FUJITSU MHW2120BS - Manual (Page 268)

Operations
(3) Slumber mode
In this mode, the (deep) Power Save mode is set for the interface circuit.
The device switches to Slumber mode when the following occurs:
The device receives the PMREQ_P signal from the host and responds with the
PMACK signal.
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The device sends the PMREQ_S signal and the host responds with PMACK
signal.
The device cannot switch to Slumber mode if the following condition is satisfied:
The device responds with the PMNAK signal because it is not waiting for
commands.
The device returns to Active mode from Slumber mode when the following
condition is satisfied:
The device receives the COMRESET or ComWake signal from the host.
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