For the 854S712I device (1:2 Fanout Buffer with Pre-Emphasis), the datasheet mentions that the maximum Output Pre-Emphasis Duration, Δtpe is 300 ps. In that case, can it still be used for a 4.5 Gbps application which has a bit duration of only 222 ps?

Yes, the part can still be used for 4.5 Gbps applications. In this case,  the pre-emphasis will take place only as long as the signal is high or low( i.e. approx. 111 ps).

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