SAM7X256_14 Datasheet, PDF (538/662 Pages) ATMEL Corporation – ARM-based Flash MCU

Table 37-1. Receive Buffer Descriptor Entry (Continued)
Bit
Function
14
Start of frame - when set the buffer contains the start of a frame. If both bits 15 and 14 are set, then the buffer contains a
whole frame.
13:12
Receive buffer offset - indicates the number of bytes by which the data in the first buffer is offset from the word address.
Updated with the current values of the network configuration register. If jumbo frame mode is enabled through bit 3 of the
network configuration register, then bits 13:12 of the receive buffer descriptor entry are used to indicate bits 13:12 of the
frame length.
11:0 Length of frame including FCS (if selected). Bits 13:12 are also used if jumbo frame mode is selected.
To receive frames, the buffer descriptors must be initialized by writing an appropriate address to bits 31 to 2 in the first
word of each list entry. Bit zero must be written with zero. Bit one is the wrap bit and indicates the last entry in the list.
The start location of the receive buffer descriptor list must be written to the receive buffer queue pointer register before
setting the receive enable bit in the network control register to enable receive. As soon as the receive block starts writing
received frame data to the receive FIFO, the receive buffer manager reads the first receive buffer location pointed to by
the receive buffer queue pointer register.
If the filter block then indicates that the frame should be copied to memory, the receive data DMA operation starts writing
data into the receive buffer. If an error occurs, the buffer is recovered. If the current buffer pointer has its wrap bit set or is
the 1024^th descriptor, the next receive buffer location is read from the beginning of the receive descriptor list. Otherwise,
the next receive buffer location is read from the next word in memory.
There is an 11-bit counter to count out the 2048 word locations of a maximum length, receive buffer descriptor list. This is
added with the value originally written to the receive buffer queue pointer register to produce a pointer into the list. A read
of the receive buffer queue pointer register returns the pointer value, which is the queue entry currently being accessed.
The counter is reset after receive status is written to a descriptor that has its wrap bit set or rolls over to zero after 1024
descriptors have been accessed. The value written to the receive buffer pointer register may be any word-aligned
address, provided that there are at least 2048 word locations available between the pointer and the top of the memory.
Section 3.6 of the AMBA^™ 2.0 specification states that bursts should not cross 1K boundaries. As receive buffer manager
writes are bursts of two words, to ensure that this does not occur, it is best to write the pointer register with the least three
significant bits set to zero. As receive buffers are used, the receive buffer manager sets bit zero of the first word of the
descriptor to indicate used. If a receive error is detected the receive buffer currently being written is recovered. Previous
buffers are not recovered. Software should search through the used bits in the buffer descriptors to find out how many
frames have been received. It should be checking the start-of-frame and end-of-frame bits, and not rely on the value
returned by the receive buffer queue pointer register which changes continuously as more buffers are used.
For CRC errored frames, excessive length frames or length field mismatched frames, all of which are counted in the
statistics registers, it is possible that a frame fragment might be stored in a sequence of receive buffers. Software can
detect this by looking for start of frame bit set in a buffer following a buffer with no end of frame bit set.
For a properly working Ethernet system, there should be no excessively long frames or frames greater than 128 bytes
with CRC/FCS errors. Collision fragments are less than 128 bytes long. Therefore, it is a rare occurrence to find a frame
fragment in a receive buffer.
If bit zero is set when the receive buffer manager reads the location of the receive buffer, then the buffer has already
been used and cannot be used again until software has processed the frame and cleared bit zero. In this case, the DMA
block sets the buffer not available bit in the receive status register and triggers an interrupt.
If bit zero is set when the receive buffer manager reads the location of the receive buffer and a frame is being received,
the frame is discarded and the receive resource error statistics register is incremented.
A receive overrun condition occurs when bus was not granted in time or because HRESP was not OK (bus error). In a
receive overrun condition, the receive overrun interrupt is asserted and the buffer currently being written is recovered.
The next frame received with an address that is recognized reuses the buffer.
If bit 17 of the network configuration register is set, the FCS of received frames shall not be copied to memory. The frame
length indicated in the receive status field shall be reduced by four bytes in this case.
SAM7X Series [DATASHEET]
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