Trouble With Crystal Clock Generation on 9S12DP256

Started by rmk1113 May 1, 2003
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



When you refer to "crystal", do you mean a quartz crystal, or an external
oscillator? 607-656-2597 -----Original Message-----
From: rmk1113 [mailto:]
Sent: Thursday, May 01, 2003 5:28 PM
To:
Subject: [68HC12] Trouble With Crystal Clock Generation on 9S12DP256 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
--------------------
">http://docs.yahoo.com/info/terms/



Is the crystal frequency upper limit for the DP256 different than that of
the DG128A?

On March 18, 2002 Motorola announced in this forum that there was an erratum
in the data sheet for the DG128A that implied that either a 16Mhz crystal or
package oscillator could be used. It turns out that for reliable
operations, a crystal can only be used for frequencies up to 8Mhz!
-----Original Message-----
From: Doron Fael [mailto:]
Sent: Friday, May 02, 2003 11:11 AM
To:
Subject: Re: [68HC12] Trouble With Crystal Clock Generation on 9S12DP256 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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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




The 9S12DP256 Colpitts crystal circuit is more stable than on the 68HC912D
parts. As far as I know, on the 9S12DP256 the Colpitts crystal can really
work to 16MHz.

With newer HCS12 parts, also the Pierce crystal configuration is possible,
which can work to 40MHz.

Doron
Nohau Corporation
HC12 In-Circuit Emulators
www.nohau.com/emul12pc.html

At 04:31 PM 5/2/2003 +0200, you wrote:

>Is the crystal frequency upper limit for the DP256 different than that of
>the DG128A?
>
>On March 18, 2002 Motorola announced in this forum that there was an erratum
>in the data sheet for the DG128A that implied that either a 16Mhz crystal or
>package oscillator could be used. It turns out that for reliable
>operations, a crystal can only be used for frequencies up to 8Mhz! >
>-----Original Message-----
>From: Doron Fael [mailto:]
>Sent: Friday, May 02, 2003 11:11 AM
>To:
>Subject: Re: [68HC12] Trouble With Crystal Clock Generation on 9S12DP256 >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


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 >
>





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



At 08:49 AM 5/5/2003 +0200, you wrote:

>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.

We finally gave up on the Colpitts with the D60A. Just too unreliable on
startup over temperature. Finally added an external osc. - even though it
cost us more money and power. IMHO, the Colpitts was not a good design change.

The D60C is supposed to have a better crystal driver design, haven't tested
it yet. And the new D60P (with the Pierce driver) doesn't seem to be
readily available.

Like you said, we decided to switch rather than fight.

jmk -----------
James M. Knox
TriSoft ph 512-385-0316
1109-A Shady Lane fax 512-366-4331
Austin, Tx 78721
-----------


At 05:11 PM 5/2/2003 +0200, you wrote:

>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.

Is that 2.5V restriction unique to the 9S12DP256? I looked at other
datasheets (like the D60A) and it looks like a 5V drive signal (basically,
anything up to Vdd) is fine.

jmk
-----------
James M. Knox
TriSoft ph 512-385-0316
1109-A Shady Lane fax 512-366-4331
Austin, Tx 78721
-----------