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clock question of MC9S12XS128

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clock question of MC9S12XS128

‎10-15-2009 02:52 AM
719 Views
skyfaller
Contributor I

I developed a board based on MC9S12XS128 and I used a 16MHz external oscillator to drive the MCU.

 

In the databook(MC9S12XS256RMV1.PDF) it's said that the external clock should be 1.8V level because the VDDPLL=1.8V from internal regulator and the maxium value is VDDPLL+0.3=2.1V.

 

At first I used 3.3V to power the external oscillator ,i.e. the clock was 3.3V level, but I found that the clock could not drive the MCU and the MCU went into selfclock-mode after reset. When I changed the power for the external oscillator from 3.3V to 5V it works properly.

 

So I am confused. According to the datasheet even the 3.3V exceed the maxium value. But how to explain the problems I meet?

 

Thanks

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‎10-16-2009 08:46 AM
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skyfaller
Contributor I

And I found that the VDDPLL output voltage varied with the clock voltage level !! When the oscillator was powered by 5V the VDDPLL output voltage rised to 2.6V instead of 1.8V.

 

So I got totally confused because the VDDPLL was the output of internal regulator and it was ought to be at fixed level of 1.8V.

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