Programming model of the library

UM0463

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2.2

Transactions on noncontrol endpoints

The user application uses noncontrol endpoints by calling a set of functions to send or

receive data. The user application calls a function to pass the data buffer that contains the

data or will receive the data from the library and calls another function to inquire if the

transaction is finished. This inquiry checks an internal flag that represents the status of the

transceiver.

The interrupt model and the polling model are implemented for the noncontrol endpoints too.

The flag of the transceiver status is set in the interrupt routine as soon as the transaction is

done in the interrupt model, but the flag is set in the USB polling routine if the polling model

is selected. Obviously, the interrupt model improves overall system performance.

2.3

Special note on MCU interrupt

The USB events are very special versus other events in the MCU application. Interrupt of

the USB events has to be answered as soon as possible, otherwise, the performance of the

USB system would be decreased and some USB events may be lost.

The library configures the MCU to work on nested interrupt mode and set the USB interrupt

as the highest interrupt level, so that the USB interrupt will be served in the fastest way.

In the interrupt model, the USB interrupt is configured as interrupt level 3 and the USB state

machine will be running in interrupt level 2. All the other interrupt sources are configured as

interrupt level 1.

In the polling model, the USB interrupt is configured as interrupt level 3 and the USB state

machine will be running in interrupt level 1. All the other interrupt sources are configured as

interrupt level 1. The user code can change the non-USB interrupt to level 2.

The user application has to be aware of this special configuration when using the interrupt

peripheral of the MCU.