AM79C874VC datasheet(25/60 Pages) AMD

25 / 60 page

Am79C874

P R E L I M INARY

External loopback can be accomplished using an ex-

ternal loopback cable with TX± connected to RX±. Ex-

ternal loopback works for both 10 Mbps and 100 Mbps

after setting Register 0, bit 8 to force full duplex and bit

13 to set the speed.

Reset

The NetPHY-1LP device can be reset in the three fol-

lowing ways:

1. During initial power on (with internal power on reset

circuit).

2. At hardware reset. A logic low signal of 10 ms pulse

width applied to the RST pin.

3. At software reset. Write a 1 to MII Register 0, bit 15.

LED Port Configuration

The NetPHY-1LP device has several pins that are used

for both device configuration and LED drivers. These

pins set the configuration of the device on the rising

edge of RST and thereafter indicate the state of the re-

spective port. See Table 4 for standard LED selections

and Table 5 for advanced LED selections.

The polarity of the LED drivers (Active-LOW or Active-

HIGH) is set at the rising edge of RST. If the pin is LOW

at the rising edge of RST, it becomes an active-HIGH

driver. If it is HIGH at the rising edge of RST, it becomes

an active-LOW driver.

Proper configuration requires pull-up or pull-down re-

sistors. As shown in the Pin Description sections, each

of the LED/Configuration pins has internal pull-up re-

sistors. If the pin’s LED functionality is not used, the pin

may still need to be terminated via an external pull-

down resistor according to the desired configuration.

The resistor value is not critical and can be in the range

of 1 k

W to 10 kW. If the corresponding LED is used, the

terminating resistor must be placed in parallel with the

LED. Suggested LED connection diagrams simplifying

the board design are shown in Figure 5 (standard) and

Figure 6 (advanced).

The value of the series resistor (RL) should be selected

to ensure sufficient illumination of the LED. It is depen-

dent on the rating of the LED.

Table 4.

Standard LED Selections

Notes:

1. 1 means on (logic level low since active low).

2. 0 means off (logic level high since active low).

3. T means toggles (will end at logic level high).

MODE

LEDSDP[0]

LEDSDP[1]

LEDLNK

LEDDPX

LEDTX

LEDRX

LEDCOL

No Link

10HD-RX

10HD-TX

10HD-COL

10FD-RX

10FD-TX

10FD-RX+TX

100HD-RX

100HD-TX

100HD-COL

100FD-RX

100FD-TX

100FD-RX+TX

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