KL24 LPO 1 kHz as a clock source for RTC

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KL24 LPO 1 kHz as a clock source for RTC

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wes33
Contributor I

Hello, I'm trying to configure Kinetis KL24 RTC to be sourced from LPO Clock but Processor Expert give me back information that RTC can be driven only by 32 kHz clock. How to configure that RTC block in PE to be driven by LPO 1 kHz clock?

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chris_brown
NXP Employee
NXP Employee

Maciej

First, I would like to reinforce what Ma Hui has said.  If you use a 32.768 kHz clock source with the RTC, it will increment every second. 

To clarify why the LPO was included as a clock source, this was done to give the user a timer option that can wake the device from VLLSx power modes (except for VLLS0).  It won't provide a real time clock in these modes, true.  However, in the use case where a simple timer is required (but not an accurate time count) to wake the part from a low power mode (except for VLLS0), the LPO could be used as the clock source to the RTC which has the capability to wake the device from these power modes.  In this case, you could setup the part to wake every 32.768 seconds.  It appears that you are more familiar with the European notations for decimals, so I want to clarify, that this wake-up interval I have just quoted is about one-half of a minute (not 9 hours).  If I misunderstood something, I apologize in advance. 

Hope this clarifies things,

Chris

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Hui_Ma
NXP TechSupport
NXP TechSupport

Hi,

I also check PE [RTC_LDD] component with RTC input clock frequency restriction. That should be the Processor Expert tool bug for KL product.

While, customer could manually modify the System Options Register 1 (SIM_SOPT1) [OSC32KSEL] bit to select the LPO clock as RTC clock source.

After customer select the LPO 1KHz clock as RTC clock source, the RTC second register value will increase every 32.768 seconds when RTC enabled.

Wish it helps.

B.R.
Ma Hui

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wes33
Contributor I

So as I understand my RTC second register will be incresed once every apx 9 hours? ( 32.768 / 3600 = 9,1022 )? It doesn,t sound like Real time clock :smileyhappy:

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chris_brown
NXP Employee
NXP Employee

Maciej

First, I would like to reinforce what Ma Hui has said.  If you use a 32.768 kHz clock source with the RTC, it will increment every second. 

To clarify why the LPO was included as a clock source, this was done to give the user a timer option that can wake the device from VLLSx power modes (except for VLLS0).  It won't provide a real time clock in these modes, true.  However, in the use case where a simple timer is required (but not an accurate time count) to wake the part from a low power mode (except for VLLS0), the LPO could be used as the clock source to the RTC which has the capability to wake the device from these power modes.  In this case, you could setup the part to wake every 32.768 seconds.  It appears that you are more familiar with the European notations for decimals, so I want to clarify, that this wake-up interval I have just quoted is about one-half of a minute (not 9 hours).  If I misunderstood something, I apologize in advance. 

Hope this clarifies things,

Chris

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Hui_Ma
NXP TechSupport
NXP TechSupport

Hi,

The typical RTC module reference clock is 32.768KHz and the RTC_TPR register with default 32768 prescaler, then it will get accuracy 1 second counter at RTC_TSR register. When you change the clock source to LPO 1KHz, the RTC_TPR still with 32768 prescaler, then the RTC_TSR register will increase with actual 32.768 seconds. If you want to use RTC as real time clock, it need the clock frequency with 32.768KHz.

Wish it helps.

B.R.

Ma Hui

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