DP83936AVF datasheet(17/104 Pages) NSC

17 / 104 page

30 Functional Description (Continued)

During transmission of a packet from the SONIC-T the

transceiver will always loop the packet back to the

SONIC-T The SONIC-T will use this to monitor the packet

as it is being transmitted The CRC and source address of

the looped back packet are checked with the CRC and

source address that were transmitted If they do not match

an error bit is set in the status of the transmitted packet (see

Packet Monitored Bad PMB in the Transmit Control Regis-

ter Section 644) Data is not written to the receive FIFO

during this monitoring process unless a Loopback mode has

been selected (see Section 37)

Receive Logic

The receive logic contains the command

control and status registers that govern the operations of

the receive section It generates the control signals for writ-

ing data to the receive FIFO processes error signals ob-

tained from the CRC checker and the deserializer activates

the ‘‘packet reject’’ signal to the RSM for rejecting packets

and posts the applicable status in the Receive Control regis-

ter

Deserializer

This section deserializes the serial input data

stream and provides a byte clock for the address compara-

tor and receive logic It also synchronizes the CRC checker

to begin operation (after SFD is detected) and checks for

proper frame alignment with respect to CRS going inactive

at the end of reception

Address Comparator

The address comparator latches the

Destination Address (during reception or loopback) or

Source Address (during transmission) and determines

whether the address matches one of the entries in the CAM

CRC Checker

The CRC checker calculates the 4-byte

Frame Check Sequence (FCS) field from the incoming data

stream and compares it with the last 4-bytes of the received

packet The CRC checker is active for both normal recep-

tion and self-reception during transmission

Content Addressable Memory (CAM)

The CAM contains

16 user programmable entries and 1 pre-programmed

Broadcast address entry for complete filtering of received

packets The CAM can be loaded with any combination of

Physical and Multicast Addresses (see Section 42) See

Section 61 for the procedure on loading the CAM registers

332 MAC Transmit Section

The transmit section

(Figure 3-7) is responsible for reading

data from the transmit FIFO and transmitting a serial data

stream onto the network in conformance with the IEEE

8023 Carrier Sense Multiple Access with Collision Detec-

tion (CSMACD) standard The Transmit Section consists of

the following blocks

Transmit State Machine (TSM)

The TSM controls the

functions of the serializer preamble generator and JAM

generator It determines the proper sequence of events that

the transmitter follows under various network conditions If

no collision occurs the transmitter prefixes a 62-bit pream-

ble and 2-bit Start of Frame Delimiter (SFD) at the beginning

of each packet and then sends the serialized data At the

end of the packet an optional 4-byte CRC pattern is ap-

pended If a collision occurs the transmitter switches from

transmitting data to sending a 4-byte Jam pattern to notify

all nodes that a collision has occurred Should the collision

occur during the preamble the transmitter waits for it to

complete before jamming After the transmission has com-

pleted the transmitter writes status in the Transmit Control

Protocol State Machine

The protocol state machine as-

sures that the SONIC-T obeys the CSMACD protocol Be-

fore transmitting this state machine monitors the carrier

sense and collision signals for network activity If any other

nodes are currently transmitting the SONIC-T defers its

transmission until the network is quiet It then transmits after

its Interframe Gap Timer (96 ms) has expired The Inter-

frame Gap time is divided into two portions During the first

64 ms any new network activity will restart the Interframe

Gap timer Beyond this time however network activity is

ignored and the state machine waits the remaining 32 ms

before transmitting If the SONIC-T experiences a collision

during a transmission it switches from transmitting data to

transmitting a 4-byte JAM pattern (4 bytes of all 1’s) before

ceasing to transmit The SONIC-T then waits a random

number of slot times (512 ms) determined by the

Truncated

Binary Exponential Backoff Algorithm before reattempting

another transmission In this algorithm the number of slot

times to delay before the nth retransmission is chosen to be

a random integer r in the range of

0 s r s 2k

where k e min(n10)

If a collision occurs on the 16th transmit attempt the

SONIC-T aborts transmitting the packet and reports an ‘‘Ex-

cessive Collisions’’ error in the Transmit Control register

TLF12597 – 10

FIGURE 3-7 MAC Transmitter

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