LAN8740 datasheet(39/136 Pages) MICROCHIP

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DS00001987A-page 39

LAN8740A/LAN8740Ai

3.8.1.6

nINTSEL and LED2 Polarity Selection

The nINTSEL configuration strap is shared with the LED2 pin. The LED2 output will automatically change polarity based

on the presence of an external pull-down resistor. If the LED2 pin is pulled high to VDD2A to select a logical high for

nINTSEL, then the LED2 output will be active low. If the LED2 pin is pulled low by an external pull-down resistor to select

a logical low for nINTSEL, the LED2 output will then be an active high output. Figure 3-12 details the LED2 polarity for

each nINTSEL configuration.

3.8.2

VARIABLE VOLTAGE I/O

The device’s digital I/O pins are variable voltage, allowing them to take advantage of low power savings from shrinking

technologies. These pins can operate from a low I/O voltage of +1.8 V up to +3.3 V. The applied I/O voltage must main-

tain its value with a tolerance of ±10%. Varying the voltage up or down after the transceiver has completed power-on

reset can cause errors in the transceiver operation. Refer to Chapter 5, "Operational Characteristics" for additional infor-

mation.

3.8.3

POWER-DOWN MODES

There are two device power-down modes: General Power-Down Mode and Energy Detect Power-Down Mode. These

modes are described in the following subsections.

3.8.3.1

General Power-Down

This power-down mode is controlled via the Power Down bit of the Basic Control Register. In this mode, the entire trans-

ceiver (except the management interface) is powered-down and remains in this mode as long as the Power Down bit is

“1”. When the Power Down bit is cleared, the transceiver powers up and is automatically reset.

3.8.3.2

Energy Detect Power-Down (EDPD)

This power-down mode is activated by setting the EDPWRDOWN bit of the Mode Control/Status Register. In this mode,

when no energy is present on the line the transceiver is powered down (except for the management interface, the

SQUELCH circuit, and the ENERGYON logic). The ENERGYON logic is used to detect the presence of valid energy

from 100BASE-TX, 10BASE-T, or Auto-negotiation signals.

In this mode, when the ENERGYON bit of the Mode Control/Status Register is low, the transceiver is powered-down

and nothing is transmitted. When energy is received via link pulses or packets, the ENERGYON bit goes high and the

transceiver powers-up. The device automatically resets into the state prior to power-down and asserts the nINT interrupt

if the ENERGYON interrupt is enabled in the Interrupt Mask Register. The first and possibly the second packet to acti-

vate ENERGYON may be lost.

When the EDPWRDOWN bit of the Mode Control/Status Register is low, energy detect power-down is disabled.

FIGURE 3-12:

LED2/nINTSEL POLARITY CONFIGURATION

Note:

Refer to Section 3.7.5, "nINTSEL: nINT/TXER/TXD4 Configuration" for additional information on the nINT-

SEL configuration strap.

Note:

Input signals must not be driven high before power is applied to the device.

~270



nINTSEL = 0

LED output = Active High

10K

~270



VDD2A

nINTSEL = 1

LED output = Active Low

LED2/nINTSEL

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