CSMA BLOCK
The CSMA/CD block is first interfaced via its control registers in order to define its operational configuration. From then on, the DMA interface between the CSMA/CD block and memory is used to transfer data to and from its data path interface.
For transmit, the CSMA/CD block will be asked to transmit frames as soon as they are ready in memory. It will continue transmissions until any of the following transmit error occurs:
a)16 collisions on same frame (half duplex mode)
b)Late collision (half duplex mode)
c)Lost Carrier sense and MON_CSN set
d)Transmit Under run
e)SQET error and STP_SQET set
In that case TXENA will be cleared and the CPU should restart the transmission by setting it again. If a transmission is successful, TXENA stays set and the CSMA/CD is provided by the DMA block with the next packet to be transmitted.
For receive, the CPU sets RXEN as a way of starting the CSMA/CD block receive process. The CSMA/CD block will send data after address filtering through the data path to the DMA block. Data is transferred into memory as it is received, and the final check on data acceptance is the CRC checking done by the CSMA/CD block. In any case, the DMA takes care of requesting/releasing memory for receive packets, as well as generating the byte count.
The receive status word is provided by the CSMA/CD block and written in the first location of the receive structure by the DMA block. If configured for storing CRC in memory, the CSMA/CD unit will transfer the CRC bytes through the DMA interface, and then will be treated like regular data bytes.
Note that the receive status word of any packet is available only through memory and is not readable through any other register. In order to let the CPU know about receive overruns, the RX_OVRN bit is latched into the Interrupt Status Register, which is readable by the CPU at any time.
The address filtering is done inside the CSMA/CD block. A packet will be received if the destination address is broadcast, or if it is addressed to the individual address of the SMC91C95, or if it is a multicast address and ALMUL bit is set, or if it is a multicast address matching one of the multicast table entries. If the PRMS bit is set, all packets are received.
The CSMA/CD block is a full duplex machine, and when working in full duplex mode, the CSMA/CD block will be simultaneously using its data path transmit and receive interfaces.
Statistical counters are kept by the CSMA/CD block, and are readable through the appropriate register. The counters are four bits each, and can generate an interrupt when reaching their maximum values. Software can use that interrupt to update statistics in memory, or it can keep the counter interrupt disabled, while relying on the transmit interrupt routine reading the counters. Given that the counters can increment only once per transmit, this technique is a good complement for the single interrupt per sequence strategy.
The interface between the CSMA/CD block and memory is word oriented. Two
Whenever a normal collision occurs (less than 16 retries - half duplex mode), the CSMA/CD will trigger the backoff logic and will indicate the DMA logic of the collision. The DMA is responsible for restarting the data transfer into the CSMA/CD block regardless of whether the collision happened on the preamble or not.
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