> The oscillator requires quite large value capacitors - try 33 pF.

It really depends on the Xtal, rather than the clock oscillator itself.

Normally you would load the crystal with its actual load capacitance.

Ultimately the caps also affect start up time and reliability of oscillation.

 

It depends on whether the (assumed Pierce) oscillator is implemented as

a NOT gate, or with a FET. (generally you can tell whether the oscillator

needs a bias resistor or not)

Sometimes the oscillator's drive can be a bit low, depending on silicon.
For some Xtals, the load caps might need optimising a bit, but with most

MCUs you don't have that luxuary since you need a 50% dutycycle

(unless you're using the PLL, then it's OK)

 

Ideally, the input cap dictates the loading of the crystal, and the output cap

sets the phase shift for the oscillator.

 

A good rule of thumb would be to use double the load value of the Xtal

on either side, so it depends on the Xtal that you're using.

A typical CL pF Xtal would eg. use 27 pF on either side.

 

You can get other problems too, like the Xtal can go into Third Overtone etc.

(Anyone ever used the P300 of Intellon ?? 3OT problems galore :-)

Best is to use a good quality, fundamental Xtal with low ESR and low load capacitance

for good startup.

 

This area is too often overlooked.

Xtal oscillators aren't just "throw it in" kind of circuits.

The industry tends to refer to "Computer Grade" Xtals, which have the higher

ESR and load capacitance, a dangerous combination at times !

 

-- Kris

 

> > It really depends on the Xtal, rather than the clock oscillator
> itself.> [deleted]
>
> Philips actually recommends using larger value capacitors, there is a
> table in the user manual on page 39 that suggests values of 58 pF for
> a 30 pF crystal load capacitance and Rs < 50R.
>
> Leon

 

I see a table with recommended caps for various load cap. crystals (10, 20, 30 pF),

but no recommendation to use _large_ caps ?

 

I didn't mean to contradict you Leon, just that Xtal oscillators are very little understood

beasties at times, and many just bang it in without knowing their actual load capacitance,

but considering it is the heart of any MCU system, it should be given MUCH more attention.....

(regarding runtime reliability). Notwithstanding layout issues...

 

I hope the brief before clarfied it a little.

 

Mayge Philips_apps could clarify whether the oscillator indeed is a Pierce, and whether it's

a NOT gate or a FET ???

This will help in future.

 

B rgds

Kris

 

Robert wrote :

 
> This seems to be one of those little spoken of issues.  Hitex had a section
> on this in one of their white papers "Insider's Guide to Planning 166
> Family Design"  (http://www.hitex.com/download.html)
>
> Robert

Some oscillators will have trouble driving a Xtal properly, some will drive a Xtal

too hard.

Since the LPC2000 oscillator specs output caps as high as 56 pF, I doubt it would

have drive issues. A crystal that is driven too hard will age much quicker, and will

ultimately fail after a relatively short time.... A series resistor can fix this. (also avoids 3OT)

In case startup and/or runtime is an issue, the Xtal does not necessarily have to be

loaded with its optimal load capacitance. This mainly sets the proper frequency within

say 50 PPM, but in an MCU that's typ. a peripheral issue.

The output caps can be decreased where a Xtal for example has a so-so ESR.

This will provide a bit more drive, and then the input cap can be adjusted accordingly

to get the suitable phase shift for reliable oscillation.

If the Xtal clock is actually cclk, then however the 2 caps need to stay ~ equal in value

to maintain a 50 % dutycycle, as otherwise it will affect R/W timing in the CPU.

In that case the values can be dropped a bit to get a better startup and sustained oscillation.

 

In a marginally high ESR scenario however, a good quality Xtal should be used for production

though, for it's hard to characterize the oscillator in production over temperature.

 

Always bear in mind that with thru-hole, HC49S (low profile) crystals have a higher ESR

than their full profile HC49 buddies.

 

-- Kris