DP83840A datasheet(8/89 Pages) NSC

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2.0 Pin Description (Continued)

National Semiconductor

Version A

2.4 CLOCK INTERFACE

Signal Name

Type

Pin #

Description

REFIN

REFERENCE INPUT: 25 MHz TTL reference clock input. This clock can be

supplied from an external oscillator module or from the CLK25M output (pin 81).

CLK25M

O, Z

25 MHz CLOCK OUTPUT: Derived from the 50 MHz OSCIN input. When not in

use, this clock output may be shut off through software by setting bit 7 of the PCS

Conﬁguration Register at address 17h. This output remains unaffected by

hardware and software reset.

OSCIN

OSCILLATOR INPUT: 50 MHz 50 ppm external TTL oscillator input. If not used,

pull down to GND (4.7 k

Ω pull down resistor suggested).

CRYSTAL

OSCILLATOR

OUTPUT:

External

MHz

0.005%

crystal

connection. Used for 10BASE-T timing. When using an external 20 MHz oscillator

connected to X1 or with no reference to X1, leave this pin unconnected.

CRYSTAL OSCILLATOR INPUT: External 20 MHz 0.005% crystal connection.

Used for 10BASE-T timing and Auto-Negotiation. If not used, this pin should be

pulled up to VCC. (4.7 kΩ pull up resistor suggested). When pulled high, the

DP83840A detects this condition, enables the internal 2.5 divider, and switches

the 10 Mb/s and Auto-Negotiation circuitry to the internally derived 20 MHz clock.

I = TTL/CMOS input

O = TTL/CMOS output

Z = TRI-STATE output

J = IEEE 1149.1 pin

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