38.6.1.1 Initialization

The USB module requires a GCLK_USB of 48 MHz ± 0.25% clock for low speed and full speed operation.

Clock recovery is achieved by a digital phase-locked loop in the USB module, which complies with the USB jitter specifications. If crystal-less operation is used in USB device mode, refer to USB Clock Recovery Module.

After a hardware reset, the USB is in the idle state. In this state:

  • The module is disabled. The USB Enable bit in the Control A register (CTRLA.ENABLE) is reset.
  • The module clock is stopped in order to minimize power consumption
  • The USB pad is in suspend mode
  • The internal states and registers of the device are reset
Figure 38-2. General States

The output drivers for the DP/DM USB line interface can be fine tuned with calibration values from production tests. The calibration values must be loaded from the NVM Software Calibration Area into the USB Pad Calibration register (PADCAL) by software, before enabling the USB, to achieve the specified accuracy. Refer to NVM Software Calibration Area Mapping for further details.

For details, refer to Pad Calibration (PADCAL) register.

The USB is enabled by writing a '1' to CTRLA.ENABLE. The USB is disabled by writing a '0' to CTRLA.ENABLE.

The USB is reset by writing a '1' to the Software Reset bit in CTRLA (CTRLA.SWRST). All registers in the USB will be reset to their initial state, and the USB will be disabled. Refer to the CTRLA register for details.

The user can configure pads and speed before enabling the USB by writing to the Operating Mode bit in the Control A register (CTRLA.MODE) and the Speed Configuration field in the Control B register (CTRLB.SPDCONF). These values are taken into account once the USB has been enabled by writing a '1' to CTRLA.ENABLE.

After writing a '1' to CTRLA.ENABLE, the USB enters device mode or host mode (according to CTRLA.MODE).

The USB can be disabled at any time by writing a '0' to CTRLA.ENABLE.

Refer to USB Device Operations for the basic operation of the device mode.

Refer to Host Operations for the basic operation of the host mode.