KSZ8775CLX Datasheet, PDF (25/132 Pages) Microchip Technology – Integrated 5–Port 10/100 Managed Ethernet Switch with Port 4 RMII and Port 5 RGMII/MII/RMII Interfaces

KSZ8775CLX
3.5.2.5.1
LPI Signaling
Low power idle signaling allows the switch to indicate to the PHY, and to the link partner, that a break in the data stream
is expected. The switch can use this information to enter power-saving modes that require additional time to resume
normal operation. LPI signaling also informs the switch when the link partner has sent such an indication. The definition
of LPI signaling uses the MAC for simplified full-duplex operation with carrier sense deferral. This provides full-duplex
operation but uses the carrier sense signal to defer transmission when the PHY is in the LPI mode.
The decision on when to signal LPI (LPI request) to the link partner is made by the switch and communicated to the PHY
through the MAC MII interface. The switch is also informed when the link partner is signaling LPI and indicating LPI acti-
vation (LPI indication) on the MAC interface. The conditions under which the switch decides to send LPI and what
actions are taken by the switch when it receives LPI from the link partner, are specified in the implementation section.
3.5.2.5.2
LPI Assertion
Without LPI assertion, the normal traffic transition continues on the MII interface. As soon as an LPI request is asserted,
the LPI assert function starts to transmit the “Assert LPI” encoding on the MII and stops the MAC from transmitting nor-
mal traffic. Once the LPI request is de-asserted, the LPI assert function starts to transmit the normal inter-frame encod-
ing on the MII again. After a delay, the MAC is allowed to start transmitting again. This delay is provided to allow the link
partner to prepare for normal operation. Figure 8 illustrates the EEE LPI between two active data idles.
3.5.2.5.3
LPI Detection
In the absence of “Assert LPI” encoding on the receive MII, the LPI detect function maps the receive MII signals as nor-
mal conditions. At the start of LPI, indicated by the transition from normal inter-frame encoding to the “Assert LPI” encod-
ing on the receive MII, the LPI detect function continues to indicate idle on interface and asserts LP_IDLE indication. At
the end of LPI, indicated by the transition from the “Assert LPI” encoding to any other encoding on the receive MII, LP_I-
DLE indication is de-asserted and the normal decoding operation resumes.
3.5.2.5.4
PHY LPI Transmit Operation
When the PHY detects the start of “Assert LPI” encoding on the MII, the PHY signals sleep to its link partner to indicate
that the local transmitter is entering LPI mode. The EEE capability requires the PHY transmitter to go quiet after sleep
is signaled. LPI requests are passed from one end of the link to the other and system energy savings can be achieved
even if the PHY link does not go into a low power mode.
The transmit function of the local PHY is enabled periodically to transmit refresh signals that are used by the link partner
to update adaptive filters and timing circuits in order to maintain link integrity. This quiet-refresh cycle continues until the
reception of the normal inter-frame encoding on the MII. The transmit function in the PHY communicates this to the link
partner by sending a wake signal for a predefined period of time. The PHY then enters the normal operating state. No
data frames are lost or corrupted during the transition to or from the LPI mode.
In 100BT/full-duplex EEE operation, refresh transmissions are used to maintain the link and the quiet periods are used
for power saving. Approximately every 20-22 ms a refresh of 200-220 µs is sent to the link partner. The refresh trans-
mission and quiet periods are shown in Figure 3-6.
FIGURE 3-6:
ACTIVE
TRAFFIC ACTIVITY AND EEE LPI OPERATIONS
LOW-POWER
ACTIVE
QUIET
QUIET
TS
TQ
TR
QUIET
TW_PHY
TW_SYSTEM
Ts = THE PERIOD OF TIME THAT THE PHY TRANSMITS THE SLEEP SIGNAL BEFORE TURNING ALL TRANSMITTERS OFF, 200 ≤ Ts ≤ 220 USED IN 100BASE-TX.
Tq = THE PERIOD OF TIME THAT THE PHY REMAINS QUIET BEFORE SENDING THE REFRESH SIGNAL, 20_000 ≤ Tq ≤ 22_000 USED IN 100BASE-TX.
Tr = DURATION OF THE REFRESH SIGNAL, 200 ≤ Tr ≤ 220 USED IN 100BASE-TX.
 2015 Microchip Technology Inc.
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