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USB superspeed peripherals

Level 5
First solution authored 100 replies posted 50 replies posted
Level 5

According to the CX3 datasheet, on page 8:

Note: REFCLK and CLKIN must have either separate clock
inputs or if the same source is used, the clock must be passed
through a buffer with two outputs and then connected to the clock

Normally, I design in this clock buffer, but today I tried unsoldering one of the buffers, and shorting across the pads with some wire. Now my CX3 has a single oscillator feeding both the REFCLK and CLKIN pins, with no buffer, and everything seems to work just fine. I'm streaming about 900Mbps of video over USB 3.0, and the 19.2MHz clock waveform looks just fine on the oscilloscope.

Why does the datasheet insist that a buffer is needed. And why does my circuit work just fine without one?

Many thanks



1 Solution
Moderator First question asked 1000 replies posted 750 replies posted

Hello Hugo,

Please refer to the following thread which already answers this question:


As mentioned in the above thread, the standard and recommended way to drive REFCLK and CLKIN is to use a clock buffer or separate clock inputs so that it has sufficient strength to drive both CLKIN and REFCLK. But, if the input clock has enough drive strength to drive both REFCLK and CLKIN, then you can directly use it to drive both these pins without using a clock buffer. 

Best Regards,

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