MA28151
MA28151
Radiation hard Programmable
Communication Interface
Replaces Jone 1999 version, DS3574-4.0
DS3574-5.0 January 2000
The MA28151 is based on the industry standard 8251A
Universal Synchronous Asynchronous Receiver/Transmitter
(USART), modified for data communications with the MAS281
microprocessor.
The MA28151 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.
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 MAS281 (MIL-STD-1750A)
Microprocessor
The MA28151 is based on the industry standard 8251A
USART, modified for use with the MAS281 processor,
incorporating the following features:
1. MA28151 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 MA28151 is not selected, the RDWN and DSN
lines do not affect the internal operation of the device.
9. The MA28151 Status can be read at any time but the status
update will be inhibited during status read.
10. The MA28151 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: MA28151 Block Diagram
1/22
MA28151
1. FUNCTIONAL DESCRIPTION
1.1 GENERAL
The MA28151 is a Universal Synchronous/Asynchronous
Receiver/Transmitter designed for use with the MAS281
microprocessor. Like other l/O devices in a microcomputer
system, its functional configuration is programmed by the
system’s software for maximum flexibility. The MA28151 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 READ/WRITE SELECT (RDWN)
A high on the RDWN input indicates a read of data or
status information from the MA28151. A low on this input
indicates a transfer of data or control words into the MA28151.
The RDWN line is valid only when DSN is low. Figure 2
summarises the MAS28151 read/write operati ons.
1.7 CONTROL/DATA (CDN)
This input, in conjunction with the DSN and RDWN inputs,
informs the MA28151 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
RDWN
1
0
1
0
x
x
DSN
0
0
0
0
1
x
CSN
0
0
0
0
0
1
ACTION
28151 TO CPU
CPU TO 28151
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 MA28151 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 MA28151. 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 DSN and RDWN
lines have no effect on the chip.
1.3 RESET
A high on this input forces the MA28151 into idle mode.
The MA28151 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 MA28151 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.
1.5 DATA STROBE (DSN)
This input indicates that a data transfer is taking place.
During a CPU write operation the MA28151 reads data from
the bus on the rising edge of DSN. During a read operation the
MA28151 can output data while DSN is low. Data is valid on
the rising edge of DSN.
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.
2/22
MA28151
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.
automatically transmitted as fillers. TxEMPTY does not go low
when the SYNC characters are being shifted out.
1.13 CLEAR TO SEND (CTS)
A low on this input enables the MA28151 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.
1.18 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 72KHz in the 16x mode
TxC
equals 7.04KHz in the 64x mode
The falling edge of
TxC
shifts the serial data out of the
MA28151.
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 DSN (with RDWN low) 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 MA28151 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 MA28151 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
1.21 RxRDY (RECEIVER READY)
This output indicates that the MA28151 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,
3/22
MA28151
the Receiver must be enabled and a character must finish
assembly and be transferred to the Data Output Register
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.
C/D
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 4800 Hz in the 16x mode
RxC
equals 19.2 KHz in the 64x mode.
Baud Rate equals 2400 Baud if
RxC
equals 2400Hz 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 MA28151 on the rising edge of
RxC.
Note: In most communications systems, the MA28151 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.
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 MA28151 to single character mode. Both
sync characters are skipped if mode instruction has
programmed the MA28151 to async mode
Figure 3: Typical data block
2. OPERATION DESCRIPTION
2.1 GENERAL
The complete functional definition of the MA28151 is
programmed by the system’s software. A set of control words
must be sent out by the CPU to initialize the MA28151 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 MA28151 is ready to perform its
communication functions. The TxRDY output is raised high to
signal the CPU that the MA28151 is ready to receive a data
character from the CPU. This output (TxRDY) is reset
automatically when the CPU writes a character into the
MA28151. Alternatively, the MA28151 receives serial data
from the MODEM or l/O device. Upon receiving an entire
character, the RxRDY output is raised high to signal the CPU
that the MA28151 has a complete character ready for the CPU
to fetch. RxRDY is reset automatically upon the CPU data read
operation.
The MA28151 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 MA28151 has located the
SYNC character in the Receive mode. If the MA28151 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 MA28151 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.
