THIS DOCUMENT IS FOR MAINTENANCE
PURPOSES ONLY AND IS NOT
RECOMMENDED FOR NEW DESIGNS
APRIL 1995
MA8251
DS3810-2.4
MA8251
RADIATION HARD PROGRAMMABLE
COMMUNICATION INTERFACE
The MA8251 is based on the industry standard 8251A
Universal Synchronous Asynchronous Receiver/Transmitter
(USART).
The MA8251 is used as a peripheral device and is
programmed by the CPU to operate using virtually any serial
data transmission technique presently in use (including IBM
“bi-sync”). The USART accepts data characters from the CPU
in parallel format and then converts them into a continuous
serial data stream for transmission.
Simultaneously, it can receive serial data streams and
convert them into parallel data characters for the CPU. The
USART signals the CPU whenever it receives a character for
transmission or whenever it receives a character for the CPU.
The CPU can read the complete status of the USART at any
time, including data transmission errors and control signals
such as SYNDET and TxEMPTY.
RD
WR
FEATURES
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Radiation Hard to 1MRad(Si)
s
Latch Up Free, High SEU Immunity
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Silicon-on-Sapphire Technology
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Synchronous 5 - 8 Bit Characters; Internal or External
Character Synchronisation; Automatic Sync Insertion
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Asynchronous 5 - 8 Bit Characters; Clock Rate - 1, 16 or
64 Times Baud Rate; Break Character Generation, 1
1
⁄
2
or
2 Stop Bits
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All Inputs and Outputs are TTL Compatible
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Compatible with the MA31750 (MIL-STD-1750A & Draft
MIL-STD-1750B Option 2) Microprocessor
The MA8251 is based on the industry standard 8251A
USART, incorporating the following features:
1. MA8251 has double-buffered data paths with separate l/O
registers for control status, data in and data out, which
considerably simplifies control programming and minimizes
CPU overhead.
2. In synchronous operations, the Receiver detects and
handles “break” automatically, relieving the CPU of this task.
3. A refined Rx initialisation prevents the Receiver from
starting when in the “break” state, preventing unwanted
interrupts from the disconnected USART.
4. At the conclusion of a transmission, the TxD line will
always return to the marking state unless SBRK is
programmed.
5. Tx Enable logic enhancement prevents a Tx Disable
command from prematurely halting transmission of the
previously written data before completion. The logic also
prevents the transmitter from turning off in the middle of a
word.
6. When external Sync Detect is programmed, Internal Sync
Detect is disabled and an External Sync Detect status is
provided via a flip-flop, which clears itself upon a status read.
7. The possibility of a false sync detect is minimized in two
ways: by ensuring that if double character sync is
programmed, the characters will be continuously detected
and by clearing the Rx register to all 1’s whenever Enter-Hunt
command is issued in Sync mode.
8. When the MA8251 is not selected, the RDN and WRN
lines do not affect the internal operation of the device.
9. The MA8251 Status can be read at any time but the status
update will be inhibited during status read.
10. The MA8251 is free from extraneous glitches, providing
higher speed and better operating margins.
11. Synchronous Baud rate is from DC to 64K.
12. Asynchronous Baud rate is from DC to 19.2K.
Figure 1: MA8251 Block Diagram
1
MA8251
1. FUNCTIONAL DESCRIPTION
1.1 GENERAL
The MA8251 is a Universal Synchronous/Asynchronous
Receiver/Transmitter designed for use with the MA31750
microprocessor. Like other l/O devices in a microcomputer
system, its functional configuration is programmed by the
system’s software for maximum flexibility. The MA8251 can
support most serial data techniques in use, including IBM bi-
sync.
In a communication environment, an interface device must
convert parallel format system data into serial format for
transmission, and convert incoming serial data into parallel
system data for reception. The interface device must also
delete or insert bits or characters that are functionally unique to
the communication technique. In essence, the interface should
appear transparent to the CPU for the simple input or output of
byte-oriented system data.
