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HDLC CONTROLLER
8.3.1 HDLC FRAME FORMAT
The HDLC transmits and receives data (address, control, information and CRC field) in a standard format called
a frame. All frames start with an opening flag (beginning of flag, BOF, 7EH) and end with a closing flag (end of
flag, EOF, 7EH). Between the opening and the closing flags, a frame contains an address (A) field, a control (C)
field, an information (I) field (optional), and a frame check sequence (FCS) field (see Table 8-1).
Opening
Flag
01111110
8 bits per byte
NOTE: The address field can be extended up to four bytes using a optional software control setting.
8.3.1.1 Flag (F)
A flag is a unique binary pattern (01111110) that is used to delimit HDLC frames. This pattern is generated
internally by the transmitter. An opening flag starts a frame and a closing flag ends the frame. Opening flags and
closing flags are automatically appended to frames.
A single flag pattern can optionally serve as both the closing flag of one frame and the opening flag of the next
one. This feature is controlled by the double-flag (FF), single-flag (F), or frame separator selection bit (the
TxSDFL bit in the HCON register).
8.3.1.2 Order of Bit Transmission
Address field, control field, and information field bytes are transferred between the CPU and the HDLC module in
parallel over the data bus. These bytes are transmitted and received LSB first. The 16-bit frame check sequence
(FCS) field is, however, transmitted and received MSB first.
8-4
Table 8-1. HDLC Data Frame Format
Address
Control
Field
Field
8 bits per byte
Information
Frame Check
Field
Sequence Field
8 bits per byte;
16 bits
variable length
S3C2500B
Closing
Flag
01111110
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