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Preliminary Data Sheet, Rev. 1
October 2000
TMXF28155 Super Mapper
155/51 Mbits/s SONET/SDH x28/x21 DS1/E1
441
Lucent Technologies Inc.
28-Channel Framer Block Functional Description
(continued)
HDLC Operation
(continued)
Flags
1
All flags have the bit pattern 01111110 and are used for frame synchronization. The framer’s HDLC block automat-
ically sends one flag at the beginning of each frame. If the FRM_HTIDLE (Table 470) bit is cleared to 0, the FLAG
byte (01111110) is continuously sent between frames if no data is present in the FIFO. If the FRM_HTIDLE bit is set
to 1, the HDLC block sends continuous FRM_IDLE (Table 384) bytes (11111111) when the transmit FIFO is empty.
Once there is data in the transmit FIFO, an opening flag is sent followed by the frame. During transmission, two
successive flags will not share the intermediate 0.
An opening flag is always generated at the beginning of a frame (indicated by the presence of data in the transmit
FIFO and the transmitter enabled). FRM_CFLAGS[1:0] (Table 470) determines which FRM_FCNT[0—3][4:0]
parameter to use. The FRM_FCNT[0—3][4:0] parameters define the number of idle flags that are sent between
HDLC packets. Data is transmitted per the HDLC protocol until a byte is read from the FIFO with Tx HDLC register
bits FRM_HTFUNC[1:0] (Table 473) = 01 set. The HDLC block follows this byte with the CRC sequence and a
closing flag.
The HDLC receiver recognizes the 01111110 pattern as a flag. Two successive flags may or may not share the
intermediate 0 bit and are identified as two flags (i.e., both 011111101111110 and 0111111001111110 are recog-
nized by the HDLC block). When another flag is identified, it is treated as the closing flag. As mentioned above, a
flag sequence in the user data or FCS fields is prevented by zero-bit insertion and deletion.
1. Regardless of the time-fill byte used, there always is an opening and closing flag with each frame. Back-to-back frames are separated by two
flags.
Aborts
The bit pattern of the abort sequence is 01111111, with 0 transmitted first. A frame can be aborted by writing setting
Tx HDLC register bits FRM_HTFUNC[1:0] = 01. This causes the last byte written to the transmit FIFO to be fol-
lowed by the abort sequence upon transmission. Once a byte is tagged by a write to Tx HDLC register bits
FRM_HTFUNC[1:0] = 01, it cannot be cleared by subsequent writes.
When receiving a frame, the receiver recognizes the abort sequence whenever it receives a 0 followed by seven
consecutive 1s. This status results in the abort bit, and possibly the bad byte count bit and/or bad CRC bits, being
set in the status of frame status byte which is appended to the receive data queue. The last bytes of user data are
assumed to be CRC bits and are placed in the queue in the regular HDLC mode. All subsequent FRM_IDLE or flag
bytes are ignored until a valid opening flag is received.
Receive IDLES
In accordance with the HDLC protocol, the HDLC block recognizes 15 or more contiguous received 1s as idle.
When the HDLC block receives 15 contiguous 1s, the receiver FRM_IDLE[7:0] bit, idle is set.