
21-6
MC68SZ328 Reference Manual
MOTOROLA
USB Device Module
USBD_VM—Input D- signal connected directly to the D-.
21.5 Interrupt Services
The USB module generates a number of interrupts to indicate conditions requiring attention from the host.
There are two categories of interrupts, USB general interrupts and endpoint-specific interrupts.
21.5.1 USB General Interrupts
The USB general interrupts indicate global configuration and status changes that pertain to the USB. All of
these interrupts are maskable. When an event occurs that causes an interrupt condition to occur, and the
corresponding bit in the interrupt mask register is 0, an interrupt signal is asserted on the module’s
interface. Writing a 1 to the associated bit in the interrupt register clears the interrupt. After a hard reset, all
interrupts are masked by default. This section describes the USB general interrupts.
MSOF—Missed start-of-frame. The MSOF interrupt informs the software that it did not service the
SOF interrupt before another SOF was received.
SOF—Start-of-frame. The SOF interrupt indicates that a start-of-frame token was received by the
device. The current USB frame number may be read from the USB_FRAME register.
RESET_STOP—End of USB Reset Signalling. The RESET_STOP interrupt indicates that reset
signalling on the USB has stopped.
RESET_START—Start of USB Reset Signalling. The RESET_START interrupt indicates that
reset signalling on the USB has begun. When reset signalling is active on the USB, the device
should not expect to receive any transactions on the bus. This interrupt results in a reset of the UDC
into the powered state, but does not cause any specific actions in the USB device module’s front-end
logic. User software must clear any pending interrupts and ensure that the module is configured
properly after the reset. This interrupt only indicates the start of reset signalling. Status of the USB
at any given moment can be verified by examining the USB_STAT register.
NOTE:
The user is cautioned that the presence of USB reset signalling invalidates
any transaction in progress. On detecting RESET_START, user software
should read any remaining valid data from the receive FIFOs and flush all
the others. When reset signalling is detected, user software must clear any
pending interrupts and ensure that the module is configured properly after
the reset.
WAKEUP—Resume Signalling Detected. This interrupt asserts when resume signalling is detected
on the USB. The device uses this interrupt to wake up from suspend mode and resume active mode.
This interrupt is independent of the clock to the module and therefore can be used when the USB
device is powered down. The status is reported in the USB_ISR register. In USB power down mode,
the USB_ISR register does not update; however, the programmer may check USB_GEN_ISR to
validate an asynchronous WAKEUP interrupt assertion.
RES—Resume Active Mode. This interrupt asserts when a state change from suspend to resume
occurs in the UDC module. The RES interrupt only indicates the change from suspended to active
mode. This interrupt will not assert when the USB device is powered down. In this case, the
programmer uses the asynchronous WAKEUP interrupt.
SUSP—USB Suspended. This interrupt asserts when the USB goes into suspend mode. Suspend
mode occurs when the device fails to receive any traffic from the USB for a period of 6 ms. Once
suspend mode is detected, the device puts itself into a low-power standby mode.
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Freescale Semiconductor, Inc.
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