Isochronous endpoints implement double-buffering
to ease application software development, using both ‘transmission’ and ‘reception’ packet
memory areas to manage buffer swapping on each successful transaction in order to have
always a complete buffer to be used by the application, while the USB peripheral fills the
other.
The memory buffer which is currently used by the USB peripheral is defined by the DTOG
bit related to the endpoint direction (DTOG_RX for ‘reception’ isochronous endpoints,
DTOG_TX for ‘transmission’ isochronous endpoints, both in the related USB_EPnR
register) according to Table 171.
同步模式下，必须使用双缓存
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