
Communication Processor Module
MOTOROLA
MPC823 USER’S MANUAL
16-321
SCC2
COMMUNICATION
16
PROCESSOR
MODULE
MAXD2—This parameter also gives you the option to stop system bus writes from
occurring after a frame has exceeded a certain size. However, the value of this register
is valid in promiscuous mode when no address match is detected. The SCC2 Ethernet
controller checks the length of an incoming Ethernet frame against the user-defined
value given in this 16-bit register that is usually set to 0x5F0. If this limit is exceeded,
the remainder of the incoming frame is discarded. The SCC2 Ethernet controller waits
until the end of the frame or until MFLR bytes have been received and reports the frame
status and length in the last RX buffer descriptor. In a monitor station, MAXD2 can be
programmed to a value much less than MAXD1 to receive entire frames addressed to
this station, but receives only the headers of the other frames.
MAXD—For internal use only.
MAX_B—For internal use only.
DMA_CNT is a temporary down-counter used to track the frame length.
GADDR1–4—These four registers are used in the hash table function of the group
addressing mode. You can write zeros to these values after reset and before the
Ethernet channel is enabled to disable all group hash address recognition functions.
The SET GROUP ADDRESS command is used to enable the hash table.
TBUF0_DATA0—For internal use only.
TBUF0_DATA1—For internal use only.
TBUF0_RBA0—For internal use only.
TBUF0_CRC—For internal use only.
TBUF0_BCNT—For internal use only.
PADDR1—You must write the 48-bit individual address of this station into this location.
PADDR1_L is the lowest order half-word and PADDR1_H is the highest order
half-word. PADDR_M is the middle half-word.
P_PER—This parameter allows the SCC2 Ethernet controller to be less aggressive
after a collision. Normally, this parameter should be set to 0x0000. To decrease the
aggressiveness of the SCC2 Ethernet controller, you can set P_PER to a value
between 1 and 9, with 9 being the least aggressive. The P_PER value is added to the
retry count in the backoff algorithm to reduce the probability of transmission on the next
time slot.
Note: Using P_PER is fully allowed in the Ethernet/802.3 specifications. In a heavily
congested Ethernet LAN, a less aggressive backoff algorithm used by multiple
stations on the LAN increases the overall LAN throughput by reducing the
probability of collisions. The SBT bit in the PSMR–SCC2 Ethernet offers another
way to reduce the aggressiveness of the SCC2 Ethernet controller.