9S12C128 comes up with SCM set even with a clean 8MHz xtal clock

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9S12C128 comes up with SCM set even with a clean 8MHz xtal clock

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ve3id
Contributor II

In my first 9S12 design, I haven't paid enough attention to the PLL loop filter since I knew I was not going to use the PLL.  On power-up of all 6 prototypes, I found SCM was setting and the BDM pod was reporting a bus frequency of 1.48 MHz instead of the 4 MHz I expected from the 8 MHz pierce oscillator with 8M2 resistor across the xtal and two C of 22p each to gnd. 

 

Looking at the board, I saw I had put the PLL filter too far awy from the chip and obviously it was picking up noise in the lands.

 

So I cut the lands to the PLL filter, bypassed the Vdd PLL with a 100nF at the chip, and all seemed well.  For a couple of months I have been successfully develping software using the 6 prototypes.  I have got a clean 8MHz clock of 2V p-p from the xtal.

 

All of a sudden today, all six boards have gone back to a 1.48MHz bus speed!  It`s like the laws of physics have changed overnight.  I can`t think of anything that has changed.

 

Since there is little risk to having a cpu failure (it is a ham radio project) I would be happy if I could just disable the clock test circuitry and have it merrily using the 8MHz xtal.  Can anybody tell me how to do this, since I can't find any way by reference to the data books.

 

Or any solution to my problem other than re-designing the board, since it is pro-bono I don't want to shoulder that expense.

 

Thanks for any help,

 

Nigel Johnson

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ve3id
Contributor II

Thanks for the reply.  Being a radio ham, I have a SW receiver handy, and I can clearly hear the 8 MHz oscillator come on and off as I power up and down the board!  Kinda eliminates the scope probe capacitance :smileyhappy:

 

I also tried clearing CME as my first line of code, but of course you can't do this if SCM is set, which happens before you get to execute any code.

 

I did discover what was different from previous weeks however: I had been using a 12V power supply with a 100 ohm resistor in series from another project, and the voltage supplied to my board was 7V instead of 12V. This meant that around 3.3 V was being applied to the chip after the voltage regulators.  So it seems to work with 3.3V and not 5V. Perhaps the 2V p-p from the xtal is not enough?  I tried my original wire-wrapped prototype which works, and found 2V there also, as well as on a tech arts board, so I am not sure what this means.

 

Again, thanks for any help, it is appreciated. Even one of Baldrick's 'cunning plans' wouldn't be amiss.

 

cheers,

 

Nigel Johnson

 

 

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kef
Specialist I

Hm, defective crystals?

Are you sure oscilator is oscilating before you touch it with scope probe? If ECLK output is not used in your application, then I would clear NECLK bit in firmware on reset and monitor ECLK output instead of crystal pins.

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