MA8251
Failure to read the received character from the Rx Data
Output Register prior to the assembly of the next Rx Data
character will set overrun condition error and the previous
character will be written over and lost. If the Rx Data is being
read by the CPU when the internal transfer is occurring, the
overrun error will be set and the old character will be Iost.
1.24 BREAK (ASYNC MODE ONLY)
This output will go high whenever the receiver remains low
through two consecutive stop bit sequences including the start
bits, data bits, and parity bits. Break Detect may also be read
as a Status bit. It is reset only upon a master chip Reset or Rx
Data returning to a “one” state.
1.22
RxC
(RECEIVER CLOCK)
The Receiver Clock controls the rate at which the character
is to be received. In Synchronous Mode the Baud Rate (1x) is
equal to the actual frequency of
RxC.
In Asynchronous Mode,
the Baud Rate is a fraction of the actual
RxC
frequency. A
portion of the mode instruction selects this factor: 1,
1
⁄
16
or
1
⁄
64
of
the Receiver Clock.
For example:
Baud Rate equals 300 Baud, if
RxC
equals 300 Hz in the 1 x mode:
RxC
equals 4.8 kHz in the 16x mode
RxC
equals 19.2 kHz in the 64x mode.
Baud Rate equals 2400 Baud if
RxC
equals 2400 Hz in the 1x mode
RxC
equals 38.4 kHz in the 16x mode;
RxC
equals 153.6 kHz in the 64x mode.
Data is sampled into the MA8251 on the rising edge of
RxC.
Note: In most communications systems, the MA8251 will
be handling both the transmission and reception operations of
a single link. Consequently the Receive and Transmit Baud
Rates will be the same. Both
TxC
and
RxC
will require identical
frequencies for this operation and can be tied together and
connected to a single frequency source (Baud Rate
Generator) to simplify the interface.
C/D
1
1
1
1
0
1
0
1
ACTION
MODE INSTRUCTION
SYNC CHARACTER 1 (SYNC ONLY) *
SYNC CHARACTER 2 (SYNC ONLY) *
COMMAND INSTRUCTION
DATA
COMMAND INSTRUCTION
DATA
COMMAND INSTRUCTION
Note: The second sync character is skipped if mode instruction
has programmed the MA8251 to single character mode. Both
sync characters are skipped if mode instruction has
programmed the MA8251 to async mode
Figure 3: Typical data block
2. OPERATION DESCRIPTION
2.1 GENERAL
The complete functional definition of the MA8251 is
programmed by the system’s software. A set of control words
must be sent out by the CPU to initialize the MA8251 to
support the desired communications format. These control
words will program the: Baud Rate, Character Length, Number
of Stop Bits, Synchronous or Asynchronous Operation, Even/
Odd/Off Parity, etc. In the Synchronous Mode, options are also
provided to select either internal or external character
synchronization.
Once programmed, the MA8251 is ready to perform its
communication functions. The TxRDY output is raised high to
signal the CPU that the MA8251 is ready to receive a data
character from the CPU. This output (TxRDY) is reset
automatically when the CPU writes a character into the
MA8251. Alternatively, the MA8251 receives serial data from
the MODEM or l/O device. Upon receiving an entire character,
the RxRDY output is raised high to signal to the CPU that the
MA8251 has a complete character ready for the CPU to fetch.
RxRDY is reset automatically upon the CPU data read
operation.
The MA8251 cannot begin transmission until the TxEnable
(Transmitter Enable) bit is set in the Command instruction and
it has received a Clear To Send (CTS) input. The TxD output
will be held in the marking state upon Reset.
1.23 SYNC/BREAK DETECT (SYNDET/BRKDET)
This pin is used in Synchronous Mode for SYNDET and
may be used as either input or output, programmable through
the Control Word. It is reset to output mode, low upon RESET.
When used as an output (internal Sync mode), the SYNDET
pin will go high to indicate that the MA8251 has located the
SYNC character in the Receive mode. If the MA8251 is
programmed to use double Sync characters (bi-sync), the
SYNDET will go high in the middle of the last bit of the second
Sync character.
SYNDET is automatically reset upon a Status Read
operation.
When used as an input (external SYNC detect mode), a
positive going signal will cause the MA8251 to start
assembling data characters on the rising edge of the next
RxC.
Once in SYNC, the high input signal can be removed. When
External SYNC Detect is programmed, Internal SYNC Detect
is disabled.
4
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