at91sam9g20-cu ATMEL Corporation, at91sam9g20-cu Datasheet - Page 623

• Manufacturer Part Number: at91sam9g20-cu
• Description: At91 Arm Thumb Microcontrollers
• Manufacturer: ATMEL Corporation

36.3.2.1
36.3.2.2
Table 36-1.
6384B–ATARM–15-Dec-08
Bit
31:2
31
30
29
1
0
Address of beginning of buffer
Wrap - marks last descriptor in receive buffer descriptor list.
Ownership - needs to be zero for the EMAC to write data to the receive buffer. The EMAC sets this to one once it has
successfully written a frame to memory.
Software has to clear this bit before the buffer can be used again.
Global all ones broadcast address detected
Multicast hash match
Unicast hash match
Receive Buffers
FIFO
Receive Buffer Descriptor Entry
The FIFO depths are 1
system clock speed, memory latency and network speed.
Data is typically transferred into and out of the FIFOs in bursts of four words. For receive, a bus
request is asserted when the FIFO contains four words and has space for 28 more. For transmit,
a bus request is generated when there is space for four words, or when there is space for 27
words if the next transfer is to be only one or two words.
Thus the bus latency must be less than the time it takes to load the FIFO and transmit or receive
three words (112 bytes) of data.
At 100 Mbit/s, it takes 8960 ns to transmit or receive 112 bytes of data. In addition, six master
clock cycles should be allowed for data to be loaded from the bus and to propagate through the
FIFOs. For a 133 MHz master clock this takes 45 ns, making the bus latency requirement 8915
ns.
Received frames, including CRC/FCS optionally, are written to receive buffers stored in mem-
ory. Each receive buffer is 128 bytes long. The start location for each receive buffer is stored in
memory in a list of receive buffer descriptors at a location pointed to by the receive buffer queue
pointer register. The receive buffer start location is a word address. For the first buffer of a
frame, the start location can be offset by up to three bytes depending on the value written to bits
14 and 15 of the network configuration register. If the start location of the buffer is offset the
available length of the first buffer of a frame is reduced by the corresponding number of bytes.
Each list entry consists of two words, the first being the address of the receive buffer and the
second being the receive status. If the length of a receive frame exceeds the buffer length, the
status word for the used buffer is written with zeroes except for the “start of frame” bit and the
offset bits, if appropriate. Bit zero of the address field is written to one to show the buffer has
been used. The receive buffer manager then reads the location of the next receive buffer and
fills that with receive frame data. The final buffer descriptor status word contains the complete
frame status. Refer to
5. Transmit buffer manager read
6. Transmit buffer manager write
Table 36-1
28
bytes for receive and 1
Word 0
Word 1
for details of the receive buffer descriptor list.
Function
AT91SAM9G20 Preliminary
28
bytes for transmit and are a function of the
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