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Intel® PXA255 Processor Developer’s Manual 12-7
USB Device Controller
12.3.5.2 Isochronous Transaction Type
Isochronous transactions ensure constant rate, error-tolerant transmission of data between the host
and UDC. The host schedules isochronous packets during every frame. USB protocol allows
isochronous transfers to take up to 90% of the USB bandwidth. Unlike bulk transactions, if
corrupted data is received, the UDC will continue to process the corrupted data that corresponds to
the current start of frame indicator. Isochronous transactions do not support a handshake phase or
retry capability. Two packet types are used to construct isochronous transactions: token and data.
The types of isochronous transactions based on data direction are shown in Table 12-8.
12.3.5.3 Control Transaction Type
The host uses control transactions to configure endpoints and query their status. Like bulk
transactions, control transactions begin with a setup packet, followed by an optional data packet,
then a handshake packet. Control transactions, by default, use DATA0 type transfers. Table 12-9
shows the four types of control transactions.
Table 12-7. Bulk Transaction Formats
Action Token Packet Data Packet Handshake Packet
Host successfully received data from UDC IN DATA0/DATA1 ACK
UDC temporarily unable to transmit data IN
None NAK
UDC endpoint needs host intervention IN
None STALL
Host detected PID, CRC, or bit stuff error IN DATA0/DATA1
None
UDC successfully received data from host OUT DATA0/DATA1 ACK
UDC temporarily unable to receive data OUT DATA0/DATA1 NAK
UDC endpoint needs host intervention OUT DATA0/DATA1 STALL
UDC detected PID, CRC, or bit stuff error OUT DATA0/DATA1
None
Packets from UDC to host are boldface
Table 12-8. Isochronous Transaction Formats
Action Token Packet Data Packet
Host received data from UDC IN DATA0
UDC received data from host OUT DATA0
Packets from UDC to host are boldface
Table 12-9. Control Transaction Formats
Action Token Packet Data Packet Handshake Packet
UDC successfully received control from host SETUP DATA0 ACK
UDC temporarily unable to receive data SETUP DATA0 NAK
UDC endpoint needs host intervention SETUP DATA0 STALL
UDC detected PID, CRC, or bit stuff error SETUP DATA0
None
Packets from UDC to host are boldface