not know where to send them. The solution to this problem is to use a broadcast address. A directed
broadcast has a
valid network address and a broadcast host address. Network routers and nodes
forward directed broadcasts to the appropriate network, where it is seen as a
MAC-level broadcast
and detected by the powered-off
client.
19.4.5.1
EMAC
WOL Support
EMAC enters WOL mode when the wake-up bit
in
Mode Register 0 is set (EMACO_MRO[WKE] = 1).
WOL mode should only be changed while the receive MAC idle bit
in
Mode Register 0 is set
(EMACO_MRO[RXI] = 1) and the receive MAC enable bit is reset (EMACO_MRO[RXE] = 0). After the
wake-up
enable bit
is
set (EMACO_MRO[WKE] = 1), the receive MAC enable bit can be set
(EMACO_MRO[RXE] = 1).
A reset (soft
or
hard) should be issued before programming EMAC to WOL mode. When programmed
to
WOL mode, EMAC does not propagate any received packet to MAL. Also, EMAC transmit channels
do not request data from MAL.
19.5 Flow Control
EMAC implements full-duplex flow control for efficient system performance by handling specific MAC
control packets contained
in
the pause opcode. EMAC supports flow control as defined in the IEEE
802.3x-1997 standard.
19.5.1 MAC Control Packet
The flow control mechanism allows the Receive FIFO control logic to automatically notify the node
transmitting packets to suspend its transmission for a defined period of time. The pause
control
packet has a fixed length defined as follows:
MinFrameSize-32 bits (60 bytes)
MAC
control packets have a unique value of Ox8808
in
the length/type field, and share the same
packet format as
normal Ethernet packets, except that the data field consists of
an
opcode field and a
parameter
field. The opcode field contains an opcode command and the parameter field contains a
value associated with the opcode command (Ox0001). The only opcode command defined by IEEE
802.3x
is the Pause opcode; the parameter field for the Pause opcode defines the pause time. MAC
control
packets with the Pause opcode, also called pause packets, can have a destination address
equal to a reserved multicast address, or can be the address of the receive station itself. The
reserved
multicast address is Ox0180C2000001.
Preliminary Ethernet Media Access Controller 19-15
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