KL25 RTC OSCILLATOR SELECTION

[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.

[Hui_Ma]

Hi,

1> You can use ABS25-32.768KHZ as the RTC reference clock.
You can check TWR-K60N512 board BOM file, which provides some selected 32.768K crystal part number.

2> When the chip in VLLSx low power mode, system oscillator is limited to low range in VLLS1/3 and Off in VLLS0. And RTC module still works in Async operation (in VLLS0 low power mode, the RTC clock source from RTC_CLKIN pin).
If you check the 300nA in VLLS0 low power mode, which does not include RTC external clock source power consumption.
The datasheet shows the Table 6. Low power mode peripheral adders — typical value about external clock source add power consumption.

3> Kinetis L RTC module can only get power form uC main power. The Kinetis K series product provide VBAT pin to provide sperated power to RTC module.

[lupogrigio]

Thanks a lot Hui_Ma and Zhe Tian for your replies, but there is still a thing that I'm not able to understand completely.

1) Whats the difference between the two pin  Crystal (Y2) used in TWR-K60N512 and the four pin (Vcc powered) Crystal suggested in the TWR-KL25 Reference Design? It's clear I have a lack of knowledge on this field.

2) The post https://community.freescale.com/docs/DOC-94734 explain how to add RTC support to FRDM-KL25 by simply adding a 32.768Khz oscillator to the board and connecting it to PTC1 (RTCCLKIN) and PTC3 (CLKOUT). Is there no need for external capacitor between the two pin of the Oscillator and ground? Could this configuration work in the lowest power mode of the uC? If the case, can you suggest me a valuable crystal (or its characteristics)?

Thanks a lot and in advance to all can help me to solve this question.

Best Regards.

Giulio G.

[Paul_Tian]

Hi, Lupogrigio

1. Y1 is non-power crystal and 4 pins oscillator on KL board is powered. Y1 need MCU provide power to it, But oscillator has power pin, so it is independent power no need MCU. Y1 should connect both to EXTAL and XTAL. But oscillator only need to connect EXTAL. On TWR-KL25 board, this oscillator is used to give clock signal to RTC. Crystal need load capacitor but oscillator no need.

2. I think your understanding is incorrect. If you use oscillator, you only need to connect PTC1 no need to connect to PTC3. You no need to add load capacitor.

3. If you use oscillator(powered), RTC can work in lowest power mode. I suggest you use 32.768KHz in your desgin.

Hope my reply can help you.

Best Regards

Paul

[lupogrigio]

Thank you Zhe Tian,

please correct me if my deductions are incorrects:

A) In order to keep alive the RTC in the lowest power mode the best solution is to use an 32.768Khz Oscillator, connecting the OUT pin to RTC_CLKIN (PTC1) pin of the uC

B) Also a 32.768Khz Crystal could be used, attaching it to the RTC_CLKIN and CLKOUT pins of the uC, and adding external capacitors. ALSO THIS CONFIGURATION WORKS ON THE LOWEST POWER MODE.

A) Is the best solution, less component count and trace signals (maybe a little more expensive than B). Both solutions work on lowest power mode.

But, which is the best solution in terms of power consumption? Keep in mind that my board when in normal power mode work with an 8Mhz Crystal.

I hope this discussion can help other than me.

Thanks for your courtesly attention.

Giulio

[Paul_Tian · Accepted solution]

Hi, Giulio

A) In order to keep alive the RTC in the lowest power mode the best solution is to use an 32.768Khz Oscillator, connecting the OUT pin to RTC_CLKIN (PTC1) pin of the uC

Answer: Correct. This connection support RTC module in lowest power mode.

B) Also a 32.768Khz Crystal could be used, attaching it to the RTC_CLKIN and CLKOUT pins of the uC, and adding external capacitors. ALSO THIS CONFIGURATION WORKS ON THE LOWEST POWER MODE.

Answer: Incorrect. You cannot attach crystal to RTC_CLKIN and CLKOUT. RTC_CLKIN only support  oscillator not crystal.

A) Is the best solution, less component count and trace signals (maybe a little more expensive than B). Both solutions work on lowest power mode.

Answer: Incorrect. If you want to use RTC in lowest power mode, you have to use connection A.

But, which is the best solution in terms of power consumption? Keep in mind that my board when in normal power mode work with an 8Mhz Crystal.

Answer: RTC_CLKIN will not affect your main clock. You can use 8MHz crystal for your normal run mode. And connect this crystal to EXTAL and XTAL pins.

Hope my reply can help you. 

Best Regards

Paul

[lupogrigio]

Thanks a lot for your attention, Mr. Zhe.

Your answers have been very useful for my design.

Best Regards

Giulio G.

[Paul_Tian]

Hi, Giulio G.

1. Your crystal is good and suitable for your application.

2. No this current is only for uC not including RTC module.

3. Your understanding is correct. KL series has not VBAT pin, so it have to provide power to chip VDD.

Hope my reply can help you.

Best Regards

Paul