Rainbow Electronics DS3131 - Manual (Page 87)

DS3131
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9.2.4 Done Queue
The DMA writes to the receive done queue when it has filled a free data buffer with packet data and has
loaded the associated packet descriptor with all the necessary information. The descriptor location is
indicated through a 16-bit pointer that the host uses with the receive descriptor base address to find the
exact 32-bit address of the associated receive descriptor.
Figure 9-7. Receive Done-Queue Descriptor
dword 0
V EOF Status(3) BUFCNT(3) 00b HDLC CH#(6) Descriptor Pointer (16)
Note: The organization of the done queue is not affected by the enabling of Big Endian.
dword 0; Bits 0 to 15/Descriptor Pointer. This 16-bit value is the offset from the receive descriptor base
address of a receive packet descriptor that has been readied by the DMA and is available for the host to begin
processing.
dword 0; Bits 16 to 21/HDLC Channel Number. HDLC channel number, which can be from 1 to 40.
000000 (00h) = HDLC channel number 1
100111 (27h) = HDLC channel number 40
dword 0; Bits 22 to 23/Unused. Set to 0 by the DMA.
dword 0; Bits 24 to 26/Buffer Count (BUFCNT). If an HDLC channel has been configured to only write to
the done queue after a packet has been completely received (i.e., the threshold field in the receive DMA
configuration RAM is set to 000), then BUFCNT is always set to 000. If the HDLC channel has been
configured through the threshold field to write to the done queue after a programmable number of buffers
(from 1 to 7) has been filled, then BUFCNT corresponds to the number of buffers that have been written to
host memory. The BUFCNT is less than the threshold field value when the incoming packet does not require
the number of buffers specified in the threshold field.
000 = indicates that a complete packet has been received (only used when threshold = 000)
001 = 1 buffer has been filled
010 = 2 buffers have been filled
111 = 7 buffers have been filled
dword 0; Bits 27 to 29/Packet Status. These three bits report the final status of an incoming packet. They
are only valid when the EOF bit is set to 1 (EOF = 1).
000 = no error, valid packet received
001 = receive FIFO overflow (remainder of the packet discarded)
010 = CRC checksum error
011 = HDLC frame abort sequence detected (remainder of the packet discarded)
100 = nonaligned byte count error (not an integral number of bytes)
101 = long frame abort (max packet length exceeded; remainder of the packet discarded)
110 = PCI abort or parity data error (remainder of the packet discarded)
111 = reserved state (never occurs in normal device operation)
dword 0; Bit 30/End of Frame (EOF). This bit is set to 1 when this receive descriptor is the last one in the
current descriptor chain. This indicates that a packet has been fully received or an error has been detected,
which has caused a premature termination.
dword 0; Bit 31/Valid Done-Queue Descriptor (V). This bit is set to 0 by the receive DMA. Instead of
reading the receive done queue read pointer to locate completed done queue descriptors, the host can use this
bit since the DMA sets the bit to 0 when it is written into the queue. If the latter scheme is used, the host must
set this bit to 1 when the done queue descriptor is read.