BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
336
Section 10: Serial Interfaces
Document
1250_1125-UM100CB-R
S
YNCHRONOUS
M
ODE
In synchronous mode, the serial data stream is accompanied by a clock and an optional gating or framing
signal. There are two sub-modes, HDLC and transparent. In HDLC sub-mode, frames within the bit stream are
recognized and processed according to ISO/IEC-3309 (High-level data link control (HDLC) procedures -
Frame structure). In transparent sub-mode, serial data is transmitted and received without modification.
In both sub-modes, the serial stream can be qualified by a gating signal. Typically, such gating would select a
subset of the bits at the physical interface for internal processing. The gating signal can be provided externally.
Alternatively, it can be generated by a table-driven sequencer that is synchronized to an external framing
pulse. Configuration options allow a choice of polarities and delays for the gating or synchronizing signals.
The synchronous serial ports can operate at speeds of 0 to 55 Mbp/s and thus can support up to T3, E3 and
OC-1 data rates. In such applications, an external framer or similar interface would typically be used to connect
to the serial line and would itself be controlled from the CPU via the generic bus interface (see
Section11: “Generic/Boot Bus” on page 362
Each port is associated with a DMA channel for transmit and another for receive. In synchronous mode, the
port and the pair of DMA channels are collectively called a serial channel.
The interface provides two identical and independent serial channels, 0 and 1. The registers and interrupts
associated with each are differentiated by appending
_0
or
_1
to their names.
F
UNCTIONAL
O
VERVIEW
The serial channels are part of the I/O subsystem on the part and connect to the ZBbus through I/O bridge 1.
shows a block diagram. Each consists of 3 major functional blocks:
•
a pair of built-in DMA controllers with FIFOs
•
a protocol engine that can operate in either HDLC or transparent mode
•
a programmable line interface
For each serial channel, there are two built-in DMA controllers, one for transmit and one for receive. The DMA
controllers interface with the I/O Bridge. They connect to the protocol engine via separate FIFOs for transmit
(TxFIFO) and receive (RxFIFO).
Except for the interpretation of link-specific option and status bits, which are summarized below, the serial DMA
channels function identically to those provided for the Ethernet MACs. Note that each serial DMA channel
supports only a single chain or ring. For a full description of DMA configuration and programming, refer to
.
The line interface unit controls the signals connecting the part to external serial devices, which might range
from codecs to simple line drivers to sophisticated external framers. The line interface is described in the next
section.
The protocol engine provides the link-level processing. It maps between DMA’s byte streams and the line
interface’s bit streams according to the selected protocol.
Section: “Framing Parameters” on page 345
Section: “HDLC Transmitter” on page 345
describe its operation for the HDLC and transparent protocols
respectively.