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

[brian_kbs]

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?

Thanks,

Brian 

[mjbcswitzerland]

Hi Brian

The M522xx with USB and RTC have one oscillator for the 48MHz quarz (advisable for USB operation) and one oscillator for RTC 32kHz. The 32kHz oscillator continues running when the main power is down and the Vbatt is supplied.

Example of M52259 in 144 pin QFP package:
Pin 105 = XTAL

Pin 106 = EXTAL

Pin 13 = RTC_EXTAL

Pin 14 = RTC_XTAL

The RTC (and battery backed up RAM) consumes about 15..20uA.

Is this not suitable?

Regards

Mark

www.uTasker.com

[brian_kbs · Accepted solution]

Hi Mark,

Thanks for the suggestion. Yes, it looks like that processor fits the requirements I posted. The trouble is that it's a bit too big & complex for my application. But I've figured out a way to do what I need with a Coldfire V1 processor (MCF51JM128) in a 44-pin LQFP package. So for anyone who's interested, here's a solution:

For the low-power RTC, connect the 32.768kHz crystal to the XTAL/EXTAL pins.

Use the USB_ALT_CLK option on pin PTG0 to send an external 48MHz clock directly to the USB module. This option is not very easy to find in the Reference Manual, but it's there! The Freescale support team tells me you still have to run the core at >12MHz while the USB is running, but that's OK.

So I have a solution, but I still wish the processor's low-frequency crystal spec went down to 31.25kHz! That would allow me to generate any integer MHz frequency. So I could use a single oscillator for both my "low-power/RTC" mode and my "USB running" mode. It would be like cutting $1 off the price of the processor. Anyone know whether Freescale has any plans to extend the range of this oscillator in the future? I wonder if it has been characterised below 32kHz?

Regards,

Brian