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USB device

The HAL USBPhy API provides physical access to the USB bus in the role of a USB device. Implementing this API enables the use of any supported class driver, such as CDC, HID and MSD.

Assumptions

Defined behavior

  • You can use any endpoint configurations that fit in the parameters of the table returned by USBPhy::endpoint_table.
  • You can use all endpoints in any valid endpoint configuration concurrently.
  • The device supports use of at least one control, bulk, interrupt and isochronous in each direction at the same time - at least 8 endpoints.
  • USBPhy supports all standard endpoint sizes (wMaxPacketSize).
  • USBPhy can handle an interrupt latency of at least 100ms if the host PC is not performing a reset or setting the device's address.
  • USBPhy only sends USBPhyEvents when it is in the initialized state.
  • When unpowered, USBPhy only sends the USBPhyEvents::power event.
  • On USB reset, all endpoints are removed except for endpoint 0.
  • A call to USBPhy::ep0_write results in the call of USBPhyEvents::in when the PC reads the data unless a power loss, reset or a call to USBPhy::disconnect occurs first.
  • A call to USBPhy::endpoint_write results in the call of USBPhyEvents::in when the PC reads the data unless a power loss, reset or a call to USBPhy::endpoint_abort occurs first.
  • A call to USBPhy::endpoint_read results in the call of USBPhyEvents::out when the PC sends data unless a power loss, reset or a call to USBPhy::endpoint_abort occurs first.
  • Endpoint 0 naks all transactions aside from setup packets until higher-level code calls one of USBPhy::ep0_read, USBPhy::ep0_write or USBPhy::ep0_stall.
  • Endpoint 0 stall automatically clears on reception of a setup packet.

Undefined behavior

  • Calling USBPhy::endpoint_add or USBPhy::endpoint_remove outside of the control requests SetInterface or SetConfiguration.
  • Calling USBPhy::endpoint_remove on an endpoint that has an ongoing read or write operation. To avoid undefined behavior, you must abort ongoing operations with USBPhy::endpoint_abort.
  • Devices behavior is undefined if latency is greater than 2ms when address is being set - see USB spec 9.2.6.3.
  • Devices behavior is undefined if latency is greater than 10ms when a reset occurs - see USB spec 7.1.7.5.
  • Calling any of the USBPhy::endpoint_* functions on endpoint 0.

Notes

  • Make sure USBPhy sends USBPhyEvents in the correct order when multiple packets are present. USBPhy must send IN endpoint events before OUT endpoint events if both are pending.
  • A host PC may resend setup packets to a USB device if there is noise on the USB line. The USBPhy should be able to handle this scenario and respond to the setup packet with an ACK.
  • Bidirectional protocols making use of alternating IN and OUT phases should not rely on the last ACK an IN transfer to indicate that the OUT phase should start. Instead, the OUT phase should be started at the same time the last IN transfer is started. This is because the ACK to the last in transfer may be dropped if there is noise on the USB line. If dropped, it will only be resent on the next IN phase. You can find more information on this in section 8.5.3.3 of the USB specification.

Implementing USB

You can find the HAL API and specification for USB in the following module:

To enable the HAL USBPhy API in Mbed OS, add the USBDEVICE label in the device_has option of the target's section in the targets.json file.

Testing

The Mbed OS HAL provides a set of conformance tests for the HAL USBPhy API. You can use these tests to validate the correctness of your implementation. To run the HAL USBPhy API tests, use the following command:

mbed test -t <toolchain> -m <target> -n mbed-os-tests-usb_device-*
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