Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support
Datasheet
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 activate ENERGYON may be lost.
When the EDPWRDOWN bit of the Mode Control/Status Register is low, energy detect power-down is
disabled.
3.8.4
3.8.5
Isolate Mode
The device data paths may be electrically isolated from the RMII interface by setting the Isolate bit of
the Basic Control Register to “1”. In isolation mode, the transceiver does not respond to the TXD,
TXEN and TXER inputs, but does respond to management transactions.
Isolation provides a means for multiple transceivers to be connected to the same RMII interface without
contention. By default, the transceiver is not isolated (on power-up (Isolate=0).
Resets
The device provides two forms of reset: Hardware and Software. The device registers are reset by
both Hardware and Software resets. Select register bits, indicated as “NASR” in the register definitions,
are not cleared by a Software reset. The registers are not reset by the power-down modes described
in Section 3.8.3.
Note: For the first 16us after coming out of reset, the RMII interface will run at 2.5 MHz. After this
time, it will switch to 25 MHz if auto-negotiation is enabled.
3.8.5.1
Hardware Reset
A Hardware reset is asserted by driving the nRST input pin low. When driven, nRST should be held
low for the minimum time detailed in Section 5.5.3, "Power-On nRST & Configuration Strap Timing,"
on page 70 to ensure a proper transceiver reset. During a Hardware reset, an external clock must be
supplied to the XTAL1/CLKIN signal.
Note: A hardware reset (nRST assertion) is required following power-up. Refer to Section 5.5.3,
"Power-On nRST & Configuration Strap Timing," on page 70 for additional information.
3.8.5.2
Software Reset
A Software reset is activated by setting the Soft Reset bit of the Basic Control Register to “1”. All
registers bits, except those indicated as “NASR” in the register definitions, are cleared by a Software
reset. The Soft Reset bit is self-clearing. Per the IEEE 802.3u standard, clause 22 (22.2.4.1.1) the reset
process will be completed within 0.5s from the setting of this bit.
3.8.6
Carrier Sense
The carrier sense (CRS) is output on the CRS_DV pin. CRS is a signal defined by the MII specification
in the IEEE 802.3u standard. The device asserts CRS based only on receive activity whenever the
transceiver is either in repeater mode or full-duplex mode. Otherwise the transceiver asserts CRS
based on either transmit or receive activity.
The carrier sense logic uses the encoded, unscrambled data to determine carrier activity status. It
activates carrier sense with the detection of 2 non-contiguous zeros within any 10 bit span. Carrier
sense terminates if a span of 10 consecutive ones is detected before a /J/K/ Start-of Stream Delimiter
pair. If an SSD pair is detected, carrier sense is asserted until either /T/R/ End–of-Stream Delimiter
pair or a pair of IDLE symbols is detected. Carrier is negated after the /T/ symbol or the first IDLE. If
/T/ is not followed by /R/, then carrier is maintained. Carrier is treated similarly for IDLE followed by
some non-IDLE symbol.
SMSC LAN8720A/LAN8720Ai
39
Revision 1.4 (08-23-12)
DATASHEET
SMSC [ SMSC CORPORATION ]
