68300

Tim,
 
When you drop the voltage on the part, you also drop the VOL and VOH voltages.  That is, the part will not have as much drive current on any of the pins.  In this case, you need to worry about XTAL.  The first (and easiest) thing for you to try is reduce the value of the series resistor in the crystal oscillator circuit.  Most Motorola documentation shows a 330 Kohm resistor in series with the crystal.  This value is too high (too many ohms) when you use 3.3 volt parts.
 
You are going to have to experiment a little yourself but here is what you want to accomplish.  You want to pick a series resistor that will cause the crystal to start in 200 - 400 milliseconds.  This time is measured from the application of power to the time you see a valid signal on CLKOUT.  DO NOT probe XTAL or EXTAL.  The scope probe will have a material effect on crystal performance.
 
I imagine that you will end up with a series resistor of about 50 Kohm or so.
 
Let me know about your progress.
 
Charlie

-----Original Message-----
From: tmccormick@... [mailto:tmccormick@...]
Sent: Thursday, June 20, 2002 2:19 PM
To: 68300@yahoogroups.com
Subject: Re: [68300] 3 volt 68332?



Hello,

We have a design using a 3.3V 68332 micro (MC68LK332ACFC16) and we are
experiencing a problem wherein on 2 boards so far, the micro stays in reset
after a power up because the 32,768KHz crystal is not oscillating.

I wonder if this is related to the attached message from Charles Melear
concerning a mask difference between the 5V and 3.3V devices in the PLL
area.

Is there a know  problem with the 3.3V device related to the PLL?

Tim McCormcik



12/14/01
>Message sent by "Charles Melear" <rdph40@...>
>to the 683xx Mailing List.

>Tim,

>I think that there may be a small tweek to the PLL but that is all.

>Charlie

>tmccormick@... wrote:
>
>  Message sent by tmccormick@...
>  to the 683xx Mailing List.
>
> Follow up question:
>
> Is the 3.3V device MC68LK332ACFC16 the same die also?
>
> Tim McCormick
>
>
>                     "Charles Melear"
>                     <rdph40@...>       To:
683xx@...
>                     Sent by:                         cc:
>                     owner-683xx@oakhill-csic.s       Subject:     Re: 25
MHz 68332 any diff?
>                     ps.mot.com
>
>
>                     12/12/01 04:58 PM
>                     Please respond to 683xx
>
>
>
>  Message sent by "Charles Melear" <rdph40@...>
>  to the 683xx Mailing List.
>
> Scott,
>
> You are the man.  You have all the bases covered with regard to using the
> High Stability
> Filter and all.
>
> Bye the bye, the 16, 20 and 25 MHz devices all come from the same die.
> There is NO - NADA
> - NYET difference between the devices.
>
> Here are two ideas.  First, does the oscillator fail to start?  This
would
> be indicated by
> the part never releasing reset and limp mode frequency (about 1.8 MHz) on
> Clockout.  The
> cure is to reduce the value of the series resistor on the crystal (the
330
> Kohm) until you
> see an 8 MHz signal on clock out about 200 to 400 milliseconds after
power
> is turned on.
> It is really hard to probe the crystal pins to see when the oscillator is
> starting because
> the scope probe really loads those crystal pins.  Look at Clockout
instead.
>
> Second, if you are changing the frequency to anything above 8 MHz, it is
> IMPERATIVE (like
> you MUST DO THIS) that the X bit in the SYNCR register be set to a logic
1.
> There is just
> no other alternative   -  well, there is none that will work.
>
> See where this gets you and post another message if this doesn't work.
>
> Regards,
>
> Charlie


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