ENC28J60T-CSLASHML datasheet(9/96 Pages) MICROCHIP

9 / 96 page

Preliminary

DS39662B-page 7

ENC28J60

2.4

Magnetics, Termination and Other

External Components

To complete the Ethernet interface, the ENC28J60

requires several standard components to be installed

externally. These components should be connected as

shown in Figure 2-4.

The internal analog circuitry in the PHY module requires

that an external 2.32 k

Ω, 1% resistor be attached from

RBIAS to ground. The resistor influences the TPOUT+/-

signal amplitude. The resistor should be placed as close

as possible to the chip with no immediately adjacent

signal traces to prevent noise capacitively coupling into

the pin and affecting the transmit behavior. It is

recommended that the resistor be a surface mount type.

Some of the device’s digital logic operates at a nominal

2.5V. An on-chip voltage regulator is incorporated to

generate this voltage. The only external component

required is an external filter capacitor, connected from

VCAP to ground. The capacitor must have low equiva-

lent-series resistance (ESR), with a typical value of

μF, and a minimum value of 1 μF. The internal regu-

lator is not designed to drive external loads.

On the TPIN+/TPIN- and TPOUT+/TPOUT- pins,

1:1 center-taped pulse transformers rated for Ethernet

operations are required. When the Ethernet module is

enabled, current is continually sunk through both

TPOUT pins. When the PHY is actively transmitting, a

differential voltage is created on the Ethernet cable by

varying the relative current sunk by TPOUT+ compared

to TPOUT-.

A common-mode choke on the TPOUT interface,

placed between the TPOUT pins and the Ethernet

transformer (not shown), is not recommend. If a com-

mon-mode choke is used to reduce EMI emissions, it

should be placed between the Ethernet transformer

and pins 1 and 2 of the RJ-45 connector. Many Ether-

net

transformer

modules

include

common-mode

chokes inside the same device package. The trans-

formers should have at least the isolation rating speci-

fied in Table 16-5 to protect against static voltages and

meet

IEEE

802.3

isolation

requirements

(see

Section 16.0 “Electrical Characteristics” for specific

transformer requirements). Both transmit and receive

interfaces additionally require two resistors and a

capacitor to properly terminate the transmission line,

minimizing signal reflections.

All power supply pins must be externally connected to

the same power source. Similarly, all ground refer-

ences must be externally connected to the same

ground node. Each VDD and VSS pin pair should have

a 0.1

μF ceramic bypass capacitor (not shown in the

schematic) placed as close to the pins as possible.

Since relatively high currents are necessary to operate

the twisted-pair interface, all wires should be kept as

short as possible. Reasonable wire widths should be

used on power wires to reduce resistive loss. If the

differential data lines cannot be kept short, they should

be routed in such a way as to have a 100

Ω character-

istic impedance.

FIGURE 2-4:

ENC28J60 ETHERNET TERMINATION AND EXTERNAL CONNECTIONS

I/O

SCK

SDO

SDI

INT0

MCU

Level

Shift

Logic(2)

SCK

INT

ENC28J60

VCAP

LEDA

LEDB

RBIAS

TPOUT

TPIN

μF

Note

Ferrite Bead should be rated for at least 80 mA.

Required only if the microcontroller is operating at 5V. See Section 2.5 “I/O Levels” for more information.

These components are installed for EMI reduction purposes.

Ferrite

Bead(1,3)

3.3V

2.32 k

Ω, 1%

RJ-45

1:1 CT

1 nF, 2 kV(3)

Ω(3) 75Ω(3) 75Ω(3) 75Ω(3)

49.9

Ω, 1%

49.9

Ω, 1%

49.9

Ω, 1%

49.9

Ω, 1%

0.1

μF(3)

0.1

μF

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