MK20 approprite clock source for CAN

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MK20 approprite clock source for CAN

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sudeepchandrase
Contributor II

I am using an MK20DV256 for one of my projects. In the hardware(PCB) there are two clock available:

1. A 32 KHz RTC crystal connected to (XTAL32 and EXTAL32)

2. A 12 MHz crystal connected to (XTAL0 and EXTAL0).

We have derived the MCGOUT clock(~70 Mhz) from the FLL operating in FEE mode.

From what I understand I can clock the Flex CAN module from the below options:

1. The 12 Mhz crystal directly(OSCERCLK)

2. ~35.99 Mhz Bus clock.

Below are a few questions that I am trying to get answers to:

1. The clock to choose for the Flex CAN module seems to be dependent on the most accurate clock. How do we calculate the accuracy of the FLL stabilized MCGOUT clock?

2. Are there any other parameters that need to be considered before we choose the MCG clock.

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3 Replies

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mjbcswitzerland
Specialist V

Hi

If the required bus speed can be derived from OSCERCLK this should be used since it has lowest jitter and so achieves best CAN operation quality.

Regards

Mark

CAN
http://www.utasker.com/docs/uTasker/uTaskerCAN.PDF

Complete K20 solutions for professionals, training and support:http://www.utasker.com/kinetis.html
Kinetis K20:
- http://www.utasker.com/kinetis/FRDM-K20D50M.html
- http://www.utasker.com/kinetis/TWR-K20D50M.html
- http://www.utasker.com/kinetis/TWR-K20D72M.html
- http://www.utasker.com/kinetis/TEENSY_3.1.html
- http://www.utasker.com/kinetis/tinyK20.html

810 Views
sudeepchandrase
Contributor II

Hi Mark,

Thanks for the reply. I am intending to operate at 1 Mbps. Is there a way to quantify jitter?

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810 Views
mjbcswitzerland
Specialist V

Hi

Jitter: https://en.wikipedia.org/wiki/Jitter
See data sheet for values.

For 1M CAN speed from 12MHz OSCERCLK set a pre-scaler of 6 and time quanta of 24 for exact 1MHz. This is the value 0x01b70007 in CAN_CTRL1.

Regards

Mark

Turn-key CANopen for Kinetis - http://www.utasker.com/index.html
CAN simulation: https://www.youtube.com/watch?v=Ha8cv_XEvco