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OPERATIONAL
DESCRIPTION
MPD8251 PROGRAMMING
MODE INSTRUCTION
COMMAND INSTRUCTION
North Star HSIO-4
A set o f control words must be sent to the pPD8251 to define the desired mode and
communications format. The control words w ill specify the BAUD RATE FACTOR
( lx , 16x, 64x), CHARACTER LENGTH (5 to 8), NUMBER OF STOP BITS
(1, 1-1/2, 2). ASYNCHRONOUS or SYNCHRONOUS MODE, SYNDET (IN or OUT),
PARITY, etc.
A fter receiving the control words, thepPD8251 is ready to communicate. TxRDY is
raised to signal the processor that the USART is ready to receive a character fo r trans
mission. When the processor writes a character to the USART, T xR D Y is auto
matically reset.
Concurrently, thepPD8251 may receive serial data; and after receiving an entire
character, the RxRDY output is raised to indicate a completed character is ready for
the processor. The processor fetch w ill automatically reset RxRDY.
Note: ThepPD8251 may provide faulty RxRDY for the first read after power-on or
for the first read after receive is re-enabled by a command instruction (RxE).
A dummy read is recommended to clear faulty RxRDY. But this is not the
case for the first read after hardware or software reset after the device
operation has once been established.
The pPD8251 cannot transmit until the TxEN (Transmitter Enable) bit has
been set by a Command Instruction and u ntil the CTS (Clear to Send) input is a
"ze ro". TxD is held in the "m arking" state after Reset awaiting new Command
Words.
The USART must be loaded w ith a group of two to four control words provided by
the processor before data reception and transmission can begin. A Reset (internal or
external) must immediately proceed the control words which are used to program the
complete operational description of the communications interface. If an external
RESET is not available, three successive 00 Hex or tw o successive 80 Hex command
instructions (C/D = 1) followed by a software reset command instruction (40 Hex)
can be used to initialize the 8251.
There are tw o control word formats:
1. Mode Instruction
2. Command Instruction
This control word specifies the general characteristics of the interface regarding the
SYNCHRONOUS or ASYNCHRONOUS MODE, BAUD RATE FACTOR, CHARACTER
LENGTH, PARITY, and NUMBER OF STOP BITS. Once the Mode Instruction
has been received, SYNC characters or Command Instructions may be inserted depend
ing on the Mode Instruction content.
This control word w ill be interpreted as a SYNC character definition if immediately
preceded by a Mode Instruction which specified a Synchronous format. A fte r the
SYNC character(s) are specified or after an Asynchronous Mode Instruction, all sub
sequent control words w ill be interpreted as an update to the Command Instruction.
Command Instruction updates may occur at any time during the data block. To
modify the Mode Instruction, a b it may be set in the Command Instruction which
causes an internal Reset which allows a new Mode Instruction to be accepted.
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User/Technical Manual
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