LAN8700 Datasheet, PDF (44/74 Pages) SMSC Corporation – ±15kV ESD Protected MII/RMII Fast-Ethernet PHY with HP Auto-MDIX and SMSC flexPWR TM in a Small Footprint

LAN8700/LAN8700i
Software (SW) reset: Activated by writing register 0, bit 15 high. This signal is self- clearing. After the register-write,
internal logic extends the reset by 256µs to allow PLL-stabilization before releasing the logic from reset.
The IEEE 802.3u standard, clause 22 (22.2.4.1.1) states that the reset process should be completed within 0.5s from
the setting of this bit.
Power-Down reset: Automatically activated when the PHY comes out of power-down mode. The internal power-down
reset is extended by 256µs after exiting the power-down mode to allow the PLLs to stabilize before the logic is released
from reset.
These 3 reset sources are combined together in the digital block to create the internal “general reset”, SYSRST, which
is an asynchronous reset and is active HIGH. This SYSRST directly drives the PCS, DSP and MII blocks. It is also input
to the Central Bias block in order to generate a short reset for the PLLs.
The SMI mechanism and registers are reset only by the Hardware and Software resets. During Power-Down, the SMI
registers are not reset. Note that some SMI register bits are not cleared by Software reset – these are marked “NASR”
in the register tables.
For the first 16us after coming out of reset, the MII will run at 2.5 MHz. After that it will switch to 25 MHz if auto-negoti-
ation is enabled.
5.4.7 LED DESCRIPTION
The PHY provides four LED signals. These provide a convenient means to determine the mode of operation of the Phy.
All LED signals are either active high or active low.
The four LED signals can be either active-high or active-low. Polarity depends upon the Phy address latched in on reset.
The LAN8700/LAN8700i senses each Phy address bit and changes the polarity of the LED signal accordingly. If the
address bit is set as level “1”, the LED polarity will be set to an active-low. If the address bit is set as level “0”, the LED
polarity will be set to an active-high.
The ACTIVITY LED output is driven active when CRS is active (high). When CRS becomes inactive, the Activity LED
output is extended by 128ms.
The LINK LED output is driven active whenever the PHY detects a valid link. The use of the 10Mbps or 100Mbps link
test status is determined by the condition of the internally determined speed selection.
The SPEED100 LED output is driven active when the operating speed is 100Mbit/s or during Auto-negotiation. This LED
will go inactive when the operating speed is 10Mbit/s or during line isolation (register 31 bit 5).
The Full-Duplex LED output is driven active low when the link is operating in Full-Duplex mode.
5.4.8 LOOPBACK OPERATION
The LAN8700/LAN8700i may be configured for near-end loopback and far loopback.
5.4.8.1 Near-end Loopback
Near-end loopback is a mode that sends the digital transmit data back out the receive data signals for testing purposes
as indicated by the blue arrows in Figure 5-2.The near-end loopback mode is enabled by setting bit register 0 bit 14 to
logic one.
A large percentage of the digital circuitry is operational near-end loopback mode, because data is routed through the
PCS and PMA layers into the PMD sublayer before it is looped back. The COL signal will be inactive in this mode, unless
collision test (bit 0.7) is active. The transmitters are powered down, regardless of the state of TXEN.
DS00002260A-page 44
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