Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support
Datasheet
3.8
Miscellaneous Functions
3.8.1
LEDs
Two LED signals are provided as a convenient means to determine the transceiver's mode of
operation. All LED signals are either active high or active low as described in Section 3.8.1.2,
"nINTSEL and LED2 Polarity Selection" and Section 3.8.1.1, "REGOFF and LED1 Polarity Selection,"
on page 37.
The LED1 output is driven active whenever the device detects a valid link, and blinks when CRS is
active (high) indicating activity.
The LED2 output is driven active when the operating speed is 100Mbps. This LED will go inactive
when the operating speed is 10Mbps or during line isolation.
Note: When pulling the LED1 and LED2 pins high, they must be tied to VDD2A, NOT VDDIO.
3.8.1.1
REGOFF and LED1 Polarity Selection
The REGOFF configuration strap is shared with the LED1 pin. The LED1 output will automatically
change polarity based on the presence of an external pull-up resistor. If the LED1 pin is pulled high to
VDD2A by an external pull-up resistor to select a logical high for REGOFF, then the LED1 output will
be active low. If the LED1 pin is pulled low by the internal pull-down resistor to select a logical low for
REGOFF, the LED1 output will then be an active high output. Figure 3.10 details the LED1 polarity for
each REGOFF configuration.
REGOFF = 1 (Regulator OFF)
LED output = Active Low
REGOFF = 0 (Regulator ON)
LED output = Active High
VDD2A
LED1/REGOFF
10K
~270 ohms
~270 ohms
LED1/REGOFF
Figure 3.10 LED1/REGOFF Polarity Configuration
Note: Refer to Section 3.7.3, "REGOFF: Internal +1.2V Regulator Configuration," on page 32 for
additional information on the REGOFF configuration strap.
3.8.1.2
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
SMSC LAN8720A/LAN8720Ai
37
Revision 1.4 (08-23-12)
DATASHEET
SMSC [ SMSC CORPORATION ]
