How to determine the HCS12 flash FCLKDIV setting

[AndersJ]

Various application notes decribe how to set FCLKDIV, based on

"oscillator frequency", "bus clock", "oscillator" etc etc.

Which frequency is it?

* Crystal frequency

* Bus frequency

* PLL frequency

* etc

With a 16MHz crystal and no PLL, how should FCLKDIV be programmed?

Thanks,

Anders J

[Lundin]

Crystal frequency = the number printed on the crystal.

Oscillator frequency = crystal frequency, unless you are using an external oscillator.

PLL frequency = if the PLL is used, it is the same as the oscillator frequency multiplied/divided by a number.

Bus frequency = either the oscillator frequency / 2 (default), or the PLL frequency.


FCLKDIV is calculated based on both bus frequency and oscillator, as you can see in the algorithm described in the manual. You need to set it to a value between 150-200kHz. Also note that FCLKDIV is a write-once register, so you need to write everything at once, not bit-by-bit.

Can't give you a register value for 16MHz, but you will probably want to make a generic algorithm working with any frequency anyway.

[Nabla69]

Hello,

I agree with all but a small thing I want to flag up.

Even the PLL clock is divided by 2 before feeding th bus.

Please see enclosed picture from S12DG128 CRG datasheet.

Alvin.

[Lundin]

Yes, you are correct. PLL clock replaces the oscillator clock and in either case it is divided by 2 to get the bus clock.