Datasheet

Micrel, Inc.
KSZ8081MNX/KSZ8081RNB
August
19, 2015 20
Revision 1.4
10Base-T Receive
On the receive side, input buffer and level detecting squelch circuits are used. A differential input receiver circuit and a
phase-locked loop (PLL) performs the decoding function. The Manchester-encoded data stream is separated into clock
signal and NRZ data. A squelch circuit rejects signals with levels less than 400mV, or with short pulse widths, to prevent
noise at the RXP and RXM inputs from falsely triggering the decoder. When the input exceeds the squelch limit, the PLL
locks onto the incoming signal and the KSZ8081MNX/RNB decodes a data frame. The receive clock is kept active during
idle periods between data receptions.
SQE and Jabber Function (10Base-T Only)
In 10Base-T operation, a short pulse is put out on the COL pin after each frame is transmitted. This SQE test is needed to
test the 10Base-T transmit/receive path. If transmit enable (TXEN) is high for more than 20ms (jabbering), the 10Base-T
transmitter is disabled and COL is asserted high. If TXEN is then driven low for more than 250ms, the 10Base-T
transmitter is re-enabled and COL is de-asserted (returns to low).
PLL Clock Synthesizer
The KSZ8081MNX/RNB generates all internal clocks and all external clocks for system timing from an external 25MHz
crystal, oscillator, or reference clock. For the KSZ8081RNB in RMII 50MHz clock mode, these clocks are generated from
an external 50MHz oscillator or system clock.
Auto-Negotiation
The KSZ8081MNX/RNB conforms to the auto-negotiation protocol, defined in Clause 28 of the IEEE 802.3 Specification.
Auto-negotiation allows unshielded twisted pair (UTP) link partners to select the highest common mode of operation.
During auto-negotiation, link partners advertise capabilities across the UTP link to each other and then compare their own
capabilities with those they received from their link partners. The highest speed and duplex setting that is common to the
two link partners is selected as the mode of operation.
The following list shows the speed and duplex operation mode from highest to lowest priority.
• Priority 1: 100Base-TX, full-duplex
• Priority 2: 100Base-TX, half-duplex
• Priority 3: 10Base-T, full-duplex
• Priority 4: 10Base-T, half-duplex
If auto-negotiation is not supported or the KSZ8081MNX/RNB link partner is forced to bypass auto-negotiation, then the
KSZ8081MNX/RNB sets its operating mode by observing the signal at its receiver. This is known as parallel detection,
which allows the KSZ8081MNX/RNB to establish a link by listening for a fixed signal protocol in the absence of the auto-
negotiation advertisement protocol.
Auto-negotiation is enabled by either hardware pin strapping (NWAYEN, Pin 42) or software (Register 0h, Bit [12]).
By default, auto-negotiation is enabled after power-up or hardware reset. After that, auto-negotiation can be enabled or
disabled by Register 0h, Bit [12]. If auto-negotiation is disabled, the speed is set by Register 0h, Bit [13], and the duplex is
set by Register 0h, Bit [8].
The auto-negotiation link-up process is shown in Figure 1.