Capacitors with 16Mhz crystal in NXP Kinetis MK66 at 180Mhz.

[luiHS]

 
Hello

I have doubts, about whether it is really necessary, to put two capacitors using a 16Mhz crystal with the NXP Kinetis MK66 microcontroller at 180Mhz.

I think I read somewhere, that capacitors are only necessary when using crystals of up to 8Mhz. For example in the Teensy 3.6 that uses a Kinetis MK66, according to the scheme there are no capacitors with the 16Mhz crystal, on the other hand I have designs with STM32 with 8Mhz crystal and it does not start if I do not put the capacitors.

So, the question is when it is necessary to put capacitors with the microcontroller's oscillator crystal.


My current schematic with MK66 (LQFP 144 pin)




Teensy 3.6 Schematic with MK66 (BGA)

[tsman]

Kinetis K66 has integrated load capacitors. Look at OSC_CR register in the datasheet.

[free_electron]

yes, what does the datasheet say ? do that ...

[luiHS]

Thanks, I have found some information in Reference Manual.

In NXP forums also suggest I check, low gain mode program in MCG_C2 for no external capacitors , or If loading capacitors (and feedback resistance) are used the high gain mode (HGO bit) must be set.

[luiHS]

 
I see in old Processor Expert software, there is option to choose between High gain and Low power for Oscillator Operating Mode, I understand this is the parameter to allow internal capacitors. I want to check where is available in MCUxpresso.

If load capacitors (and feedback resistance) are used, the High Gain mode (HGO bit) must be set. Thus, I infer that the Low Power mode needs to be selected to use the internal capacitors. I'll check.

[pgo]

The choice of internal or external capacitors has nothing to do with the power level chosen.
The power level influences the choice of crystal and whether a bias resistor is required.
It also affects  the start-up time of the oscillator and stability (influence from external affects).
Please see chapter 28 of the MK66 reference manual.
If your search the NXP (Freescale) site there are some application notes that discuss OSC in more detail (from memory).

I know of no reason to prefer an external cap over the selectable internal values.
I can only speculate on reasons:

• desire for a lower tolerance on the cap since it affects frequency e.g. NPO caps might be more temp stable than internal
• the value is outside the internal range - unlikely (range is 0-32pF in 2pF steps)

bye

[luiHS]

The choice of internal or external capacitors has nothing to do with the power level chosen.

So, how do you configure, if it really is necessary, to avoid having to install external capacitors with the crystal oscillator?

I know of no reason to prefer an external cap over the selectable internal values.

I do not want external capacitors, I want to remove them from my designs with Kinetis, the question is if I need to make some special configuration to be able to eliminate them, or I simply remove them and it will work the same.