Yes, the first thing we tried was exactly the "recommended"
circuit (Figure
15-1), with no capacitors. Extreme care was taken in the PCB layout (shortest
possible traces, good 4-layer board, etc.). It failed miserably.
We tried crystals with different characteristics, without success.
We tried everything in FAQ-20214, without success.
We spent weeks of effort to try to get the on-chip oscillator to start up
reliably at low temperatures, without success. We tried all possible
combinations in the oscillator circuit, with 0, 1, or 2 capacitors of various
values (10pF to 47pF) to ground (at EXTAL and/or XTAL), different feedback
resistor values from 1M to 22M ohms, and with or without the series resistor
(Rs
in Motorola Application Note AN2049). None of these circuits started up
reliably at low temperatures, so we reluctantly decided (as strongly
recommended
in Motorola Application Note AN2049) that the only solution was to use a
separate oscillator circuit that was guaranteed to start up at low
temperatures.
That solved all of our problems.
Regards,
Art Johnson
Senior Systems Analyst
PMC Prime Mover Controls Inc.
3600 Gilmore Way
Burnaby, B.C., Canada
V5G 4R8
Phone: 604 433-4644
FAX: 604 433-5570
Email:
http://www.pmc-controls.com
-----Original Message-----
From: Charlie W [mailto:]
Sent: Thursday, June 26, 2003 5:50 PM
To: Pete Becher;
Subject: Re: [motoroladsp] Re: Crystal not working
Please be aware that leg capacitors - most MCU
manufactures suggest 22pf-33pf - can kill crystal.
Motorola does not recommend leg caps for its DSP56F80x
devices.
Charlie W
--- Pete Becher <> wrote:
> Hi Rob,
>
> I have had problems with the crystal oscillator
> starting reliable
> also (at room temp). Out of 4 prototype boards I
> was testing 1 would
> start reliably, 1 would start randomly and 2 would
> not start at all.
> I did a lot of testing with various capacitors,
> resistors and
> different crystals with no good luck. I finally
> went with an
> external clock oscillator after reading Art's posts.
> All prototype
> and production boards have been working great since.
>
> Pete
>
> --- In , Hutchings
> William-p23437
> <william.hutchings@m...> wrote:
> > Rob -
> >
> > I do not know exactly how you have connected up
> the crystal, but we
> have a FAQ that might be of interest.
> >
> > FAQ-20214
> >
> >
> > Question:
> >
> > I am experiencing some crystal or resonator start
> up yield
> problems. Is their something you can recommend to
> resolve these
> issues ?
> >
> > answer
> >
> > Answer:
> >
> > The internal oscillator is designed to interface
> with a parallel-
> resonant crystal resonator in the frequency range
> specified for the
> external crystal in the data sheet. In the data
> sheet a recommended
> crystal oscillator circuit is shown. Follow the
> crystal
> supplier's recommendations when selecting a
> crystal, because
> crystal parameters determine the component values
> required to provide
> maximum stability and reliable start-up. The crystal
> and associated
> components should be mounted as close as possible to
> the EXTAL and
> XTAL pins to minimize output distortion and start-up
> stabilization
> time. The internal 56F800 oscillator circuitry is
> designed to have no
> external load capacitors present. As shown in the
> data sheet no
> external load capacitors should be used. If your
> circuit has external
> load capacitors these will cause crystal start up
> problems. Removing
> these capacitors should resolve your crystal start
> up issues
> >
> > If your circuit does not have external load
> capacitors and you are
> still experiencing a small yield issue you can also
> try limiting the
> ESR of value of the crystal you have chosen. The ESR
> of the crystal
> can affect the over all gain of the circuit and
> hence the start up
> characteristics. Lowering the ESR provides greater
> gain. An example
> would be if your crystal manufacturer specifies a
> maximum of 70 ohms
> ESR you could request a screen of the parts supplied
> to you for a
> maximum of 60 ESR and test to see if this resolves
> your start up
> yield issue. Crystal start up is affected by many
> different board,
> manufacturing, and crystal parameters and the values
> for ESR in this
> FAQ should not be taken as a maximum or recommended
> value.
> >
> >
> >
> > Related Products
> >
> > DSP56F826 <http://e-
>
www.motorola.com/webapp/sps/site/prod_summary.jsp?code=DSP56F826>
>
> > DSP56F827 <http://e-
>
www.motorola.com/webapp/sps/site/prod_summary.jsp?code=DSP56F827>
>
> > DSP56F801 <http://e-
>
www.motorola.com/webapp/sps/site/prod_summary.jsp?code=DSP56F801>
>
> > DSP56F803 <http://e-
>
www.motorola.com/webapp/sps/site/prod_summary.jsp?code=DSP56F803>
>
> > DSP56F805 <http://e-
>
www.motorola.com/webapp/sps/site/prod_summary.jsp?code=DSP56F805>
>
> > DSP56F807 <http://e-
>
www.motorola.com/webapp/sps/site/prod_summary.jsp?code=DSP56F807>
>
> >
> >
> > -----Original Message-----
> > From: Art Johnson [mailto:art@p...]
> > Sent: Wednesday, June 25, 2003 2:43 PM
> > To: dibosco;
> > Subject: RE: [motoroladsp] Crystal not working
> >
> >
> > Hi Rob,
> >
> > We had some oscillator startup failures (although
> not at room
> temperature) on our 56F807 products, so we went to
> an external
> oscillator. This is described in the following
> messages I sent on
> September 5 and 6 of 2002:
> > September 5, 2002, about 7:50am:
> > "RE: [motoroladsp] 56F807 expected life cycle"
> > September 6, 2002, about 6:53am:
> > "RE: [motoroladsp] Application Note AN2049 vs.
> 56800/56800E
> User's Manuals"
> >
> > We never had an oscillator startup failure at room
> temperature, so
> your problem may be a bad chip or a bad crystal.
> >
> > Regards,
> >
> > Art Johnson
> > Senior Systems Analyst
> > PMC Prime Mover Controls Inc.
> > 3600 Gilmore Way
> > Burnaby, B.C., Canada
> > V5G 4R8
> > Phone: 604 433-4644
> > FAX: 604 433-5570
> > Email: art@p...
> > http://www.pmc-controls.com
> <http://www.pmc-controls.com>
> >
> >
> >
> > -----Original Message-----
> > From: dibosco [mailto:Robert.Wood@a...]
> > Sent: Wednesday, June 25, 2003 2:02 PM
> > To:
> > Subject: [motoroladsp] Crystal not working
> >
> >
> > Hi folks,
> >
> > I've just built up my new target board (first one
> with a 56Fxxx) and
> > am having no joy getting the crystal going - which
> is a bit
> depressing!
> >
> > I've got a 56F803, with an ACT crystal and 10M
> across the crystal,
> > just as the manual suggests and the EVM has.
> >
> > The crystal is an HC49/U - datasheet can be found
> on the link below.
> >
> >
>
http://www.act.co.uk/products/crystals/PTH_Crystals/actHC49_U.html
>
>
<http://www.act.co.uk/products/crystals/PTH_Crystals/actHC49_U.html>
>
> >
> > The voltages on the pins of the crytal sit at
> around 2.6V and Vcc
> (3.3V).
> >
> > I assume a blank device should still get the
> crystal oscillating? If
> > not, I guess the problem could well be with the
> JTAG set up.
> >
> > With that sketchy amount of info, anyone got any
> suggestions?
> >
> > Cheers,
> >
> > Rob.
> >
> >
> >
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