Ryan,
In my experience, the HCS12 will work fine if you give it an external clock
generator square wave in the range of 0.5MHz - 16MHz, and wrongly select
the Colpitts configuration by puling XCLKS high during reset. This is a
normal behavior in my experience, and doesn't mean the HCS12 doesn't
expects a Colpitts crystal rather than an external clock generator. It
could help if a status bit would exist in an internal HCS12 register and
would tell what was the sampled state of XCLKS during Reset. I looked in
the manual for such a bit but couldn't find one. I guess after verifying
you indeed pull PE7/XCLKS high during Reset (note that high needs to be
higher than 0.65 * VDDX = 3.25V typically in the case of the HCS12), you
need to believe the Colpitts crystal configuration is selected.
In your case, I would suspect especially the DC blocking capacitor. What
value are you using? Try changing its value and see if it makes a
difference. If this doesn't help try using a different crystal from another
manufacturer.
We have had many customers that had problems with getting the Colpitts
crystal to work. Some replaced crystals and capacitors and got it working
eventually, and some gave up and switched to a Pierce crystal configuration
with one of the newer HCS12 devices.
Hope this helps,
Doron
Nohau Corporation
HC12 In-Circuit Emulators
www.nohau.com/emul12pc.html
At 01:18 AM 5/5/2003 +0000, you wrote:
>Doron,
>
>Thanks for the information.
>
>I am using a 4Mhz citizen crystal on the board, and the Colpitts
>design from the motorola data books. I think that the crystal
>circuit is OK, it is just this weird problem with the HCS12 not
>seeming to "acknowledge the voltage" on XCLKS, and only use the
>external. Any idea why this might be?
>
>Ryan
>--- In , Doron Fael <doronf@n...> wrote:
> > Ryan,
> >
> > There are a few things to notice when designing with a crystal for
>the
> > 9S12DP256:
> >
> > When PE7/XCLKS is pulled high during Reset, the DP256 expects an
>external
> > Colpitts crystal configuration to generate its clock. The Colpitts
>crystal
> > configuration requires different external crystal connections as
>compared
> > with the more common Pierce crystal configuration used by most
> > micro-processor systems. In the Colpitts configuration, the crystal
>in
> > mounted between EXTAL and VSSPLL, a capacitor is mounted between
>EXTAL and
> > XTAL, and another capacitor is mounted between XTAL and VSSPLL. The
> > Colpitts configuration is used to preserve power, as the generated
>voltage
> > swing with it is only a few hundred mili-volts, around a positive
>DC
> > voltage level. The down-side of this quality is that it makes the
>Colpitts
> > oscillator also more susceptible to noise, than the Pierce
>configuration,
> > and so careful layout of the crystal circuit next to the DP256 is
>needed,
> > to reduce possible influence of noise and leakage current (Motorola
>include
> > recommended crystal layout in their data sheets). The Colpits
>configuration
> > is also limited in the case of the DP256 to use crystals in the
>range of
> > 0.5MHz to 16MHz. Attempting to use a higher frequency crystal,
>above 16MHz,
> > will likely lead to a clock failure. An additional complication of
>the
> > Colpitts configuration is that a DC voltage is applied across the
>crystal,
> > which may shorten the life expectancy of many crystals, and may
>also
> > prevent some crystals from starting to oscillate. The way to
>overcome these
> > problems is by installing a 3rd capacitor in the crystal circuit
>between
> > the EXTAL pin and the crystal, to block the DC level from reaching
>the crystal.
> >
> > Some users choose not to use the Colpits crystal configuration due
>to these
> > complications mentioned above. If you also choose not to use the
>Colpits
> > configuration you have the following two alternatives:
> >
> > 1. Move to another HCS12 part, that can also work with the Piece
>crystal
> > configuration and has much less complications.
> >
> > 2. Use an external clock generator.
> > The external clock generator also has several possible
>complications that
> > need to be taken care of:
> > It requires a square wave voltage levels of 0V and 2.5V which
>requires also
> > a 2.5V external voltage regulator on your board. If going into
>power-down
> > modes and especially STOP power-down, the internal generate VDD and
>VDDPLL
> > in the DP256 drop significantly bellow 2.5V (to preserve power). If
>your
> > clock generator continues to supply 2.5V square wave during this
>period
> > damage may occur to the DP256 internal clock circuit. The two
>possible ways
> > to overcome this problem, are: A) supply VDD and VDDPLL from an
>external
> > 2.5V supply that does not drop its voltage when going into the STOP
>power
> > down mode (and connect VREGEN to GND). B) Connect two Schottky
>diodes from
> > your external 2.5V regulator to VDD and VDDPLL, to prevent these
>voltages
> > to drop more than 0.3V bellow the external clock generator square
>wave.
> > Another thing to note when using an external clock generator is to
>not
> > generate a 50MHz external clock using it, as its specified rise
>time and
> > fall time need to be less than 1nSEC, and the duty cycle needs to
>be 47.5%
> > - 52.5% at this maximum frequency. If a lower frequency is used
>instead
> > (40MHz or bellow), the rise time, fall time and duty cycle can be
>much more
> > relaxed and easier to generate at 0V - 2.5V voltage levels. You can
>then
> > still use the internal PLL to up the clock to the maximum allowed
>25MHz bus
> > speed.
> >
> > I hope you find this explanation useful, and am sorry it is not any
>less
> > complicated.
> >
> > Doron
> > Nohau Corporation
> > HC12 In-Circuit Emulators
> > www.nohau.com/emul12pc.html
> >
> > At 09:28 PM 5/1/2003 +0000, you wrote:
> > >Hi All,
> > >
> > >I'm in the process of designing an MCU board using a
9S12DP256. I
> > >want to use a crystal and generate the clock internally. I have
>used
> > >to recommended values (from Motorola) to set up the PLL filter
on
>the
> > >XFC pin, but I currently have an interesting problem:
> > >
> > >As you all know, the HCS12 can user either a crystal and
internal
> > >generator (!XCLKS line = VCC) or an external clock (!XLCKS =
GND).
> > >
> > >I have the board assembled and it seems to work just fine using
a
> > >function generator for an external clock. When I change the
!XCLKS
> > >voltage to use the crystal, the board no longer works. The
quirk
>is
> > >that when !XCLKS is set up for internal, applying the external
>clock
> > >will still work, and the board will fully be functional.
> > >
> > >I have tested this on an Axiom Manufacturing board with the
same
> > >chip, and when the !XCLKS line is VCC (for internal) the HCS12
will
> > >ignore the external clock from the generators.
> > >
> > >Any help greatly appreciated,
> > >
> > >Ryan Kruse
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