3. When the TOKEN is complete, USBFS updates the BDT and, if KEEP=0, changes
the OWN bit to 0.
4. The STAT register is updated and the TOK_DNE interrupt is set.
5. When the processor processes the TOK_DNE interrupt, it reads from the status
register all the information needed to process the endpoint.
6. At this point, the processor allocates a new BD so that additional USB data can be
transmitted or received for that endpoint, and then processes the last BD.
The following figure shows a timeline of how a typical USB token is processed after the
BDT is read and OWN=1.
SETUP TOKEN
DATA
ACK
USB RST
SOF
IN TOKEN
DATA
ACK
OUT TOKEN
DATA
ACK
USB_RST
Interrupt Generated
USB Host
Function
TOK_DNE
Interrupt Generated
SOF Interrupt Generated
TOK_DNE
Interrupt Generated
TOK_DNE
Interrupt Generated
Figure 42-4. USB token transaction
The USB has two sources for the DMA overrun error:
Memory Latency
The memory latency may be too high and cause the receive FIFO to overflow. This is
predominantly a hardware performance issue, usually caused by transient memory access
issues.
Oversized Packets
The packet received may be larger than the negotiated MaxPacket size. Typically, this is
caused by a software bug. For DMA overrun errors due to oversized data packets, the
USB specification is ambiguous. It assumes correct software drivers on both sides.
NAKing the packet can result in retransmission of the already oversized packet data.
Therefore, in response to oversized packets, the USB core continues ACKing the packet
for non-isochronous transfers.
Programmers interface
K22F Sub-Family Reference Manual , Rev. 3, 7/2014
998
Freescale Semiconductor, Inc.
Содержание MK22FN256VDC12
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