MC9S12G64: Using external oscillator (not crystal)

[ZdenoQC]

Hi everyone,

I'm trying to use an external oscillator for the G64. The source is an RF chip that give me 13.56MHz. I calculate all the value for the CPMUSYNR, CPMUREFDIV but the PLL never lock. By going backward, I find that the bit CPMUFLG_UPOSC is always zero. Like if the micro doesn't detect the oscillation.

The clock source is plug to the EXTAL pin and XTAL pin is not plug.

Do I have some register to set for the UPOSC bit to go 1 or it's just not possible with this clock configuration?

Here is my code for now:

CPMUOSC = 0x80; //Enable oscillator

CPMUSYNR = 0x16; // VCO from 32 to 48MHz, SYNDIV = 22

CPMUREFDIV = 0x0C; // Fref from 1 to 2MHz, Fref = Fosc/13

while(!CPMUFLG_LOCK); // Wait for lock !!! Stop here !!!

CPMUCLKS_PLLSEL = 1; //Switch to PLL

Thanks you

[bigmac]

Hello,

You might try the following configuration -

Connect a 1Mohm resistor between the XTAL and EXTAL pins of the MCU.  Then couple the external signal from the RF chip to the EXTAL pin using a series capacitor, say 100 pF.

Regards,

Mac

[iggi · Accepted solution]

The main thing is that S12G device does not support external oscillator, only crystal in LCP connection. The

MC9S12G Family Reference Manual and Data Sheet clearly states on page 44:

Main External Oscillator (XOSCLCP)

• Loop control Pierce oscillator using a 4 MHz to 16 MHz crystal

Btw, it seems like the VCOFRQ[1:0] values is incorrect. There should be CPMUSYNR = 0x56;

In the reference manual, find the chapter 10.1.2 Modes of Operation. There are described 3 different PLL modes: PEI, PEE, PBE. You can test these modes using SW example codes made for S12G devices. This is the part of the application note AN4455 MC9S12G64 Demonstration Lab Training. Download link below:

http://cache.freescale.com/files/microcontrollers/doc/app_note/AN4455.pdf

http://cache.freescale.com/files/microcontrollers/doc/app_note/AN4455SW.zip

Regards,

Ivan

[ZdenoQC]

Thanks you for the info. I would have like this to work, but I will have to work without it.

I have tried some level translator to lower the oscillator voltage, but at the end, it was unstable.

Thanks again!

William