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KL25 RTC OSCILLATOR SELECTION

Question asked by lupogrigio on May 29, 2013
Latest reply on May 30, 2013 by lupogrigio

Dear Freescaler,

 

I'm designing an electronic board equipped with Kinetis KL25 uC. I'm following the documentation and the reference designs available on the Freescale website (TWR-KL25, FRDM-KL25 etc). Working around my project are arised some doubts in my mind, that I want to share with you, in the hope that my issues can help other people that are facing similar situations.

 

I start with the issue to have RTC and uC powered from a small battery like CR1206 (or a 0.33F 5.5V Supercap)...

 

1)

What kind of oscillator I have to choose for the RTC input pin?I'm very interested in the possibility to keep my board in very low power mode when disconnected from the main supply, and to wake it up every hour to do some rapid tasks. I also need to keep alive the RTC. Looking at Farnell I' ve found

ABS25-32.768KHZ (Code: 2308714), with the following characteristics:


Freq = 32.768Khz

Freq. Tolerance:      +-10ppm

Load Capacitance:   12.5 pF   (also found with 6pF)

Operating Temp.       -40 to +85 C

 

Is it suitable for the application?


2)

Looking at the documentation I have read about the 300nA current consumption in low power mode of the uC. Is the current consumption of the oscillator irrilevant referenced to the 300nA of the uC?


3)

My board will be installed on logistic containers, which are connected to the power supply some hours every day. So I'm planning to use a Supercap (0,33F) or a small CR1206 battery to keep the microcontroller alive in very low power mode. The problem is that on the 3.3V rail I have some other electronics IC (and a 802.11 module). The Kinetis K uC have a dedicated pin to supply only the RTC module when not powered from the main supply. In kinetis KL the 3.3V voltage rail is unique, so, powering the uC I will also power all other electronics stuff attached to this voltage rail. Or am I misanderstanding something?


The solution could be using a Micrel MIC20xx to control the powering of the wifi module, but this will add a switching ic to my project, and isn' t a good choice for noise and EMI...and for costs.


Well,  maybe I have write too much things. I'm aware of the fact that some questions aren't inherent at 100% with the topic of this forum, but I think that others could appreciate it. If it isn't the case, just advise me.


Best Regards.

Giulio G.







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