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A fte r the functional definition of the /
Instruction and the SYNC character(s) have been entered, if in SYNC mode, the
USART is ready to receive Command Instructions and begin communication. A
Command Instruction is used to control the specific operation o f the format selected
by the Mode Instruction. Enable Transmit, Enable Receive, Error Reset and Modem
Controls are controlled by the Command Instruction.
A fter the Mode Instruction and the SYNC character(s), as needed, are loaded, all
subsequent "control w rites" (C/D = 1) w ill load or overwrite the Command Instruction
register. A Reset operation (internal via CMD IR or external via the RESET input)
w ill cause the^PD8251 to interpret the next "control w rite ", which must immediately
follow the reset, as a Mode Instruction.
It is frequently necessary fo r the processor to examine the "status" of an active
interface device to determine if errors have occurred or to notice other conditions
which require a response from the processor. The;uPD8251 has features which
allow the processor to "re ad " the device status at any time. A data fetch is issued
by the processor while holding the C/D input "h ig h " to obtain device Status Infor
mation. Many of the bits in the status register are copies of external pins. This dual
status arrangement allows the /iPD8251 to be used in both Polled and interrupt
driven environments. Status update can have a maximum delay o f a 16 clock period.
When a parity error is detected, the PE flag is set, and is cleared by setting the
ER b it in a subsequent Command Instruction. PE being set does not inhibit USART
operation.
If the processor fails to read a data character before the one follow ing is available,
the OE flag is set, and is cleared by setting the ER b it in a subsequent Command
Instruction. Although OE being set does not in h ib it USART operation, the
previously received character is overwritten and lost.
If a valid STOP bit is not detected at the end o f a character, the FE flag is set, and
is cleared by setting the ER bit in a subsequent Command Instruction. FE being set
does not inhibit USART operation.
Note: (D ASYNC mode only.
North Star HSIO-4
PD8251 has been specified by the Mode
j
89
COMMAND INSTRUCTION
FORMAT
STATUS READ FORMAT
PARITY ERROR
OVERRUN ERROR
FRAMING ERROR ©
User/Technical Manual
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