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The Receiver Buffer accepts serial data input at the RxD pin and converts the data
from serial to parallel format. Bits or characters required fo r the specific communica
tion technique in use are checked and then an eight-bit "assembled" character is
readied for the processor. For communication techniques which require less than
eight bits, the pPD8251 sets the extra bits to "zero."
PIN
NAME
NO.
SYMBOL
Receiver Control Logic
14
Receiver Ready
RxRDY
Receiver Clock
25
RxC
3
RxD
Receiver Data
16
SYNDET
Sync Detect
Note: ©
Since the pPD8251 will frequently be handling both the reception and transmission
for a given link, the Receive and Transmit Baud Rates will be same. RxC and TxC
then require the same frequency and may be tied together and connected to a single
clock source or Baud Rate Generator.
Examples:
If the Baud Rate equals 110 (Async):
RxC or TxC equals 110 Hz (lx )
RxC or TxC equals 1.76 KHz (16x)
RxC or TxC equals 7.04 KHz (64x)
North Star HSIO-4
This block manages all activities related to
incoming data.
The Receiver Ready output indicates that the
Receiver Buffer is ready with an "assembled''
character for input to the processor. For Polled
operation, the processor can check RxRDY
using a Status Read or RxRDY can be con
nected to the processor interrupt structure.
Note that reading the character to the pro
cessor automatically resets RxRDY.
The Receiver Clock is the rate at which the
incoming character is received. In the Asynchro
nous mode, the RxC frequency may be 1,16
or 64 times the actual Baud Rate but in the
Synchronous mode the RxC frequency must
equal the Baud Rate. Two bits in the mode
instruction select Asynchronous at lx , 16x or
64x or Synchronous operation at lx the Baud
Rate.
Unlike TxC. data is sampled hy the
on the rising edge of RxC. ©
A composite serial data stream is received by
the Receiver Control Logic on this pin.
The SYNC Detect pin is only used in the
Synchronous mode. The *
PD8251 may be pro
i
grammed through the Mode Instruction to
operate in either the internal or external Sync
mode and SYNDET then functions as an output
or input respectively. In the internal Sync mode,
the SYNDET output will go to a "one" when
the pPD8251 has located the SYNC character
in the Receive mode. If double SYNC
character (bi-sync) operation has been pro
grammed, SYNDET will go to "one" in the
middle of the last bit of the second SYNC
character. SYNDET is automatically reset to
"zero" upon a Status Read or RESET. In the
external SYNC mode, a "zero" to "one" transi
tion on the SYNDET input will cause the
pPD8251 to start assembling data character
on the next falling edge of RxC. The length of
the SYNDET input should be at least one RxC
period, but may be removed once the
pPD8251 is in SYNC.
If the Baud Rate equals 300:
RxC or TxC equals 300 Hz (lx ) A or S
RxC or TxC equals 4800 Hz (16x) A only
RxC or TxC equals 19.2 KHz (64x) A only
83
RECEIVER BUFFER
PIN IDENTIFICATION
(CONT.)
PD8251
m
User/Technical Manual
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