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USB Controller Host and Peripheral Modes Operation | www.ti.com |
∙End of queue (EOQ) (only valid on EOP)
∙Packet Length (only valid with SOP)
Transmit buffer descriptors contain 16 bytes (4 words) and must begin on
Four Words of Transmit Buffer Descriptor are described below.
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| Table 6. Transmit Buffer Descriptor Word 0 |
Bits | Name | Description |
31:0 | Next Descriptor | The |
| Pointer | mechanism used to reference the next buffer descriptor from the current buffer descriptor. If the |
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| value of this pointer is zero then the current buffer is the last buffer in the queue. The software sets |
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| the Next Descriptor Pointer. |
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| Table 7. Transmit Buffer Descriptor Word 1 |
Bits | Name | Description |
31:0 | Buffer Pointer | The Buffer Pointer is the byte aligned memory address of the buffer associated with the buffer |
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| descriptor. The software sets the Buffer Pointer. |
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| Table 8. Transmit Buffer Descriptor Word 2 |
Bits | Name | Description |
31:16 | Buffer Offset | The Buffer Offset indicates how many unused bytes are at the start of the buffer (SOP buffers only). |
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| A value of zero indicates that there are no unused bytes at the start of the buffer and that valid data |
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| begins on the first byte of the buffer. A value of 000Fh (decimal 15) indicates that the first 15 bytes |
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| of the buffer are to be ignored by the DMA controller while transmitting and that valid buffer data |
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| starts on byte 16 of the buffer. The Buffer Offset is valid only on Start Of Packet buffer descriptors |
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| and must be zero otherwise. The software sets the Buffer Offset. The Buffer Offset must be less |
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| than the Buffer Length. |
15:0 | Buffer Length | The Buffer Length field indicates how many valid data bytes are in the buffer. Unused or protocol |
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| specific bytes at the beginning of the buffer are not counted in the Buffer Length field. The software |
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| sets the Buffer Length. |
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| Table 9. Transmit Buffer Descriptor Word 3 |
Bits | Name | Value | Description |
31 | SOP |
| Start of Packet: SOP Indicates that the descriptor buffer is the first buffer in the packet. The |
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| software sets the SOP bit. |
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| 0 | Not start of packet buffer |
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| 1 | Start of packet buffer |
30 | EOP |
| End of Packet: EOP Indicates that the descriptor buffer is the last buffer in the packet. The |
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| software sets the EOP bit. It is valid to set both SOP and EOP in the same descriptor. |
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| 0 | Not end of packet buffer |
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| 1 | End of packet buffer |
29 | Ownership |
| The Ownership bit indicates ownership of the DMA packet and is valid only on SOP. This bit |
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| is set by the software and cleared by the DMA controller when the packet has been |
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| transmitted. The software can use this bit to reclaim buffers. |
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| 0 | The packet is owned by the host processor |
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| 1 | The packet is owned by the DMA controller |
58 | Universal Serial Bus (USB) Controller |