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BF547 - Connect CLKBUF to USB_XI?

Question asked by AndrewN on Sep 7, 2010
Latest reply on Sep 18, 2010 by AndrewN



I'm currently working on a design using a BF547. I would like to drive USB_XI using CLKBUF, so that I won't need to use a separate USB crystal. It appears this is what is intended, but the BF547 datasheet isn't quite clear that it will work... Also, EE-281_rev2 - "Hardware Design Checklist for the Blackfin Processors" -  contains the following:  "Some peripheral  XTAL pins may also be driven by the CLKBUF output of some processors", implying that CLKBUF can be used for some peripherals but not others, and I can't see which particular peripherals are suitable...


So, my questions are:


1. If I use a crystal attached to CLKIN which has operates at a USB 2.0 compatible frequency & stability (e.g, 24MHz, +/- 30 ppm), can I then connect CLKBUF to USB_XI, so that I don't need to use a separate USB crystal?


2. Will this cause any issues with being able to wake up from hibernate?


3. What is the maximum recommended ESR of the crystal? (I'm looking at useing approx 100 ohm, but I can't find any crystal ESR tolerances on the Blackfin Datasteet...)


Thanks in advance for any advice!