Datasheet

554
6120F–ATARM–03-Oct-06
AT91SAM7X512/256/128 Preliminary
A valid pause frame is defined as having a destination address that matches either the address
stored in specific address register 1 or matches 0x0180C2000001 and has the MAC control
frame type ID of 0x8808 and the pause opcode of 0x0001. Pause frames that have FCS or other
errors are treated as invalid and are discarded. Valid pause frames received increment the
Pause Frame Received statistic register.
The pause time register decrements every 512 bit times (i.e., 128 rx_clks in nibble mode)
once transmission has stopped. For test purposes, the register decrements every rx_clk cycle
once transmission has stopped if bit 12 (retry test) is set in the network configuration register. If
the pause enable bit (13) is not set in the network configuration register, then the decrementing
occurs regardless of whether transmission has stopped or not.
An interrupt (13) is asserted whenever the pause time register decrements to zero (assuming it
is enabled in the interrupt mask register).
37.3.4 Receive Block
The receive block checks for valid preamble, FCS, alignment and length, presents received
frames to the DMA block and stores the frames destination address for use by the address
checking block. If, during frame reception, the frame is found to be too long or rx_er is asserted,
a bad frame indication is sent to the DMA block. The DMA block then ceases sending data to
memory. At the end of frame reception, the receive block indicates to the DMA block whether the
frame is good or bad. The DMA block recovers the current receive buffer if the frame was bad.
The receive block signals the register block to increment the alignment error, the CRC (FCS)
error, the short frame, long frame, jabber error, the receive symbol error statistics and the length
field mismatch statistics.
The enable bit for jumbo frames in the network configuration register allows the EMAC to receive
jumbo frames of up to 10240 bytes in size. This operation does not form part of the IEEE802.3
specification and is disabled by default. When jumbo frames are enabled, frames received with a
frame size greater than 10240 bytes are discarded.
37.3.5 Address Checking Block
The address checking (or filter) block indicates to the DMA block which receive frames should
be copied to memory. Whether a frame is copied depends on what is enabled in the network
configuration register, the state of the external match pin, the contents of the specific address
and hash registers and the frame’s destination address. In this implementation of the EMAC, the
frame’s source address is not checked. Provided that bit 18 of the Network Configuration regis-
ter is not set, a frame is not copied to memory if the EMAC is transmitting in half duplex mode at
the time a destination address is received. If bit 18 of the Network Configuration register is set,
frames can be received while transmitting in half-duplex mode.
Ethernet frames are transmitted a byte at a time, least significant bit first. The first six bytes (48
bits) of an Ethernet frame make up the destination address. The first bit of the destination
address, the LSB of the first byte of the frame, is the group/individual bit: this is One for multicast
addresses and Zero for unicast. The All Ones address is the broadcast address, and a special
case of multicast.
The EMAC supports recognition of four specific addresses. Each specific address requires two
registers, specific address register bottom and specific address register top. Specific address
register bottom stores the first four bytes of the destination address and specific address register
top contains the last two bytes. The addresses stored can be specific, group, local or universal.