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Field SUSPEN writer - Suspend state enable Device mode Software can set this bit when the SUSP interrupt is received, which is issued when no traffic is received by the USB peripheral for 3Â ms. Software can also set this bit when the L1REQ interrupt is received with positive acknowledge sent. As soon as the suspend state is propagated internally all device activity is stopped, USB clock is gated, USB transceiver is set into low power mode and the SUSPRDY bit is set by hardware. In the case that device application wants to purse more aggressive power saving by stopping the USB clock source and by moving the microcontroller to stop mode, as in the case of bus powered device application, it must first wait few cycles to see the SUSPRDY=1 acknowledge the suspend request. This bit is cleared by hardware simultaneous with the WAKEUP flag set. Host mode Software can set this bit when Host application has nothing scheduled for the next frames and wants to enter long term power saving. When set, it stops immediately SOF generation and any other host activity, gates the USB clock and sets the transceiver in low power mode. If any USB transaction is on-going at the time SUSPEN is set, suspend is entered at the end of the current transaction. As soon as suspend state is propagated internally and gets effective the SUSPRDY bit is set. In the case that host application wants to purse more aggressive power saving by stopping the USB clock source and by moving the micro-controller to STOP mode, it must first wait few cycles to see SUSPRDY=1 acknowledge to the suspend request. This bit is cleared by hardware simultaneous with the WAKEUP flag set.