Lpc2000

Hi:

I am Alejandro from Argentina.
I am starting with 2138 uC.

I saw a sch from Jan Szymanski, I have a question for you.

You put a 560K resistor between the 32.768khz xtal and de uC.

Do you found that configuration in a Philips AN?

Can you explain it form me?

Best regards, 
Alejandro

Hi Alejandro,

> I am Alejandro from Argentina.
> I am starting with 2138 uC.
> 
> I saw a sch from Jan Szymanski, I have a question for you.
> 
> You put a 560K resistor between the 32.768khz xtal and de uC.
> 
> Do you found that configuration in a Philips AN?
> 
> Can you explain it form me?
> 
> Best regards, 
> Alejandro

I haven't seen the schematic, but it is not uncommon for some oscillators to need a biasing resistor.
Basic crystal oscillators like these typically consist of a Pierce oscillatior.
A Pierce in silicon can be done with an inverting-gate style setup, or just a FET.
Depending on this, the oscillator circuit might need DC biasing to ensure it starts up reliably - 
therefore sometimes resistor between XIN and XOUT is used.
If you want to look closer at this, google for 'Pierce oscillator'. Should have lots of hits.

B rgds
Kris


[Non-text portions of this message have been removed]

From: "Alejandro Fabricio Fernandez"
> I am starting with 2138 uC.
> I saw a sch from Jan Szymanski, I have a question for you.
> You put a 560K resistor between the 32.768khz xtal and de uC.
> Do you found that configuration in a Philips AN?

Next time please could post a link to the circuit, to prevent unnecessary
speculations...

If the resistor is electrically between output and crystal, it is used to
limit the drive to the crystal. Expecially the 32 kHz crystals can easily be
overdriven and destroyed. But 560 K looks a bit high - I normally use 220
kOhm.

If is is beteen the input and output of the oscillator amplifier, it is a
biasing resistor - and should normally be between 2.2 and 10M ohm.

Regards,
Arie de Muynck

I am sorry.
The link is http://www.keil.com/mcb2130/mcb2130-schematics.pdf

I understand that in this case the resistor was used for limiting the drive.
I will take your suggestion about the value. (220K)

Thank you very much.

  ----- Original Message ----- 
  From: Arie de Muynck 
  To: lpc2000@yahoogroups.com 
  Sent: Wednesday, March 23, 2005 5:42 PM
  Subject: Re: [lpc2000] 560K in RTC XTAL circuit


  From: "Alejandro Fabricio Fernandez"
  > I am starting with 2138 uC.
  > I saw a sch from Jan Szymanski, I have a question for you.
  > You put a 560K resistor between the 32.768khz xtal and de uC.
  > Do you found that configuration in a Philips AN?

  Next time please could post a link to the circuit, to prevent unnecessary
  speculations...

  If the resistor is electrically between output and crystal, it is used to
  limit the drive to the crystal. Expecially the 32 kHz crystals can easily be
  overdriven and destroyed. But 560 K looks a bit high - I normally use 220
  kOhm.

  If is is beteen the input and output of the oscillator amplifier, it is a
  biasing resistor - and should normally be between 2.2 and 10M ohm.

  Regards,
  Arie de Muynck


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[Non-text portions of this message have been removed]

Arie wrote :

> If the resistor is electrically between output and crystal, it is used to
> limit the drive to the crystal. Expecially the 32 kHz crystals can easily be
> overdriven and destroyed. But 560 K looks a bit high - I normally use 220
> kOhm.

Arie brings up a good point too, and it's easily overlooked.
Crystals that are driven too hard age fast, will excessively drift in frequency much quicker 
(not such an issue w/ MCU in general I guess), can also go in overtone over Vcc 
and even just break.

> If is is beteen the input and output of the oscillator amplifier, it is a
> biasing resistor - and should normally be between 2.2 and 10M ohm.

As before.
Can be as low as 1 Mohm though :-)

B rgds
Kris


[Non-text portions of this message have been removed]

From: <aff@...>
> The link is http://www.keil.com/mcb2130/mcb2130-schematics.pdf

Thanks.

WHAAATT? I expect the resistor between the output of the oscillator and the
connection of the crystal with the 22 pF capacitor (assuming RTXC2 is the
output, it should be on the left of C19, not on the right side). The way
shown it DECREASES the Q (resonance quality) of the crystal - and
oscillation is not likely. In series with the output it lowers the drive and
INCREASES the Q. This is a strange circuit indeed - and I expect a drawing
error...

The usual circuit  is (see in fixed font!):

    --------------------
   |                    |
   | |\                 |
    -| >o-- R ----- X --|
     |/         |       |
                C       C
                |       |
      GND ---------------

> I understand that in this case the resistor was used for limiting the
drive.
> I will take your suggestion about the value. (220K)

Start with my circuit with R is 560K - if the Keil circuit works at all,
that should be OK. If it does not start, try lower values. Don't go below
220K.

Regards,
Arie de Muynck

From: "microbit" <microbit@...>
> Crystals that are driven too hard age fast, will excessively drift in
frequency much quicker
> (not such an issue w/ MCU in general I guess), can also go in overtone
over Vcc
> and even just break.

Literally - I've seen it happen before my eyes, with the old Statek crystals
in a ceramic case with glass lid. They were $10 back then   :-(

About the biasing resistor:
> Can be as low as 1 Mohm though :-)

Only if needed when the input has leakage - a lower value also lowers the
change of oscillation (impedance of the inverting feedback path through the
crystal should be lower than that though this resistor, and if they are
close together the oscillator stage needs more gain).

Regards,
Arie de Muynck

> Literally - I've seen it happen before my eyes, with the old Statek
crystals
> in a ceramic case with glass lid. They were $10 back then   :-(
>
> Regards,
> Arie de Muynck

In my teens I had a ;pile of the good old FT-243 Xtals, from my father.
(I think I have the name right, it's like 30 years ago)
They were cool ! Just unscrew, and you could even _carefully_ alter the
frequency a bit by "etching" etc.

Cheers,
Kris