Datasheet

KSZ8795CLX
DS00002112B-page 24  2016-2017 Microchip Technology Inc.
3.4.5.1 LPI Signaling
LPI signaling allows switch to indicate to the PHY, and to the link partner, that a break in the data stream is expected,
and switch can use this information to enter power-saving modes that require additional time to resume normal opera-
tion. LPI signaling also informs the switch when the link partner has sent such an indication. The definition of LPI sig-
naling uses of 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 MAC MII interface. The switch is also informed when the link partner is signaling LPI, indication of LPI activation
(LPI indication) on the MAC interface. The conditions under which switch decides to send LPI, and what actions are
taken by switch when it receives LPI from the link partner, are specified in implementation section.
3.4.5.2 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 stop the MAC from transmitting normal
traffic. Once the LPI request is de-asserted, the LPI assert function starts to transmit the normal inter-frame encoding
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 3-6 illustrates the EEE LPI between two active data idles.
3.4.5.3 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_IDLE indication is de-asserted and the normal decoding operation resumes.
3.4.5.4 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 periodically enabled in order to transmit refresh signals that are used by the
link partner to update adaptive filters and timing circuits. This maintains 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 transmission are used to maintain link and the quiet periods are used for
the power saving. Approximately, every 20 ms to 22 ms a refresh of 200 µs to 220 µs is sent to the link partner. The
refresh transmission and quiet periods are shown in Figure 3-6.