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USB support is implemented using the STM32 USB Device library.
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The library is copied into the core project to make customizations easier to maintain
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across different ports. The USBD library supports all versions of the HAL and LL.
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GEX uses USB classes CDC/ACM and MSC/SCSI.
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The two classes are combined into a composite class with association descriptors.
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USB interrupts are processed by the USBD library and endpoint callbacks in the composite
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class are fired. To avoid race conditions (and because DAPlink did it the same way), the
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events are notified to the USB thread (TaskMain) which calls endpoint handlers in the
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corresponding class drivers.
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VFS is handled synchronously on the main thread. CDC messages (TinyFrame data) are queued
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and processed by the message queue thread. This makes it possible to query hardware
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(e.g. slow USART or NeoPixel) without stalling the USB communication. This arrangement
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also makes it possible to wait on a binary semaphore when sending data back to host. The
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semaphore is set from the CDC TxComplete callback and taken by the TinyFrame write
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function, serving as a form of flow control.
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--------------------------------------------
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COMM API:
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GEX supports alternate command interfaces.
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The active interface is set in the global variable gActiveComport
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Due to special init procedures, the com_switch_transfer() function must be called to change it.
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The TX function is defined in TF_Integration.c and any RX'd data is sent through rxQuePostMsg()
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