The USBD peripheral implements isochronous (ISO) endpoints.
The ISO endpoints have a fixed USB endpoint number, summarized in the following table.
| ISO endpoint # | USB IN endpoint | USB OUT endpoint |
|---|---|---|
| [0] | 0x88 | 0x08 |
An isochronous transaction consists of a single, non-acknowledged Data stage. The host sends out a start of frame at a regular interval (1 ms), and data follows IN or OUT tokens within each frame.
EasyDMA allows transferring ISO data directly from and to memory. EasyDMA transfers must be initiated by the software, which can synchronize with the SOF (start of frame) events.
Because the timing of the start of frame is very accurate, the SOF event can be used for jobs such as synchronizing a local timer through the SOF event and PPI. The SOF event gets synchronized to the 16 MHz clock prior to being made available to the PPI.
Every start of frame increments a free-running counter, which can be read by software through the FRAMECNTR register.
Each IN or OUT ISO data endpoint has to be explicitly enabled by software through register EPINEN or EPOUTEN, according to the configuration declared by the USB device and selected by the USB host through the SetConfig command. A disabled ISO IN data endpoint will not respond to any traffic from the USB host. A disabled ISO OUT data endpoint will ignore any incoming traffic from the host.
The USBD peripheral has an internal 1 kB buffer associated with ISO endpoints. The user can either allocate the full amount to the IN or the OUT endpoint, or split the buffer allocation between the two using register ISOSPLIT.
The internal buffer also sets the maximum size of the ISO OUT and ISO IN transfers: 1023 bytes when the full buffer is dedicated to either ISO OUT or ISO IN, and half when the buffer is split between the two.