Low Frequency Periodic Signaling (LFPS) In USB 3.x

LFPS is used for side band communication between the two ports across a link that is in a low power link state. It is also used when a link is under training, or when a downstream port issues Warm Reset to reset the link.

When Phy generates LFPS?

The control of LFPS generation is with MAC layer, So when MAC layer assert LFPS generation conditions on the pipe (pipe spec section 7.23) interface, then phy will drive LFPS on the differential pair of lines TxP/TxN over the period of time the LFPS conditions asserted by MAC.

Interface LFPS generation conditions-

     1.  PowerDown=3'b000 && TxDetectRx==1 && TxElecIdle==1

     2.  POwerDown !=3'b000 && TxELecIdle==0 (TxDetectRx == Don't care)

When Phy receives LFPS?

On the receiver differential pair of lines RxP/RxN, When Phy detects a received bit steam for LFPS tperiodduration limit and that matches with the Duty cycle of LFPS, means LFPS detected.

Signalling to MAC of LFPS detection-

    1.   RxElecIdle=0 means LFPS detected

    2.   RxElecIdle=1 means LFPS not detected (that can be ElecIdle or Data on the basis of RxDataValid signal value).

USB LFPS related terms and its values-

Below snapshots from the USB3.0 specification shows LFPS signaling diagram and table which with different types of LFPS in USB.

1. tperiod -  The period of an LFPS cycle.

      Value - Min 20ns

                  Max 100ns

2. tBurst - is the transmission of continuous LFPS signal over a period of time asserted by MAC layer.

       Value - Its value vary as per type of LFPS signalling.(Spec section Table 6-21)
                  Ex- Polling LFPS, Ping LFPS, warm reset LFPS, U1/U2/U3 LFPS

3. tRepeat - An LFPS sequence is defined by the transmission of a single LFPS burst of duration tBurst over a period of time defined by tRepeat.

       Value - Its value vary as per type of LFPS signalling.(Spec section Table 6-21)
                  Ex- Polling LFPS, Ping LFPS, warm reset LFPS, U1/U2/U3 LFPS