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8331B–AVR–03/12
Atmel AVR XMEGA AU
20.10.2
Bus Event Interrupt
The bus event (BUSEVENT) interrupt is used for all interrupts that signal various types of USB
line events or error conditions. These interrupts are related to the USB lines, and are generated
for the USB module and per endpoint. The following eight interrupts use the interrupt vector:
20.10.3
Events
The USB module can generate several events, and these are available to the event system,
allowing latency-free signaling to other peripherals or performance analysis of USB operation.
20.11 VBUS Detection
Atmel AVR XMEGA devices can use any general purpose I/O pin to implement a VBUS detec-
tion function, and do not use a dedicated VBUS detect pin.
20.12 On-chip Debug
When a break point is reached during on-chip debug (OCD) sessions, the CPU clock can be
below 12MHz. If this happens, the USB module will behave as follows:
USB OCD break mode disabled: The USB module immediately acknowledges any OCD break
request. The USB module will not be able to follow up on transactions received from the USB
host, and its behaviour from the host point of view is not predictable.
USB OCD break mode enabled: The USB module will immediately acknowledge any OCD break
request only if there are no ongoing USB transactions. If there is an ongoing USB transaction,
the USB module will acknowledge any OCD break request only when the ongoing USB transac-
tion has been completed. The USB module will NACK any further transactions received from the
USB host, whether they are SETUP, IN (ISO, BULK), or OUT (ISO, BULK).
Table 20-2.
Bus event interrupt source.
Interrupt source
Description
Start of frame (SOFIF)
A SOF token has been received
Suspend (SUSPENDIF)
The bus has been idle for 3ms
Resume (RESUMEIF)
A non-idle state is detected when the bus is suspended.
The interrupt is asynchronous and can wake the device from all
sleep modes
Reset (RSTIF)
A reset condition has been detected on the bus
Isochronous CRC error (CRCIF)
A CRC or bit-stuff error has been detected in an incoming packet
to an isochronous endpoint
Underflow (UNFIF)
An endpoint is unable to return data to the host
Overflow (OVFIF)
An endpoint is unable to accept data from the host
STALL (STALLIF)
A STALL handshake has been returned to the host
Table 20-3.
Event sources.
Event source
Description
SETUP
SETUPIF
Start of Frame
SOFIF
CRC error
CRCIF
Underflow/overflow
UNFIF and OVFIF