DS3134 datasheet(104/203 Pages) DALLAS

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DS3134

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Receive Packet Descriptors Figure 8.1.2B

dword 0

Data Buffer Address (32)

dword 1

BUFS (3)

Byte Count (13)

Next Descriptor Pointer (16)

dword 2

Timestamp (24)

HDLC Channel (8)

dword 3

unused (32)

Note: The 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 receive 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.

Note: This is an index, not absolute address.

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 will determine the buffer offset (if any) via the Buffer Offset field in the Receive DMA

Configuration RAM (see Section 8.1.5).

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 7 / HDLC Channel Number. HDLC channel number, which can be from 1 to 256.

00000000 (00h) = HDLC Channel Number 1

11111111 (FFh) = HDLC Channel Number 256

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 33 MHz PCLK, the timestamp will increment every 485 ns and will

rollover every 8.13 seconds. For a 25 MHz clock, the timestamp will increment every 640 ns and will

rollover every 10.7 seconds. The host can calculate the difference in arrival times of packets 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 types of applications, the Host should not use dword 3 of the

Receive Packet Descriptor.

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