2. F e a tur e s
• The Intel 8251 is an universal synchronous and
asynchronous communication controller.
• It supports standard asynchronous protocol with:
5 to 8 bit character format
Odd, even or no parity generation and detection
Baud rate of 19.2 KBaud
False start bit detection
Automatically break detect and handling
Break character generator
3. • It has built in baud rate generator.
• It supports standard synchronous
protocol with:
5 to 8 bit character format
Internal and external character
synchronization
Automatic sync insertion
Baud rate of 64 KBaud
4. • It allows full duplex transmission and
reception.
• It provider error detection logic, which
detects parity, overrun and framing errors.
• It has Modem Control Logic, which supports
basic data set control signals.
• It is compatible with an extended range of
Intel microprocessors.
5. • It provides separate clock inputs for
receiver and transmitter sections, thus
providing an option of fixing different
baud rates for the transmitter and
receiver section.
• It is 28 pin IC package.
7. pin de s c r ipt ion
• D0 to D7 (I/O Terminal): This is
bidirectional data bus which
receive control words, transmits
data from the CPU, sends status
words and received data to CPU.
8. • RESET (Input Terminal): A “High” on
this input forces the 8251 into “reset
status.” The device waits for the
writing of “mode instruction”
• CLK (Input Terminal): CLK signal is
used to generate internal device timing.
CLK signal is independent of RXC or
TXC.
9. • WR (Input Terminal): This is the "active low"
input terminal which receives a signal for
writing transmit data and control words from
the CPU into the 8251.
• RD (Input Terminal): This is the "active low"
input terminal which receives a signal for
reading receive data and status words from
the 8251.
10. • C/D (Input Terminal): This is an input terminal
which receives a signal for selecting data or
command words and status words when the 8251 is
accessed by the CPU. If C/D = low, data will be
accessed. If C/D = high, command word or status
word will be accessed.
• CS (Input Terminal): This is the "active low" input
terminal which selects the 8251 at low level when
the CPU accesses.
11. • TXD (Output Terminal): This is an
output terminal for transmitting data
from which serial-converted data is
sent out.
• TXRDY (Output Terminal): This is an
output terminal which indicates that
the 8251 is ready to accept a
transmitted data character.
12. • TXEMPTY (Output Terminal): This is an
output terminal which indicates that the 8251
has transmitted all the characters and had no
data character.
• TXC (Input Terminal): This is a clock input
signal which determines the transfer speed of
transmitted data. In "synchronous mode," the
baud rate will be the same as the frequency
of TXC.
13. • RXD (Input Terminal): This is a terminal
which receives serial data.
• RXRDY (Output Terminal): This is a
terminal which indicates that the 8251
contains a character that is ready to READ.
• RXC (Input Terminal): This is a clock input
signal which determines the transfer speed of
received data. In "synchronous mode," the
baud rate is the same as the frequency of
RXC.
14. • SYNDET (Sync Detect)/BRKDET (Break
Detect) (Input or Output Terminal): This
pin is used in synchronous mode for
detection of synchronous characters.
In asynchronous mode this pin goes high
to indicate a break in the data stream.
15. • DSR (Data Set Ready) (Input Terminal):
This is an input port for MODEM
interface. The input status of the terminal
can be recognized by the CPU reading
status words.
• DTR (Data Terminal Ready) (Output
Terminal): This is an output port for
MODEM interface. It is possible to set
the status of DTR by a command.
16. • CTS (Clear To Send) (Input Terminal):
This is an input terminal for MODEM
interface which is used for controlling a
transmit circuit.
• RTS (Request To Send) (Output
Terminal): This is an output port for
MODEM interface. It is possible to set
the status RTS by a command.
17. Bloc k di a g r am
It includes:-
Data bus buffer
Read/Write control
logic
Modem Control
Transmit Buffer
Transmit Control
Receiver Buffer
Receiver Control
