Rainbow Electronics DS3131 - Manual (Page 81)

DS3131
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Figure 9-4. Receive Packet Descriptors
dword 0
Data Buffer Address (32)
dword 1
BUFS (3) Byte Count (13) Next Descriptor Pointer (16)
dword 2
Timestamp (24) 00b HDLC CH#(6)
dword 3
unused (32)
Note: T he organization of the receive descriptor is not affected by the enabling of Big Endian.
dword 0; Bits 0 to 31/Data Buffer Address. Direct 32-bit starting address of the data buffer that is associated
with this receives descriptor.
dword 1; Bits 0 to 15/Next Descriptor Pointer. This 16-bit value is the offset from the receive descriptor base
address of the next descriptor in the chain. Only valid if buffer status = 001 or 010.
dword 1; Bits 16 to 28/Byte Count. Number of bytes stored in the data buffer. Maximum is 8191 Bytes (0000h =
0 Bytes / 1FFFh = 8191 Bytes). This byte count does not include the buffer offset. The host determines the buffer
offset (if any) through the buffer offset field in the receive DMA configuration RAM (Section
9.2.5 ).
dword 1; Bits 29 to 31/Buffer Status. Must be one of the three states listed below.
001 = first buffer of a multiple buffer packet
010 = middle buffer of a multiple buffer packet
100 = last buffer of a multiple or single buffer packet (equivalent to EOF)
dword 2; Bits 0 to 5/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 2; Bits 6, 7/Unused. Set to 00b by the DMA.
dword 2; Bits 8 to 31/Timestamp. When each descriptor is written into memory by the DMA, this 24-bit
timestamp is provided to keep track of packet arrival times. The timestamp is based on the PCLK frequency
divided by 16. For a 33MHz PCLK, the timestamp increments every 485ns and rolls over every 8.13s. The host
can calculate the difference in packets’ arrival times by knowing the PCLK frequency and then taking the
difference in timestamp readings between consecutive packet descriptors.
dword 3; Bits 0 to 31/Unused. Not written to by the DMA. Can be used by the host. Application Note: dword 3
is used by the transmit DMA and, in store and forward applications, the receive and transmit packet descriptors
have been designed to eliminate the need for the host to groom the descriptors before transmission. In these type of
applications, the host should not use dword 3 of the receive packet descriptor.