NXP Semiconductors K22F series - Page 1081

1. USBFS reads the BDT.
2. The SIE transfers the data via the DMA to or from the buffer pointed to by the
ADDR field of the BD.
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 43-7. 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.
Chapter 43 Universal Serial Bus Full Speed OTG Controller (USBFSOTG)
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
NXP Semiconductors 1081