DP83850CVF datasheet(10/37 Pages) NSC

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3.0 Functional Description

The following sections describe the different functional

blocks of the DP83850C 100 Mb/s Repeater Interface Con-

troller. The IEEE 802.3u repeater speciﬁcation details a

number of functions a repeater system is required to per-

form. These functions are split between those tasks that

are common to all data channels and those that are spe-

ciﬁc to each individual channel. The DP83850C follows

this split task approach for implementing the required func-

tions. Where necessary, the difference between the TX and

T4 modes is discussed.

3.1 Repeater State Machine

The Repeater State Machine (RSM) is the main block that

governs the overall operation of the repeater. At any one

time, the RSM is in one of the following states: Idle,

Repeat, Collision, One Port Left, or Noise.

3.1.1 Idle State

The RSM enters this state after reset or when there is no

activity on the network and the carrier sense is not present.

The RSM exits from this state if the above conditions are

no longer true.

3.1.2 Repeat State

This state is entered when there is a reception on only one

of the ports, port N. While in this state, the data is transmit-

ted to all the ports except the receiving port (port N). The

RSM either returns to Idle state when the reception ends,

or transitions to Collision state if there is reception activity

on more than one port.

3.1.3 Collision State

When there is receive activity on more than one port of the

repeater, the RSM moves to Collision state. In this state,

transmit data is replaced by Jam and sent out to all ports

including the original port N.

There are two ways for the repeater to leave the Collision

state. The ﬁrst is when there is no receive activity on any

of the ports. In this case, the repeater moves to Idle state.

The second is when there is only one port experiencing

collision in which case the repeater enters the One Port

Left state.

3.1.4 One Port Left State

This state is entered only from the Collision state. It guar-

antees that repeaters connected hierarchically will not jam

each other indeﬁnitely. While in this state, Jam is sent out

to all ports except the port that has the receive activity. If

more receive activity occurs on any other port, then the

repeater moves to Collision state.

Otherwise, the repeater will transition to Idle state when the

receive activity ends.

3.1.5 Noise State

When there is an Elasticity Buffer overﬂow or underﬂow

during packet reception, then the repeater enters the Noise

state. During this state, the Jam pattern is sent to all trans-

mitting ports.

The repeater leaves this state by moving

either to the Idle state, if there is no receive activity on any

ports, or to the Collision state, if there is a collision on one

of its segments.

3.2 RXE Control

When only one port has receive activity, the RXE signal

(receive enable) is activated. If multiple ports are active

(i.e. a collision scenario), then RXE will not be enabled for

any port. The Port Select Logic asserts the open-collector

outputs /IR_COL_OUT and /IR_ACTIVE to indicate to

other cascaded DP83850Cs that there is collision or

receive activity present on this DP83850C.

The polarity of the RXE signal can be set through an exter-

nal pull down resistor placed at the RSM[3] pin. That is, if

the RSM[3] pin is unconnected or pulled high, then the

RXE is active high and when the RSM[3] is pulled low, then

the RXE is active low.

3.3 TXE Control

This control logic enables the appropriate ports for data

transmission according to the four states of the RSM. For

example, during Idle state, no ports are enabled; during

Repeat state, all ports but port N are enabled; in Collision

state, all ports including port N are enabled ; during One

Port Left state, all ports except the port experiencing the

collision will be enabled.

3.4 Data Path

After the Port Selection logic has enabled the active port,

receive data (RXD), receive clock (RXC), receive error

(RX_ER) and receive data valid (RX_DV) will ﬂow through

the chip from that port out onto the Inter Repeater (IR) bus

if no collisions are present. The signals on the IR bus ﬂow

either in to or out of the chip depending upon the

Repeater’s state.

If the DP83850C is currently receiving and no collisions are

present, the IR signals ﬂow out of the chip. The

DP83850C's Arbitration Logic guarantees that only one

DP83850C will gain ownership of the IR bus. In all other

states, the IR signals are inputs.

When IR signals are inputs, the signals ﬂow into the Elas-

ticity Buffer (EB). Here, the data is re-timed and then sent

out to the transmit ports. The Transmit Control logic deter-

mines which ports are enabled for data transmission.

If a collision occurs, a Jam pattern is sent out from the EB

instead of the data. The Jam pattern (3,4,3,4,..... from the

DP83850C, encoded by the Physical Layer device as

1,0,1,0,.....) is transmitted for the duration of the collision

activity.

If the repeater is conﬁgured in the preamble regeneration

mode (T4 mode), approximately 12 clock cycles after the

assertion of /IR_ACTIVE (indicating a packet reception on

a segment), the 100RIC begins to transmit the preamble

pattern onto the other network segments. While the pre-

amble is being transmitted, the EB monitors the received

clock and data signals. When the start of the frame delim-

iter "SFD" is detected, the received data stream is written

into the EB. After this point, data from the EB is sent out to

the Transmit interface. The preamble is always generated

in its entirety (i.e. ﬁfteen 5’s and one D) even if a collision

occurs.

3.5 Elasticity Buffer

The elasticity buffer, or a logical FIFO buffer, is used to

compensate for the variations and timing differences

between the recovered Receive Clock and the local clock.

This buffer supports maximum clock skews of 200ppm for

the preamble regeneration (T4) mode, and 100ppm for the

TX mode, within a maximum packet size of 1518 bytes.

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