MA8251
3. PROGRAMMING THE MA8251
3.3 TEST MODE
3.1 MODE AND COMMAND INSTRUCTIONS
Prior to starting data transmission or reception, the
MA8251 must be loaded with a set of control words generated
by the CPU. These control signals define the complete
functional definition of the MA8251 and must immediately
follow a Reset operation (internal or external).
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 more information on test mode please
contact GEC Plessey Semiconductors.
The control words are split into two formats:
1. Mode Instruction
2. Command Instruction
3.4 ASYNCHRONOUS MODE (TRANSMISSION)
Whenever a data character is sent by the CPU the MA8251
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.
3.1.1 Mode Instruction
This instruction defines the general operational
characteristics of the MA8251. It must follow a Reset operation
(internal or external). Once the Mode instruction has been
written into the MA8251 by the CPU, SYNC characters or
Command Instructions may be written.
3.1.2 Command Instruction
When no data characters have been loaded into the
MA8251 the TxD output remains high (marking) unless a
Break (continuously low) has been programmed.
This instruction defines a word that is used to control the
actual operation of the MA8251.
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 MA8251 for data communications.
All control words written into the MA8251 after the Mode
Instruction will load the Command Instruction. Command
Instructions can be written into the MA8251 at any time in the
data block during the operation of the MA8251. 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 MA8251 back into the
Mode Instruction format. Command Instructions must follow
the Mode Instructions or Sync characters.
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 MA8251. The RxRDY pin is raised to signal the
CPU that a character is ready to be fetched.
3.2 MODE INSTRUCTION DEFINITION
The MA8251 can be used for either Asynchronous or
Synchronous data communications. To understand how the
Mode Instruction defines the functional operation of the
MA8251, 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 MA8251, and will be zeros when reading the
data from the MA8251.
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 MA8251.
5
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
