DP83907 datasheet(20/70 Pages) NSC

20 / 70 page

40 Functional Description (Continued)

TLF12082 – 13

FIGURE 18 EncoderDecode Block Diagram

410 ENCODERDECODER (ENDEC) MODULE

The ENDEC consists of four main logical blocks

a) The oscillator generates the 10 MHz transmit clock sig-

nal for system timing

b) The Manchester encoder accepts NRZ data from the

controller encodes the data to Manchester and trans-

mits the data differentially to the transceiver through the

differential transmit driver

c) The Manchester decoder receives Manchester data from

the transceiver converts it to NRZ data and clock puls-

es and sends it to the controller

d) The collision translator indicates to the controller the

presence of a valid 10 MHz collision signal to the PLL

Oscillator

The oscillator is controlled by a 20 MHz parallel resonant

crystal connected between X1 and X2 or by an external

clock on X1 The 20 MHz output of the oscillator is divided

by 2 to generate the 10 MHz transmit clock for the control-

ler The oscillator also provides internal clock signals to the

encoding and decoding circuits

CRYSTAL OPERATION

If the crystal used with the internal oscillator circuit is not

properly selected the DP83907 oscillator may not reliably

start oscillation under all conditions

If this occurs it could be deceiving to a designer since his

prototypes may work fine However when the designer

does qualification testing or starts production he may en-

counter a higher than expected board yield loss due to the

oscillator not starting

The DP83907’s oscillator circuit

clocks the Encoder-Decoder logic The DP83907’s oscilla-

tor also clocks the twisted pair interface block If the oscilla-

tor does not start the DP83907 will not be able to transmit

or receive

TLF12082 – 14

FIGURE 19 Crystal Connection to DP83907

(see text for component values)

DP83907 Datasheet(PDF) 20 Page - National Semiconductor (TI)