Digi NS9215 - Manual (Page 415)

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S E R I A L C O N T R O L M O D U L E : H D L C
Receive and transmit operations
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Serial Control Module: HDLC
C H A P T E R 1 1
T
he HDLC module allows full-duplex synchronous communication. Both the
receiver and transmitter can select either an internal or external clock. The HDLC
module encapsulates data within opening and closing flags, and sixteen bits of CRC
precedes the closing flag. All information between the opening and closing flag is
zero-stuffed; that is, if five consecutive ones occur, independent of byte
boundaries, a zero is automatically inserted by the transmitter and automatically
deleted by the receiver. This allows a flag byte (07Eh) to be unique within a serial
stream. The standard CRC-CCITT polynomial
(x16 + x12 + x5 + 1) is implemented, with
the generator and checker preset to all ones.
HDLC module
structure
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
R e c e i v e a n d t r a n s m i t o p e r a t i o n s
Both receive and transmit operations are essentially automatic.
Receive
FIFO
Interface
Transmit
FIFO
Interface
Wrapper
be
[1
:0
]
d
a
ta
[31:
0]
read wr
it
e
be
[1
:0
]
d
a
ta
[31:
0]
st
at
us
[6
:0]
AHB Bus
ref_clk
va
lid
int
HDLC
RCLK
TXD
RXD
IO Hub
TCLK