User Manual
BCM1250/BCM1125/BCM1125H
10/21/02
B r o a d c o m C o r p o r a t i o n
Document
1250_1125-UM100CB-R
Section 10: Serial Interfaces Page
349
Transmitter in Transparent Mode
In Transparent mode, the protocol engine does not perform bit-stuffing or Flag/Abort insertion. Thus the DMA
abort option is ignored. The append_CRC and append_PAD options remain available.
The TIN signal is used as described in
Section: “Output Line Interface” on page 342
either as a level sensitive
enable or to synchronize the table driven output sequencer. The transmit module will detect the start of packet
indicator from the DMA engine as data comes out of the TxFIFO and align it to the line. If TIN is used as an
enable, the edge_det bit in the
ser_mode
register selects if the start of a packet will be aligned with the next
inactive to active edge of enable or the next active level (allowing packets to go our back-to-back).
If TIN is used as a synchronization pulse the start of a packet will be aligned with the first entry of the sequencer
table being selected, which can be based on either the edge of the pulse or its level. These two rules ensure
that packet boundaries in the DMA stream get aligned with the frame boundaries used on the line. If there are
any idle cycles while a new frame is being aligned the DOUT pin will be set high impedance.
In other respects, operation is identical to operation of the transmitter in HDLC mode.
Receiver in Transparent Mode
Transparent mode operates similarly to HDLC mode except that neither Flag detection/deletion nor removal
of bit stuffing is performed. CRC checking and address filtering remain available. Note that since there is no
way to disable the CRC check, device drivers for protocols that do not have CRCs must ignore the CRC error
flag (and therefore cannot use the good packet bit) in the DMA descriptor status information.
Frames are delimited by transitions of the gating signals in each direction. If there are no transitions, transfers
make no progress.
If an external enable signal is used, it serves as an envelope delimiting the frame. The initial bit in the frame is
the one in the first bit time after transition of the (optionally delayed) enable signal to its active level. The final
bit in the frame is the bit preceding the opposite transition.
If the serial sequencer is used, the table is traversed exactly once to delimit the frame. The synchronization
pulse (optionally delayed) will start processing bits from entry zero of the table and mark the start of a new
frame. The first bit in the received frame will therefore be the first bit that is enabled in the table. The final bit
in the frame will be the last bit in the last entry of the table with the Enable bit set, following reception of this bit
the sequencer will suspend until the next sync pulse and the packet will be complete. A sync pulse that occurs
during traversal of the table will be logged as an rx_sync_error in the
ser_error
register but is otherwise
ignored.
In other respects, operation is identical to operation of the receiver in HDLC mode.