CN8478 Datasheet, PDF (162/221 Pages) Conexant Systems, Inc – Multichannel Synchronous Communications Controller (MUSYCC)

6.0 Basic Operation
6.4 Protocol Support
CN8478/CN8474A/CN8472A/CN8471A
Multichannel Synchronous Communications Controller (MUSYCC™)
6.4 Protocol Support
6.4.1 Frame Check Sequence
MUSYCC is configurable to calculate either a 16- or 32-bit Frame Check
Sequence (FCS) for HDLC packets ranging in size from a minimum of 2 octets to
a maximum of 16,384 octets. The FCS always applies to the entire packet length.
For all HDLC modes which require FCS calculations, the polynomials used to
calculate the FCS are according to ITU-T Q.921 and ISO 3309-1984.
• CRC-16:
x¹⁶ + x¹² + x⁵ + 1
• CRC-32:
x³² + x²⁶ + x²³ + x²² + x¹⁶ + x¹² + x¹¹ + x¹⁰ + x⁸ + x⁷ + x⁵ + x⁴ + x² + x + 1
6.4.2 Opening/Closing Flags
For HDLC modes only, MUSYCC supports the use of opening and closing
message flags. The 7Eh (01111110b) flag is the opening and closing flag. An
HDLC message is always bounded by this flag at the beginning and the end of the
message.
MUSYCC supports receiving a shared flag where the closing flag of one
message can act as the opening of the next message. MUSYCC also supports
receiving a shared 0 bit between two flags, i.e., the last 0 bit of one flag is used as
the first 0 bit of the next flag.
MUSYCC can be configured to transmit a shared flag between successive
messages by configuring the bit field PADEN in each Transmit Buffer Descriptor.
MUSYCC does not transmit shared 0 bits between successive flags.
6.4.3 Abort Codes
Seven consecutive 1s constitute an abort flag. Receiving the abort code causes the
current frame processing to be aborted and terminates further data transfer into
shared memory. After detecting the abort code, MUSYCC enters a mode
searching for a new opening flag.
Notification of this detected condition is provided by the receive Buffer Status
Descriptor or an Interrupt Descriptor, indicating the error condition Abort Flag
Termination.
In cases where received idle codes transition to an abort code, an Interrupt
Descriptor is generated toward the host indicating the informational event Change
to Abort Code. All received abort codes are discarded.
6-28
Conexant
100660E