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external 32 kHz crystal issue

Question asked by Dorian Hildebrandt on Mar 29, 2016

Hi everybody

i have a question regarding the power modes.
Already read a lot about that topic
https://www.google.ch/search?q=kinetis+pll+fll+current+adder&gws_rd=cr&ei=TJT6VrTjFIGa6ATq946ABw#q=kinetis+optimal+low+power
but its still not clear to me.

Basically i have a application that wakes my µC every second, does some stuff and goes sleep again.

I dont need to much compute-power, but accurate timers in the order of 1µs.

I already build my application with following configuration:
I have an external crystal with 4Mhz, then i'm using BLPE, my measurements are done in ADC DMA mode (triggered by timer ticks) while the µC is stopped.

Everything works fine.

I'm using the Kinetis Design Studio 3.0.

 

Now i saw in the Datasheet (i'm using the MKW01) in the section "Low power mode peripheral adders"
that my 4 Mhz external crystal (IEREFSTEN4MHz) is consuming 262µA. That is really
too much for my application. Switching to internal 4Mhz still uses 52µA what is also too much for me.
So my only option is to switch to an external 32 kHz crystal, which consumes less than 1 µA.

 

Now i have some questions.
1. As 32 kHz is not fast enough for my calculations and timers, i need to enable the FLL.
How much current consumption do the FLL add? As long as i need my accurate
timers of 1 µs i guess i have to leave the FLL enabled. That means like 30% of the hole time.

2. How much is the difference in consuption between BLPI/BLPE and FEE?

3. If i select "FEE" in the MCG mode, i get a Error Message that i cannot use VLPR and only BLPI or BLPE
are allowed. But i want to use VLPR and VLPS to save as much power as possible. During VLPS i have to use
the 1 µs timers to activate my DMA (the 32 kHz is too slow), so i cannot turn off the FLL and Switch to BLPE mode.
What can i do about it? Is it still possible to use the 32 kHz external crystal for my application?

One option would be to use the 32 kHz external crystal to wake me every second, and then use the 4 MHz IRC for my

DMA (during VLPS/BLPI), is that right? But i really would like to find a better solution.

 

Best regards
Dorian

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