the
devices
is
effected
by
the
SCL
(serial
clock
line).
For
further
information
about
I2C
see:
Philips
data
handbook;
Integrated
circuits
for
digital
systems
in
radio,
audio
and
video
equipment.
f

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