NXP Semiconductors LPC24XX UM10237 - Manual (Page 382)

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User manual Rev. 04 — 26 August 2009 382 of 792
NXP Semiconductors
UM10237
Chapter 13: LPC24XX USB device controller
The isochronous packet size is stored in memory as shown in Figure 13–49 . Each word in
the packet size memory shown is divided into fields: Frame_number (bits 31 to 17),
Packet_valid (bit 16), and Packet_length (bits 15 to 0). The space allocated for the packet
size memory for a given DD should be DMA_buffer_length words in size – one word for
each packet to transfer.
OUT endpoints
At the completion of each frame, the packet size is written to the address location in
Isochronous_packet_size_memory_address, and
Isochronous_packet_size_memory_address is incremented by 4.
IN endpoints
Only the Packet_length field of the isochronous packet size word is used. For each frame,
an isochronous data packet of size specified by this field is transferred from the USB
device to the host, and Isochronous_packet_size_memory_address is incremented by 4
at the end of the packet transfer. If Packet_length is zero, an empty packet will be sent by
the USB device.
14.6.4 DMA descriptor completion
DDs for isochronous endpoints can only end with a status code of NormalCompletion
since there is no short packet on Isochronous endpoints, and the USB transfer continues
indefinitely until a SystemError occurs. There is no DataOverrun detection for isochronous
endpoints.
14.6.5 Isochronous OUT Endpoint Operation Example
Assume that an isochronous endpoint is programmed for the transfer of 10 frames and
that the transfer begins when the frame number is 21. After transferring four frames with
packet sizes of 10,15, 8 and 20 bytes without errors, the descriptor and memory map
appear as shown in
Figure 13–49 .
The_total_number_of_bytes_transferred = 0x0A + 0x0F + 0x08 + 0x14 = 0x35.
The Packet_valid bit (bit 16) of all the words in the packet length memory is set to 1.