1.6 WRITE STROBE (WRN)
A low on this signal line indicates that the CPU is writing
data or control information to the MA8251. The MA8251 clocks
data into its data input buffers on a rising edge of WRN.
1.7 CONTROL/DATA (CDN)
This input, in conjunction with the WRN and RDN inputs,
informs the MA8251 that the word on the Data Bus is either a
data character, control word or status information.
1 = CONTROL/STATUS; 0= DATA
CDN
0
0
1
1
x
x
RDN
0
1
0
1
1
x
WRN
1
0
1
0
1
x
CSN
0
0
0
0
0
1
ACTION
MA8251 to CPU
CPU to MA8251
Status to CPU
CPU to Control
Bus Tristate
Bus Tristate
1.2 DATA BUS BUFFER
This 3-state, bidirectional, 8-bit buffer is used to interface
the MA8251 to the system data bus. Data is transmitted or
received by the buffer upon execution of OUTput or INput
instructions from the CPU.
Control word, Command words and Status information are
also transferred through the Data Bus Buffer. The Command
Status, Data-in and Data-out registers are separate 8-bit
registers, communicating with the system bus through the
Data Bus Buffer.
This functional block accepts inputs from the system
control bus and generates control signals for overall device
operation. It contains the Control Word Register and
Command Word Register, which store the various control
formats for the device’s functional definition.
Figure 2: Read/Write Control
1.8 CHIP SELECT (CSN)
A low on this input selects the MA8251. No reading or
writing will occur unless the device is selected. When CSN is
high, the Data Bus is in the float state and the RDN and WRN
lines have no effect on the chip.
1.3 RESET
A high on this input forces the MA8251 into idle mode. The
MA8251 will remain at idle until its functional definition is
programmed with a new set of control words. Minimum RESET
pulse width is 6 tcy (clock must be running).
The device can also be put into the idle state by a
command reset operation .
1.9 MODEM CONTROL
The MA8251 has a set of control inputs and outputs which
can be used to simplify the interface to almost any modem.
The modem control signals are general purpose in nature and
can be used for functions other than modem control, if
necessary.
1.4 CLOCK (CLK)
The CLK input is used to generate internal device timing
and is normally connected to the clock generator (OSC) of the
system.
Please note: None of the external inputs or outputs are
referenced to CLK but the frequency of CLK must be greater
than 30 times the Receiver or Transmitter data bit rates.
1.10 DATA SET READY (DSR)
The
DSR
input signal is a general-purpose, 1-bit inverting
input port. Its condition can be tested by the CPU using a
Status Read operation. The
DSR
input is normally used to test
modem conditions such as Data Set Ready.
1.11 DATA TERMINAL READY (DTR)
The
DTR
output signal is a general purpose, 1-bit inverting
output port. It can be set low by programming the appropriate
bit in the Command instruction word. The
DTR
output signal is
normally used for modem control such as Data Terminal
Ready.
1.5 READ STROBE (RDN)
A low on this signal line indicates that the CPU is reading
data or status information from the MA8251. The MA8251
drives output data onto its data bus whilst this signal remains
low.
2
MA8251
1.12 REQUEST TO SEND (RTS)
The
RTS
output signal is a general purpose, 1-bit inverting
output port. It can be set low by programming the appropriate
bit in the Command instruction word. The
RTS
output signal is
normally used for modem control such as Request To Send.
1.18 TRANSMITTER CLOCK (TxC)
The Transmitter Clock controls the rate at which the
character is to be transmitted. In the Synchronous
transmission mode, the Baud Rate (1x) is equal to the
TxC
frequency. In Asynchronous transmission mode, the baud rate
is a fraction of the actual
TxC
frequency. A portion of the mode
instruction selects this factor; it can be 1,1/16 or 1/64 the
TxC.
For Example:
If Baud Rate equals 110 Baud
TxC
equals 110Hz in the 1x mode
TxC
equals 1.76kHz in the 16x mode
TxC
equals 7.04kHz in the 64x mode
The falling edge of
TxC
shifts the serial data out of the
MA8251.