4/22
MA28151
3. PROGRAMMING THE MA28151
3.1 MODE AND COMMAND INSTRUCTIONS
Prior to starting data transmission or reception, the
MA28151 must be loaded with a set of control words
generated by the CPU. These control signals define the
complete functional definition of the MA28151 and must
immediately follow a Reset operation (internal or external).
The control words are split into two formats:
1. Mode Instruction
2. Command Instruction
3.1.1 Mode Instruction
This instruction defines the general operational
characteristics of the MA28151. It must follow a Reset
operation (internal or external). Once the Mode instruction has
been written into the MA28151 by the CPU, SYNC characters
or Command Instructions may be written.
3.1.2 Command Instruction
This instruction defines a word that is used to control the
actual operation of the MA28151.
Both the Mode and Command Instruction must conform to
a specified sequence for proper device operation. The Mode
instruction must be written immediately following a Reset
operation, prior to using the MA28151 for data
communications.
All control words written into the MA28151 after the Mode
Instruction will load the Command Instruction. Command
Instructions can be written into the MA28151 at any time in the
data block during the operation of the MA28151. To return to
the Mode Instruction format, the master Reset bit in the
Command Instruction word can be set to initiate an internal
Reset operation. This automatically places the MA28151 back
into the Mode Instruction format. Command Instructions must
follow the Mode Instructions or Sync characters.
3.3 TEST MODE
The Mode Instruction can be used to select a scan path
test facility. In this mode a test vector is read in through RxD
and read out in TxD. For further information of test mode
please contact GEC Plessey Semiconductors.
3.4 ASYNCHRONOUS MODE (TRANSMISSION)
Whenever a data character is sent by the CPU the
MA28151 automatically adds a Start bit (low level), followed by
the data bits (least significant bit first,) and the programmed
number of Stop bits to each character. Also, an even or odd
Parity bit is inserted prior to the Stop bit(s), as defined by the
Mode Instruction. The Character is then transmitted as a serial
data stream on the TxD output. The serial data is shifted out on
the falling edge of
TxC
at a rate equal to 1,
1
⁄
16
or
1
⁄
64
times that
of the
TxC,
as defined by the Mode Instruction. BREAK
characters can be continuously sent to the TxD if commanded
to do so.
When no data characters have been loaded into the
MA28151 the TxD output remains high (marking) unless a
Break (continuously low) has been programmed.
3.2 MODE INSTRUCTION DEFINITION
The MA28151 can be used for either Asynchronous or
Synchronous data communications. To understand how the
Mode Instruction defines the functional operation of the
MA28151, the designer can best view the device as two
separate components, one Asynchronous and the other
Synchronous, sharing the same package. The format
definition can be changed only after a master chip Reset. For
explanation purposes the two formats will be isolated.
NOTE: When parity is enabled it is not considered as one
of the data bits for the purpose of programming the word
length. The actual parity bit received on the Rx Data line
cannot be read on the Data Bus. In the case of a programmed
character length of less than 8 bits, the least significant data
bus bits will hold the data; unused bits are ‘don’t care’ when
writing data to the MA28151, and will be zeros when reading
the data from the MA28151.
3.5 ASYNCHRONOUS MODE (RECEIVE)
The RxD line is normally high. A falling edge on this line
triggers the beginning of a START bit. The validity of this
START bit is checked by again strobing this bit at its nominal
center (16x or 64X mode only). If a low is detected again, it is a
valid START bit, and the bit counter will start counting. The bit
counter thus locates the center of the data bits, the parity bit (if
it exists) and the stop bits. If a parity error occurs, the parity
error flag is set. Data and parity bits are sampled on the RxD
pin with the rising edge of
RxC.
If a low level is detected as the
STOP bit, the Framing Error flag will be set. The STOP bit
signals the end of a character. Note that the receiver requires
only one stop bit, regardless of the number of stop bits
programmed. This character is then loaded into the parallel l/O
buffer of the MA28151. The RxRDY pin is raised to signal the
CPU that a character is ready to be fetched.
If a previous character has not been fetched by the CPU,
the present character replaces it in the l/O buffer, and the
OVERRUN Error flag is raised (thus the previous character is
lost). All of the error flags can be reset by an Error Reset
Instruction. The occurrence of any of these errors will not affect
the operation of the MA28151.
5/22
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