DS3134 datasheet(105/203 Pages) DALLAS

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DS3134

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8.1.3 FREE QUEUE

The Host will write to the Receive Free Queue, the 32-bit addresses of the available (i.e. free) data buffers

and their associated Packet Descriptors. The descriptor space is indicated via a 16-bit pointer which the

DMA will use along with the Receive Packet Descriptor Base Address to find the exact 32-bit address of

the associated Receive Packet Descriptor.

Receive Free Queue Descriptor Figure 8.1.3A

dword 0

Free Data Buffer Address (32)

dword 1

Unused (16)

Free Packet Descriptor Pointer (16)

Note: The organization of the Free Queue is not affected by the enabling of Big Endian

dword 0; Bits 0 to 31 / Data Buffer Address. Direct 32-bit starting address of a free data buffer.

dword 1; Bits 0 to 15 / Free Packet Descriptor Pointer. This 16-bit value is the offset from the

Receive Descriptor Base Address of the free descriptor space associated with the free data buffer in dword

0. Note: This is an index not an absolute address.

dword 1; Bits 16 to 31 / Unused. Not used by the DMA. Can be set to any value by the Host and will

be ignored by the Receive DMA.

The Receive DMA will read from the Receive Free Queue Descriptor circular queue which data buffers

and their associated descriptors are available for use by the DMA.

The Receive Free Queue Descriptor is actually a set of two circular queues. See Figure 8.1.3B. There is

one circular queue that indicates where free large buffers and their associated free descriptors exist and

there is another circular queue that indicates where free small buffers and their associated free descriptors

exist.

Large and Small Buffer Size Handling

Via the Receive Configuration RAM Buffer Size field, the DMA knows for a particular HDLC channel,

whether the incoming packets should be stored in the large or the small free data buffers. The Host

informs the DMA of the size of both the large and small buffers via the Receive Large and Small Buffer

Size (RLBS/RSBS) registers. For example, when the DMA knows that data is ready to be written onto

the PCI Bus, it checks to see if the data is to be sent to a large buffer or a small buffer and then it goes to

the appropriate Free Queue Descriptor and pulls the next available free buffer address and free descriptor

pointer. If the Host wishes to have only one buffer size, then the Receive Free Queue Small Buffer Start

Address will be set equal to the Receive Free Queue End Address and in the Receive Configuration

RAM, none of the active HDLC channels will be configured for the small buffer size.

To keep track of the addresses of the dual circular queues in the Receive Free Queue, there are a set of

internal addresses within the device that are accessed by both the Host and the DMA. On initialization,

the Host will configure all of the registers shown in Table 8.1.3B. After initialization, the DMA will only

write to (i.e. change) the read pointers and the Host will only write to the write pointers.

DS3134 Datasheet(PDF) 105 Page - Dallas Semiconductor