NEC switch - Manual (Page 129)

CHAPTER 8 USB FUNCTION
User’s Manual U12978EJ3V0UD
129
8.6.2 Cautions when using interrupts
Pay attention to the following when using an interrupt request generated by the USB function.
(1) Because USBREIF is set by transition from the J state to the K state on the bus, it is also set during sync
detection or packet reception. Thus, disable the generation of interrupt requests by setting the interrupt mask
flag (USBREMK) during bus idle or control transfer.
(2) Because USBREIF is set by transition from the J state to the SE0 state, or by transition from the K state to
the SE0 state on the bus, it is also set during EOP reception or bus idle retention EOP reception. Thus,
disable the generation of interrupt requests by setting the interrupt mask flag (USBREMK) during bus idle or
control transfer.
(3) When clearing the interrupt mask flag (USBREMK), do so immediately before the transition to bus suspend
mode (immediately before the STOP instruction execution).
(4) When the USB reset signal or the Resume signal is received, the USB clock starts operating. As a
consequence, USBTMIF and UWDERR are set after the elapse of a certain period (85.3
µs: f
X = 6.0 MHz
operation) of time from reception start. Therefore, when shifting to bus suspend mode (execution of stop
instruction), set the interrupt mask flag (USBTMMK) and disable the generation of interrupt requests.