1.13 CLEAR TO SEND (CTS)
A low on this input enables the MA8251 to transmit serial
data if the Tx Enable bit in the Command byte is set to a high.
If either a Tx Enable off or
CTS
off condition occurs while the
Tx is in operation, the Tx will transmit all the data in the
USART, written prior to Tx disable command, before shutting
down.
1.14 TRANSMITTER BUFFER
The Transmitter Buffer accepts parallel data from the Data
Bus Buffer, converts it to a serial bit stream, inserts the
appropriate characters or bits (based on the communication
technique) and outputs a composite serial stream of data on
the TxD output pin on the falling edge of
TxC.
The transmitter
will begin transmission upon being enabled if
CTS
= 0. The
TxD line will be held in the marking state immediately upon a
master Reset, or when Tx Enable or
CTS
= 1, or the
transmitter is empty.
1.19 RECEIVER BUFFER
The Receiver accepts serial data, converts the data to
parallel format, checks for bits or characters that are unique to
the communications techniques and sends an assembled
character to the CPU. Serial data is input to the RxD pin and is
clocked in on the rising edge of RxC.
1.15 TRANSMITTER CONTROL
The Transmitter Control manages all activities associated
with the transmission of serial data. It accepts and issues
signals both externally and internally to accomplish this
function.
1.16 TRANSMITTER READY (TxRDY)
This output signals the CPU that the transmitter is ready to
accept a data character. The TxRDY output pin can be used as
an interrupt to the system since it is masked by TxEnable; or,
for Polled operation, the CPU can check TxRDY using a Status
Read operation. TxRDY is automatically reset by the falling
edge of WRN when a data character is loaded from the CPU.
Note that when using the polled operation, the TxRDY
status bit is not masked by TxEnable, but will only indicate the
Empty/Full Status of the Tx Data input Register.
1.20 RECEIVER CONTROL
This functional block manages all receiver-related activities
which consist of the following features:
The RxD initialisation circuit prevents the MA8251 from
mistaking an unused input line for an active low data line in the
break condition. Before starting to receive serial characters on
the RxD line, a valid 1 must first be detected after a chip master
Reset. Once this has been determined, a search for a valid low
(start bit) is enabled. This feature is only active in the
asynchronous mode and is only done once for each master
Reset.
The False Start bit detection circuit prevents false starts as
the result of a transient noise spike by first detecting the falling
edge and then strobing the nominal center of the Start bit (RxD
= low).
Parity error detection sets the corresponding status bit.
The Framing Error status bit is set if the Stop bit is absent
at the end of the data byte (asynchronous mode).
1.17 TRANSMITTER EMPTY (TxE)
When the MA8251 has no characters to send, the
TxEMPTY output will go high. It resets upon receiving a
character from CPU if the transmitter is enabled. TxEMPTY
remains high when the transmitter is disabled. TxEMPTY can
be used to indicate the end of transmission mode, so that the
CPU can turn the line around in the half-duplex operational
mode.
In the Synchronous mode, a high on the TxEMPTY output
indicates that a character has not been loaded and the SYNC
character or characters are about to be or are being
automatically transmitted as fillers. TxEMPTY does not go low
when the SYNC characters are being shifted out.
1.21 RxRDY (RECEIVER READY)
This output indicates that the MA8251 contains a character
that is ready to be input to the CPU. RxRDY can be connected
to the interrupt structure of the CPU or, for polled operation,
the CPU can check the condition of RxRDY using a Status
Read operation. RxEnable, when off holds RxRDY in the
Reset Condition. For Asynchronous mode, to set RxRDY, the
Receiver must be enabled to sense a Start Bit and a complete
character must be assembled and transferred to the Data
Output Register. For Synchronous mode, to set RxRDY, the
Receiver must be enabled and a character must finish
assembly and be transferred to the Data Output Register.
3
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.
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