K64 input clocks

[roymessinger]

I'm designing a board according to the FRDM-K64F eval board. I saw 2 external clocks are used in the design (32.768kHz and 50MHz). The 50MHz is passed from the PHY (which should receive a 25MHz) and the 32kHz is passed from external oscillator.

If I'm not using the Ethernet communcation, do I still need to supply 50MHz clock?

A friend of mine also designed such board. He did not used the 32kHz input (left it open) but rather supplied a 50MHz to EXTAL0 (pin 50) input - in the eval design it is passed from the PHY - even though he's not using Ethernet.

All is working well, which kinda left me confused... Any highlights on this matter?

Thanks.

[roymessinger]

Ok guys, Thank you very much! what a great forum it is!

[Masmiseim · Accepted solution]

Hey Roy,

If you want to use the integrated RTC you have to put the 32kHz crystal an EXTAL32/XTAL32.  This is the only clock source for the RTC.

The Additional Crystal/Oscillator on EXTAL/XTAL is optional but the recommended clock source for the PLL. But you could also use the IRC48 internal oscillator as a clock source for the PLL or use the FLL instead.

The 25 / 50 MHz Crystal/Oscillator is only mandatory if you want to connect a Phy via MII /RMII. It must be the same clock for the Phy and Controller, as they use it for synchronization. If you don’t need the connection to the Phy you could also use different clocks as 8 or 12 MHz.

Take a look at the “Clock Distribution” Chapter in the Reference manual.

Regards

[mjbcswitzerland]

Hi

It is possible to clock the K64 from multiple sources, including internal ones without any external clocks/crystals: See http://www.utasker.com/kinetis/MCG.html
The 50MHz clock is only needed for Ethernet.

Regards

Mark

http://www.utasker.com/kinetis/FRDM-K64F.html
http://www.utasker.com/kinetis/TWR-K64F120M.html
http://www.utasker.com/kinetis/TEENSY_3.5.html