MA8251
3.10 COMMAND INSTRUCTION DEFINITION
Once the functional definition of the MA8251 has been
programmed by the Mode Instruction and the sync characters
are loaded (if in Sync Mode) then the device is ready to be
used for data communications. The Command Instruction
controls the actual operation of the selected format. Functions
such as: Enable Transmit/Receive, Error Reset and Modem
Controls are provided by the Command Instruction.
Once the Mode Instruction has been written into the
MA8251 and Sync characters inserted, if necessary, then all
further “control writes” (CDN=1) will load a Command
Instruction. A Reset Operation (internal or external) will return
the MA8251 to the Mode instruction format.
Note: Internal Reset on Power-up. When power is first
applied, the MA8251 may come up in the Mode, Sync
character or Command format. To guarantee that the device is
in the Command instruction format before the Reset command
is issued, it is safest to execute the worst-case initialization
sequence (sync mode with two sync characters). Loading
three 00Hs consecutively into the device with CDN=1
configures sync operation and writes two dummy 00H sync
characters. An internal reset command (40H) may then be
issued to return the device to the idle state.
3.11 COMMAND INSTRUCTION FORMAT
D7
D6
IR
D5
D4
D3
D2
D1
D0
EH
RTS
ER SBRK RxE DTR TxEN
TRANSMIT ENABLE
1 = ENABLE
0 = DISABLE
DATA TERMINAL READY
HIGH WILL FORCE DTR
OUTPUT TO ZERO
RECEIVE ENABLE
1 = ENABLE
0 = DISABLE
SEND BREAK CHARACTER
1 = FORCES TxD LOW
0 = NORMAL OPERATION
ERROR RESET
1 = RESET ERROR FLAGS
PE, OE, FE
REQUEST TO SEND
HIGH WILL FORCE RTS
OUTPUT TO ZERO
INTERNAL RESET
HIGH RETURNS THE MA8251 TO
MODE INSTRUCTION FORMAT
ENTER HUNT MODE*
HIGH ENABLES SEARCH FOR SYNC
CHARACTERS (HAS NO EFFECT IN ASYNC MODE)
*NOTE: ERROR RESET must be performed whenever RxENABLE and ENTER-HUNT
are programmed.
Figure 12: Command Instruction Format
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ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
