Information

Universal Serial Bus Interface
MPC8308 PowerQUICC II Pro Processor Reference Manual, Rev. 1
Freescale Semiconductor 13-141
13.8.3.6 Isochronous Endpoint Operational Model
Isochronous endpoints are used for real-time scheduled delivery of data and their operational model is
significantly different than the host throttled Bulk, Interrupt, and Control data pipes. Real time delivery by
the USB_DR will is accomplished by the following:
• Exactly MULT Packets per (micro)Frame are transmitted/received. Note that MULT is a two-bit
field in the device Queue Head. The variable length packet protocol is not used on isochronous
endpoints.
• NAK responses are not used. Instead, zero length packets and sent in response to an IN request to
an unprimed endpoints. For unprimed RX endpoints, the response to an OUT transaction is to
ignore the packet within the device controller.
• Prime requests always schedule the transfer described in the dTD for the next (micro)frame. If the
ISO-dTD is still active after that frame, then the ISO-dTD is held ready until executed or canceled
by the DCD.
The USB_DR in host mode uses the periodic frame list to schedule data exchanges to Isochronous
endpoints. The operational model for device mode does not use such a data structure. Instead, the same
dTD used for Control/Bulk/Interrupt endpoints is also used for isochronous endpoints. The difference is
in the handling of the dTD.
The first difference between bulk and ISO-endpoints is that priming an ISO-endpoint is a delayed
operation such that an endpoint will become primed only after a SOF is received. After the DCD writes
the prime bit, the prime bit is cleared as usual to indicate to software that the device controller completed
a priming the dTD for transfer. Internal to the design, the device controller hardware masks that prime start
until the next frame boundary. This behavior is hidden from the DCD but occurs so that the device
controller can match the dTD to a specific (micro)frame.
Another difference with isochronous endpoints is that the transaction must wholly complete in a
(micro)frame. Once an ISO transaction is started in a (micro)frame it will retire the corresponding dTD
when MULT transactions occur or the device controller finds a fulfillment condition.
Ping
STALL NAK ACK N/A N/A N/A
Invalid
Ignore Ignore Ignore Ignore Ignore Ignore
1
SYSERR—System error should never occur when the latency FIFOs are correctly sized and
the DCD is responsive.
2
Force Bit Stuff Error.
3
NYET/ACK—NYET unless the Transfer Descriptor has packets remaining according to the
USB variable length protocol then ACK.
Table 13-89. Control Endpoint Bus Response Matrix (continued)
Token
Type
Endpoint State
Setup
Lockout
Stall
Not
Primed
Primed Underflow Overflow