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USB-1: Dealing with USB Events
The USBD peripheral on the nRF52840 contains a series of registers, called EVENTS registers, that indicate the reason for entering the USBD event handler. These events must be handled by the application to complete the enumeration process.
Open the firmware/src/bin/usb-1.rs file. In this starter code the USBD peripheral is initialized in init and a task, named main, is bound to the interrupt signal USBD. This task will be called every time a new USBD event needs to be handled. The main task uses usbd::next_event() to check all the event registers; if any event is set (occurred) then the function returns the event, represented by the Event enum, wrapped in the Some variant. This Event is then passed to the on_event function for further processing.
Connect the USB cable to the port J3 then run the starter code.
In this section as a warm-up exercise you'll need to deal with the following USB events until you reach the EP0SETUP event.
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USBRESET. This event indicates that the host issued a USB reset signal. According to the USB specification this will move the device from any state to theDefaultstate. Since we are currently not dealing with any other state, you can handle this state by doing nothing. -
EP0SETUP. The USBD peripheral has detected the SETUP stage of a control transfer. If you get to this point move to the next section. -
EP0DATADONE. The USBD peripheral is signaling the end of the DATA stage of a control transfer. You won't encounter this event just yet.
When you are done you should see this output:
(..)
INFO:usb_1 -- USB: UsbEp0Setup
INFO:usb_1 -- goal reached; move to the next section
Do not overthink this exercise; it is not a trick question. There is very little to do and no new functionality to add.
You can find the solution in the usb-1-solution.rs file.
Before we continue we need to discuss how data transfers work under the USB protocol.