CP2201 datasheet(59/108 Pages) SILABS

59 / 108 page

CP2200/1

Rev. 1.0

Note: The value of CPADDRH:CPADDRL may be invalid if an overflow event occurs. After an overflow, the FIFOHEADH:FIFO-

HEADL pointer should be used to determine the starting address of the current packet. CPLEN will always remain valid

even after an overflow event.

Note: If the Receive FIFO Full Interrupt is triggered, the interrupt flag must be cleared to re-enable packet reception. The

Receive FIFO Full Interrupt is triggered based on the size of packets or on the number of packets. If triggered based on

the number of packets, then pointer corruption has occurred.

12.4. Initializing the Receive Buffer, Filter and Hash Table

After a device reset, the receive buffer is empty and the filter is configured to accept broadcast packets and

multicast packets matching a hash value of 0x0400. This hash value allows PAUSE control packets to pass

through the receive filter.

The receive buffer does not require any additional initialization. The receive filter can be configured to accept or

ignore broadcast packets, multicast packets, runt packets (Ethernet Frame smaller than 64 bytes), and packets

with a CRC error. The receive filter is configured using the RXFILT register.

The device can be configured to accept broadcast packets and packets addressed to the controller’s MAC address

without using the hash table. If multicast packets need to be accepted, then the hash table can be programmed to

accept packets addressed to specific address ranges.

The CP2200/1 implements a 16-bit hash table to represent all possible addresses in the 64-bit address space.

Each of the possible 65536 possible values for the hash table represent a range of MAC addresses. If all 16 bits

are set to ‘1’, all multicast addresses will be accepted. If all 16-bits are set to ‘0’, then all multicast addresses will be

rejected.The following procedure can be used to determine which bits to set for a specific address:

Step 1: Perform a 32-bit CRC on the 6-bytes of the address using 0xC704DD7B as the polynomial.

Step 2: Record the least significant 4 bits of the CRC result (Hash Index).

Step 3: The Hash Index determines the bit that should be set in the hash table that will allow the address to

be received. For example, if the least significant 4-bits of the CRC result are 101b (5d), then setting

bit 5 of the 16-bit hash table will allow all MAC addresses whose CRC result is 5d to be accepted.

CP2201 Datasheet(PDF) 59 Page - Silicon Laboratories