there is a start bit (logic 0) signaling
the beginning of a word transfer and
a stop bit (logic 1) that determines
the end of the word. A parity bit is
sometimes added between the end
of the word and the stop bit for error
This word format is implemented
by a circuit called a universal
asynchronous receiver transmitter, or
UART. It may be a single IC or
integrated on another chip such as a
microcontroller. It handles all of the
parallel-to-serial and serial-to-parallel
data conversions, as well as adding
and removing the start, stop, and
parity bits. It also sets the speed.
As for the speed or data rate (R),
it is simply the reciprocal of the time
for one bit (t):
R = 1/t
As an example, for a bit time of
104.167 microseconds, the data rate
is 9600 bits per second (bps or b/s).
Knowing the data rate, you can
find the bit time:
t = 1/R
If the data rate is 1 Mbps, the bit
time is one microsecond.
The RS-232 interface defines
multiple data rates depending on the
application. Some common standard
rates are 9600 bps, 19. 2 kbps, and
115.2 kbps. The upper limit is really
determined by the cable length. Long
cables have lots of capacitance, so
rounding off the leading edges of the
binary pulses minimizes the data
speed. Cable lengths are generally
less than 50 feet.
The RS-232 standard also
specifies a connector. The early
versions used a 25-pin connector,
but today a nine-pin connector is
the most common (refer to Figure 2).
Early applications used several of the
control lines to implement
communication protocols, but today
mostly only the transmit (TD) and
receive (RD) lines are used.
I2C, or the Inter-Interconnect bus
is an interface developed by Philips
(now NXP Semiconductor) to
connect one complex IC to another
on a printed circuit board (PCB). The
transmission medium is just the
copper pattern on the PCB. It can
also use a short cable to connect two
PCBs or other devices.
The physical arrangement
consists of a master control IC like a
microcontroller and one or more
slave devices that receive the data or
send some of its own back to the
master (Figure 3). Note that there is a
data line (SDA), a clock line (SCL),
and a ground wire forming a bus. Up
to about 30 slave devices can be
accommodated if the bus
connections are not too long. Bus
length is usually less than one foot on
a PCB or no more than several feet
for a short cable. Bus length limits
Data is sent by way of a frame or
packet that contains a start bit, a
seven- or 10-bit slave address, a
read/write bit followed by the data
bytes to be sent with each followed
by an acknowledge bit (Ack), and
finally a stop bit. The basic data rate
is 100 kbps, but higher speeds such
as 400 kbps, 1 Mbps, and 3. 4 Mbps
can be used if the bus capacitance is
not too great.
There are several variations of
the I2C bus. A common one is the
system management bus (SMB)
developed by Intel. It is used to
communicate between chips on PC
motherboards. A variation of the
SMB is the power management bus
(PMB) that is used for controlling and
monitoring power supplies and ICs
like regulators or DC-DC converters.
Another version is the two-wire
interface (TWI) used by Atmel.
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January 2015 63
THE LATEST IN NETWORKING AND WIRELESS TECHNOLOGIES
; FIGURE 2. The most commonly used
connector for the RS-232 interface uses
nine pins and is called a DB9 or DE9.
Usually, only the RD and TD data lines
are utilized, but other control lines are
adopted for some protocols.
; FIGURE 3. The popular I2C interface uses a master and one or more slave
devices. External pull-up resistors are used on the MOSFET drivers inside
each chip. The typical application is intercommunications between ICs.
1 DCD Data carrier detect
2 RD Receive data
3 TD Transmit data
4 DTR Data terminal ready
5 GND Signal ground
6 DSR Data set ready
7 RTS Request to send
8 CTS Clear to send
9 RI Ring indicator