Communication Processor Module
MOTOROLA
MPC823 USER’S MANUAL
16-317
SCC2
COMMUNICATION
16
PROCESSOR
MODULE
When the end of the current buffer descriptor is reached and the L bit in the TX buffer
descriptor is set, the FCS bytes of the Ethernet frame are appended (if the TC bit is set in
the TX buffer descriptor), and TENA is negated. This notifies the EEST of the need to
generate the illegal Manchester encoding that signifies the end of an Ethernet frame. After
CRC transmission, the SCC2 Ethernet controller writes the frame status bits into the buffer
descriptor and clears the R bit. When the end of the current buffer descriptor is reached and
the L bit is not set, only the R bit is cleared.
In either mode, an interrupt can be issued, depending on how the I bit is set in the TX buffer
descriptor. The SCC2 Ethernet controller then proceeds to the next TX buffer descriptor in
the table. You can be interrupted after each frame, after each buffer, or after a specific buffer
is transmitted. The SCC2 Ethernet controller can add pad characters to short frames. If the
PAD bit is set in the TX buffer descriptor, the frame is padded up to the value of the minimum
frame length register.
To rearrange the transmit queue before the communication processor module finishes
transmitting all the frames, issue the GRACEFUL STOP TRANSMIT command. This
technique can be useful for transmitting expedited data before previously linked buffers or
for error situations. When the GRACEFUL STOP TRANSMIT command is issued, the
Ethernet controller stops immediately if no transmission is in progress or it will keep
transmitting until the current frame either finishes or terminates with a collision. When the
Ethernet controller receives the RESTART TRANSMIT command, it resumes transmission.
The Ethernet controller transmits bytes least-significant bit first.
16.9.22.6 SCC2 ETHERNET CHANNEL FRAME RECEPTION PROCESS. The Ethernet
receiver is designed to work with almost no intervention from the core and can perform
address recognition, CRC checking, short frame checking, maximum DMA transfer
checking, and maximum frame length checking.
When the core enables the Ethernet receiver, it enters hunt mode as soon as the RENA
signal is asserted if CLSN is negated. In hunt mode, as data is shifted into the receive shift
register one bit at a time, the contents of the register are compared to the contents of the
SYN1 field in the data synchronization register. This compare function becomes valid a
certain number of clocks after the start of the frame (depending on the NIB bits in the
PSMR–Ethernet). If the two are not equal, the next bit is shifted in and the comparison is
repeated. If a double zero or double one fault is detected between bits 14 to 21 from the start
of the frame, it is rejected. If a double zero fault is detected after 21 bits from the start of the
frame and before detection of the start frame delimiter, the frame is also rejected. When the
registers match, hunt mode is terminated and character assembly begins.
When the receiver detects the first bytes of the frame, the Ethernet controller performs
address recognition functions on the frame. The receiver can receive physical (individual),
group (multicast), and broadcast addresses. Ethernet receive frame data is not written to
memory until the internal address recognition algorithm is complete, which improves bus
utilization with frames not addressed to this station.