MCF52259 RTC Crystal

[swash]

The data sheet and reference manual do not say whether shunt (to ground) capacitors are required on the RTC Crystal.  Can you provide some guidance? 

Also, is there a particular type of crystal for this function?

Finally, in addition to the RTC, are any other areas (like RAM) maintained by VSTBY when power is removed?

Thanks!

[mjbcswitzerland]

Hi

I would use the EVB and DEMO board schematics as reference. These use a standard 32kHz quarz with 2 x 18pF to ground, plus a resistor in parallel (10M..18M).

The first 16k of SRAM is battery backed-up. This is not specifically described in either data-sheet or user's manual but has been confirmed by the moderator "MACL" - see https://community.freescale.com/thread/51895

Quote

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Vstby backs up the first 16K of SRAM and the RTC when the main power is shut down.  We are working to clarify the voltage spec but experimental results show that it works fine down to somewhere near 1.8V.  When the main power is turned on, the SRAM and RTC get powered from Vdd.  It's only when this source is removed that Vstby kicks in.  Its good to ensure that Vstby is always above 1.8V before the main power is turned on.

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Regards

Mark

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[vespaman]

I have had some issues with the crystal/cap/resistor setup on my board. I am now running   with a 18pF crystall, one 10pF cap on EXTAL input and 20M across.

But I don't really like the way the waveforms looks. But then again, I am not at all familiar with the low power clock drivers of Freescale. 

On the XTAL, I have somthing like a square wave output with very soft upper corners, about 1.4 Volt t-t. On the EXTAL, things looks more normal, with a sinus, about 1.5Volt t-t.

If  I tweak cap's I can either get the XTAL somewhat higher, but then the EXTAL goes down (an vice versa).

In my design, I need the RTC to drift very little, so I am quite worried about the lack of documentation of this part.

Is what I am seeing normal?

Any input much appreciated,

   Micael