UM10360 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
User manual Rev. 3 — 19 December 2013 269 of 841
NXP Semiconductors
UM10360
Chapter 11: LPC176x/5x USB device controller
11. Both B_1 and B_2 are empty, and the active buffer is B_2. The next packet written by
software will go into B_2.
In DMA mode, switching of the active buffer is handled automatically in hardware. For
Bulk IN endpoints, proactively filling an endpoint buffer to take advantage of the double
buffering can be accomplished by manually starting a packet transfer using the
USBDMARSet register.
11.16.2 Isochronous endpoints
For isochronous endpoints, the active data buffer is switched by hardware when the
FRAME interrupt occurs. The SIE Clear Buffer and Validate Buffer commands do not
cause the active buffer to be switched.
Double buffering allows the software to make full use of the frame interval writing or
reading a packet to or from the active buffer, while the packet in the other buffer is being
sent or received on the bus.
For an OUT isochronous endpoint, any data not read from the active buffer before the end
of the frame is lost when it switches.
For an IN isochronous endpoint, if the active buffer is not validated before the end of the
frame, an empty packet is sent on the bus when the active buffer is switched, and its
contents will be overwritten when it becomes active again.
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