DP83840AVCE datasheet(33/91 Pages) TI1

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3.0 Functional Description (Continued)

National Semiconductor

Version A

The Manchester encoder accepts NRZ data from the

controller or repeater, encodes the data to Manchester,

and transmits it differentially to the transceiver, through the

differential transmit driver.

The Manchester decoder receives Manchester data from

the transceiver, converts it to NRZ data and recovers clock

pulses for synchronous data transfer to the controller or

repeater.

The collision monitor indicates to the controller the

presence of a valid 10 MHz collision signal.

3.7.13.1 Manchester Encoder and differential driver

The encoder begins operation when the Transmit Enable

input (TX_EN) goes high and converts the NRZ data to

pre-emphasized Manchester data for the transceiver. For

the duration of TXE remaining high, the Transmit Data

(TXD) is encoded for the transmit-driver pair (TXU+/- or

TXS+/-). TXD must be valid on the rising edge of Transmit

Clock (TXC). Transmission ends when TX_EN deasserts.

The last transition is always positive; it occurs at the center

of the bit cell if the last bit is a one, or at the end of the bit

cell if the last bit is a zero.

3.7.13.2 Manchester Decoder

The decoder consists of a differential receiver and a PLL to

separate a Manchester encoded data stream into internal

clock signals and data. The differential input must be

externally terminated with either a differential 100

Ω or

differential 150

Ω termination network to accommodate

either UTP or STP cable respectively. Refer to Figure 16

for further detail.

The decoder detects the end of a frame when no more

mid-bit transitions are detected. Within one and a half bit

times after the last bit, carrier sense is de-asserted.

Receive clock stays active for five more bit times after CRS

goes low, to guarantee the receive timings of the controller

or repeater.

3.7.14 REQ and RTX

These pins allow for the direct control of both the pre-

emphasis (REQ) and the transmit amplitude (RTX) of the

10BASE-T transmit signal. These pins should normally be

left ﬂoating, however, in applications where lower transmit

amplitudes are required, these pins should be pulled-down

to ground resistively. Conversely, for applications that

require higher transmit amplitudes, these pins should be

pulled-up to Vcc resistively. Figure 16 provides a simpliﬁed

functional diagram.

Some experimentation is required in order to fully evaluate

the extent of transmit amplitude (and corresponding pre-

emphasis) variation due to the system to system variations

in external components (e.g. transformers and termination

networks). It is important to use the same resistor value for

both RTX and REQ (pulled to the same rail) in order to

allow the pre-emphasis to track the transmit amplitude.

In general terms, a value of approximately 50K

Ω for these

pins (either pulled-up or pulled-down) will result in a

transmit amplitude (and pre-emphasis) change on the

order

10%.

Again,

experimentation

recommended.

RTX

REQ

Transmit

Amplitude

Control

Transmit

Pre-emphasis

Control

Fixed

Current

Source

Fixed

Current

Source

GND

Vcc

FIGURE 16. REQ and RTX Operation

DP83840AVCE Datasheet(PDF) 33 Page - Texas Instruments

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