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32kHz RTC and 48MHz USB together: Is it possible?

Question asked by Brian Carroll on Jan 18, 2010
Latest reply on Feb 9, 2010 by Brian Carroll

Hi all,

I am looking for a Freescale processor that can implement both 32.768kHz RTC and a USB device (requiring 48.000MHz bus clock).  I've looked long and hard at the MCF51JM and MCF52xx families and I haven't been able to find one. Has anyone found a solution to do this?


The problem is these products only have one XTAL port. That means I have to choose either something around 32kHz or something above 1MHz. I would like to run my RTC in the 32kHz range because I have very tight power consumption constraints. But if I use a 32.768kHz crystal, I can't generate exactly 48MHz to run the USB device. Same story for 38.4kHz. If I could use 31.25kHz it would work, but the XPSC module is only specified above 32.000kHz.


I suppose I could run the RTC at 1MHz but I have not found any information on how the RTC power consumption varies with frequency. For example the MCF51JM128 datasheet gives a "typical" figure of 300nA but does not state what frequency this is for. Is it 32kHz? Does that mean it will be ~10uA for a 1MHz crystal? Also, there is no maximum specification for the RTC current consumption figure.


Has anyone had the same issue? How did you solve it?