Can OSC32KCLK continue to run through a WAKEUP reset from VLLS3?

[ryanchaves]

I'm developing an application with the MKL03Z where I am using an external 32.768 kHz crystal connected to PTA3/EXTAL0 and PTA4/XTAL0. I have enabled the oscillator via RTC_CR[OSCE] and waited for the oscillator startup time as specified in Sections 33.3.5 and 33.4.1.1 of the reference manual.

It appears that the OSC32KCLK is going away after a WAKEUP reset from VLLS3 and takes ~125ms to recover. During this time, any modules that use ERCLK32K (RTC and LPTMR) do not get clocks. Thus it is possible for the RTC to lose 125ms every time the system comes out of VLLS3. Additionally, the LPTMR will be delayed by at least 125ms for any configured timeout.

This 125ms period equates to 4096 cycles at 32768 Hz, which leads me to believe it is associated with the startup time of the oscillator module discussed in section 25.9.1.2 of the reference manual.

Is there a way to avoid having to wait the 4096 cycles for the oscillator to stabilize after a WAKEUP reset?

I have reproduced this behavior in the "power_manager_hal_demo_frdmkl03z" sample application for the FRDM-KL03Z board by setting SIM_SOPT1[OSC32KOUT]=01 before the while(1) loop in main. This is not quite the same as I am doing in my application, since in mine I am handling WAKEUP resets different from POR/PIN resets and am only clearing the ACKISO bit in PMC_REGSC for a WAKEUP reset.

The language in the OSC (Chapter 25) and RTC (Chapter 33) sections of the reference manual are not clear to me with regard to how I should have expected OSC32KCLK to behave through a WAKEUP reset. Section 33.1 states:

"RTC_CR[OSCE] can override the configuration of the System OSC, configuring the

OSC for 32 kHz crystal operation in all power modes except VLLS0, and through any

System Reset."

Additionally, Section 25.1 states:

"RTC_CR[OSCE] has overriding control over the MCG_Lite and OSC_CR enable

functions. When RTC_CR[OSCE] is set, the OSC is configured for low frequency, low

power and RTC_CR[SCxP] override OSC_CR[SCxP] to control the internal capacitance

configuration. See the RTC chapter for more details."

However, Section 25.11 (Low power modes operation) says:

"After waking up from Very Low Leakage Stop (VLLSx) modes, all OSC register bits are

reset and initialization is required through software."

So, Section 25.11 and 33.1 seem to be at odds in terms of the reset behavior of the oscillator.

Thanks.

[ryanchaves · Accepted solution]

I received the following response from a Freescale FAE:

I have checked into this and I am afraid that this is the way the device operates.

It looks as though there are two workarounds for this.

One is to use LLS mode instead of VLLS3.

The other relates as to whether you clear the ACKISO bit. If there is no need to clear ACKISO on a wake up, don’t clear it. If you do need to clear ACKISO (to access the IO) then wait >125ms before clearing it. This will ensure that the clock stays running.

We will work on getting an errata published for this

You are correct LLS mode is not supported in the KL03 and the closest low power mode is VLPS.  If you cant wait 125mS to clear ACKISO then we are left with the two options you suggest:

1. 1.- Sleep mode = VLPS

Or

1. 2.- Sleep mode = VLLS3 and compensate for the missing 125mS in software.

[ryanchaves]

I should clarify that it is technically ERCLK32K that is selected as the input to the RTC and LPTMR via SIM_SOPT1[OSC32KSEL] = 00 and LPTMR0_PSR[PCS] = 2.

[jeremyzhou]

Hi Ryan,

The ERCLK32K can't hold enabled state when during the boot up time.

Just as the Section 25.11 and 33.1 state, the ERCLK32K will be disable when the MCU resets, as the previous configuration go away and all the OSC register bits become to reset state.

Hope it helps.
Have a great day,
Ping

-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------