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UM10316_0
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 00.06 — 17 December 2008
197 of 571
NXP Semiconductors
UM10316
Chapter 13: LPC29xx USB device
DDs are placed in the USB RAM. These descriptors can be located anywhere in the USB
RAM at word-aligned addresses. USB RAM is part of the system memory that is used for
the USB purposes. It is located at address 0x7FD0 0000 and is 8 kB in size.
DDs for non-isochronous endpoints are four words long. DDs for isochronous endpoints
are five words long.
The parameters associated with a DMA transfer are:
•
The start address of the DMA buffer
•
The length of the DMA buffer
•
The start address of the next DMA descriptor
•
Control information
•
Count information (number of bytes transferred)
•
Status information
lists the DMA descriptor fields.
Table 180. DMA descriptor
Word
position
Access
(H/W)
Access
(S/W)
Bit
position
Description
0
R
R/W
31:0
Next_DD_pointer (USB RAM address)
1
R
R/W
1:0
DMA_mode (00 -Normal; 01 - ATLE)
R
R/W
2
Next_DD_valid (1 - valid; 0 - invalid)
-
-
3
Reserved
R
R/W
4
Isochronous_endpoint (1 - isochronous;
0 - non-isochronous)
R
R/W
15:5
Max_packet_size
R/W
31:16
DMA_buffer_length
This value is specified in bytes for non-isochronous
endpoints and in number of packets for isochronous
endpoints.
2
R/W
R/W
31:0
DMA_buffer_start_addr