AM79C874VC datasheet(12/60 Pages) AMD

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Am79C874

P R E L I M INARY

REFCLK

Clock Input

Input, Pull-Down

This pin connects to a 25-MHz +50 ppm clock source

with a 40% to 60% duty cycle. When a crystal input is

used, this pin should be pulled low via a 1 k

W resistor.

XTL±

Crystal Inputs

Analog Input

These pins should be connected to a 25-MHz crystal.

The crystal should be parallel resonant and have a fre-

quency stability of +100 ppm and a frequency tolerance

of +50 ppm. REFCLK (Pin 5) should be pulled low

when the crystal is used as a clock source.

These pins may be left unconnected when REFCLK is

used as a clock source.

CLK25

25 MHz Clock

Output

When the CLK25EN pin is pulled low, the CLK25 pin

provides a continuous 25 MHz clock to the MAC.

BURN_IN

Test Enable

Input, Pull-Down

When pulled high via a 10 k

W resistor, this pin forces

the NetPHY-1LP device into Burn-in mode for reliability

assurance control. When left unconnected the device

operates normally.

TEST2

Test Output

Analog Output

When BURN_IN (pin 7) is pulled high, this pin serves

as a test mode output monitor pin. TEST2 can be left

unconnected when the device is operating.

RST

Reset

Input, Pull-Up

A LOW input forces the NetPHY-1LP device to a known

reset state. The chip can also be reset through internal

power-on-reset or MII Register 0, bit 15.

PWRDN

Power Down

Input, Pull-Down

If this pin is pulled high via a 10 k

W resistor on the rising

edge of reset, the device will power down the analog

modules and reset the digital circuits. However, the de-

vice will still respond to MDC/MDIO data. The same

power-down state can also be achieved through the MII

tivity on the PWRDN pin even when bit 11 is not set.

When left unconnected, the device operates normally.

PHYAD[4:0]

PHY Address

Input/Output, Pull-Up

These pins allow 32 configurable PHY addresses. The

PHYAD will also determine the scramble seed, which

helps to reduce EMI when there are multiple ports

switching at the same time (repeater/switch applica-

tions). Each pin should either be pulled low via a 1 k

- 4.7 kW resistor (set bit to zero) or left unconnected

(set bit to 1) in order to achieve the desired PHY ad-

dress. New address changes take effect after a reset

has been issued, or at power up.

In PCS bypass mode, PHYAD[4:0] and GPIO[1:0]

serves as 10BASE-T serial input and output.

Note: In GPSI mode, the PHYAD pins must be set to

addresses other than 00h.

GPIO[0]/10TXD--/7Wire

General Purpose I/O 0

Input/Output, Pull-Up

If this pin is pulled low via a 10 k

W resistor, on the rising

edge of reset, the device will operate in 10BASE-T

7-wire (GPSI) mode. If this pin is left unconnected dur-

ing the rising edge of reset, the device will operate in

standard MII mode.

After the reset operation has completed, this pin can

function as an input or an output (dependent on the

value of GPIO[0] DIR (MII Register 16, bit 6). If MII

input value on the GPIO[0] pin will be reflected in MII

6 is set LOW, GPIO[0] is an output. The value of MII

output pin.

GPIO[1]/TP125

General Purpose I/O 1

Input/Output, Pull-Down

If this pin is pulled high via a 10 k

W resistor, on the ris-

ing edge of reset, the device will be enabled for use

with a 1.25:1 transmit ratio transformer. If this pin is left

unconnected during the rising edge of reset, the device

will be enabled for use with a 1:1 transmit ratio

transformer.

After the reset operation has completed, this pin can

function as an input or an output (dependent on the

value of GPIO[1] DIR – MII Register 16, bit 8). If MII

input value on the GPIO[1] pin will be reflected in MII

bit 8 is set LOW, GPIO[1] is an output. The value of MII

output pin.

MDIO

Management Data Input/Output

Pull-Down

This pin is a bidirectional data interface used by the

MAC to access management registers within the Net-

PHY-1LP device. This pin has an internal pull-down,

therefore, it requires a 1.5 k

W pull-up resistor as speci-

fied in IEEE 802.3 when interfaced with a MAC. This

pin can be left unconnected when management is not

used.

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