Kirk Humphries

oscillator

Discussion created by Kirk Humphries Employee on Jan 29, 2006

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Posted: Sat Sep 24, 2005  4:03 pm

 

I have a problem with the mc9s12A256 oscillator. I try to download

software but it doesn't work.

So i have test all the vdd and it is correct.

But i have nothing on the extal and xtal pin.

 

I use the pierce oscillator with pe7=0

I have c1=c2=22pf and R=4.7Mohm and crystal=16Mhz

 

When the micro is corretly powered should i see something on xtal or extal?

 


 

Posted: Sat Sep 24, 2005  4:46 pm

 

> I have a problem with the mc9s12A256 oscillator. I try to download

> software but it doesn't work.

> So i have test all the vdd and it is correct.

> But i have nothing on the extal and xtal pin.

>

 

> I use the pierce oscillator with pe7=0

> I have c1=c2=22pf and R=4.7Mohm and crystal=16Mhz

>

 

> When the micro is corretly powered should i see something on xtal or extal?

 

Yes, the oscilloscope should show you sinus wave on both pins. EXTAL is input and XTAL is output of the amplifier, XTAL amplitude will be higher than EXTAL amplitude. I'm currently looking at the scope display and that's what I see on XTAL: amplitude about 1.6V, DC offset 1.3V (so the swing is from 0.5V to 2.1V DC).  Frequency matches XTAL nominal, of course.

 


 

Posted: Sun Sep 25, 2005  7:44 pm

 

I suspect the mask-set you are using does not support the Pierce Crystal configuration at all.

 

Please note that the early silicons MC9S12A256B / C parts - mask sets 0K36N, 0K79X, 1K79X and 2K79X don't support the Pierce crystal configuration. They only support the Colpitts crystal or an external square wave clock with 0V and 2.5V voltage levels. (the mask set is marked on the part you are using, so it should be easy to read it with the naked eye, or a magnifying glass)

 

The ability for a Pierce crystal configuration has been added only in the later MC9S12A256 silicon - mask-sets: 0L91N - 4L91N and 0L01Y.

 

Try checking which mask set your are using, and if it turns out to be one of the older mask-sets, you will need to replace it with a newer one to work with the Colpitts crystal configuration.

 

This specific issue exist only with the S12D_256 and S12A256 parts. For the S12D_512, S12D_128, S12D_64 and all mask sets work with both the Pierce and the Colpitts crystal configurations.

 

Generally speaking, the Pierce crystal configuration is less sensitive to layout problems (as long as you don't have loops of 10 inches all around the board), especially as compared to the Colpitts crystal configuration which is more susceptible to layout problems. The reason is the Pirce configuration involves higher voltage swing while the Colpitts HCS12 implementation involves very low voltage swing, so small ripple or noise can easily affect the clock signal seen by the HCS12.

 

Hope this helps,

 


 

Posted: Mon Sep 26, 2005  7:09 am

 

Thank you. I will have a look on the mask.

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