Universal serial bus full-speed device interface (USB) RM0008
616/1096 Doc ID 13902 Rev 12
USB interrupt status register (USB_ISTR)
Address offset: 0x44
Reset value: 0x0000 0000
This register contains the status of all the interrupt sources allowing application software to
determine, which events caused an interrupt request.
The upper part of this register contains single bits, each of them representing a specific
event. These bits are set by the hardware when the related event occurs; if the
corresponding bit in the USB_CNTR register is set, a generic interrupt request is generated.
The interrupt routine, examining each bit, will perform all necessary actions, and finally it will
clear the serviced bits. If any of them is not cleared, the interrupt is considered to be still
pending, and the interrupt line will be kept high again. If several bits are set simultaneously,
only a single interrupt will be generated.
Endpoint transaction completion can be handled in a different way to reduce interrupt
response latency. The CTR bit is set by the hardware as soon as an endpoint successfully
completes a transaction, generating a generic interrupt request if the corresponding bit in
USB_CNTR is set. An endpoint dedicated interrupt condition is activated independently
from the CTRM bit in the USB_CNTR register. Both interrupt conditions remain active until
software clears the pending bit in the corresponding USB_EPnR register (the CTR bit is
actually a read only bit). For endpoint-related interrupts, the software can use the Direction
of Transaction (DIR) and EP_ID read-only bits to identify, which endpoint made the last
interrupt request and called the corresponding interrupt service routine.
The user can choose the relative priority of simultaneously pending USB_ISTR events by
specifying the order in which software checks USB_ISTR bits in an interrupt service routine.
Only the bits related to events, which are serviced, are cleared. At the end of the service
routine, another interrupt will be requested, to service the remaining conditions.
To avoid spurious clearing of some bits, it is recommended to clear them with a load
instruction where all bits which must not be altered are written with 1, and all bits to be
cleared are written with ‘0 (these bits can only be cleared by software). Read-modify-write
cycles should be avoided because between the read and the write operations another bit
could be set by the hardware and the next write will clear it before the microprocessor has
the time to serve the event.
1514131211109876543210
CTR
PMA
OVR
ERR WKUP SUSP RESET SOF ESOF
Reserved
DIR EP_ID[3:0]
rrc_w0rc_w0rc_w0rc_w0rc_w0rc_w0rc_w0 rrrrr
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