Is it safe to use an ATS sized crystal with an 89C66x?


I'd like to use an ATS sized crystal (e.g. CTS ATS111) with an 89C664
because it's much smaller than the traditional HC49 case. However the
datasheet for the crystal specified a maximum drive level of 100uW
(yes, microwatts!). I've no idea how much power the 89C66x on-chip
oscillator actually drives the crystal with, but 100uW doesn't sound
like much :-) Any advice?
Thanks,
Bob
Reply to
Bob Armstrong
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Find the crystal parameters (Rs, especially) and calculate the crystal power dissipation at your supply voltage, frequency etc.
Yes, it's quite possibly a problem.
Best regards, Spehro Pefhany
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Reply to
Spehro Pefhany
Here's the data sheet for the crystal
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Unfortunatley it says nothing about Rs. I did find another reference on the CTS page which specifies the crystal "ESR" as 40..150 ohms, depending on freqency.
I also found this interesting paper on the topic
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Taking Rs around 50 ohms and f=18.423Mhz, it sounds like we're talking about at least 1mW. Doesn't look good ...
Is there any other option besides one of those giant HC49s??
Bob
Reply to
Bob Armstrong
Can you use an SMT ceramic resonator?
Reply to
Spehro Pefhany
Can't find one from any mail order source (e.g. Digi-Key or Mouser) for 18.432Mhz....
Bob
Reply to
Bob Armstrong
Yes, that looks like a problem if you can't use a more common frequency.
Well, there are SMT oscillators, and they are a bit expensive and not all that much smaller (7 x 5mm).
Best regards, Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany
Unfortunately, it's not quite that simple - recall that the crystal model includes motional inductance (Ls), motional capacitance (Cs), shunt capacitance (Co), and series resistance (Rs). It's exceptionally difficult to calculate the real power disappation in the crystal.
100uW is probably higher than your on-chip oscillator is supplying. It's typically in the 1's or 10's of uW. Some very low-power IC's are in the 100's of nW.
I'd recommend the following:
(0) use load caps for the crystal that are double the value of the crystal's specified Cl. For example, if the crystal is spec'd for 12pF load capacitance, you should use two 22pF caps. Always 'round down' to the next standard value - you've got stray C on your PCB. Your crystal type should be a PARALLEL RESONANT type, and should be a FUNDAMENTAL type (NOT an OVERTONE type!!).
(1) insert an external fixed resistor in series with the crystal. Place this resistor between the crystal terminal and the OUTPUT of the on-chip oscillator. The value of this resistor should be 4x the ESR of the crystal you're intending to use.
(2) temporarily include a ~5K potentiometer in series with your resistor. So, you should have, in order: IC output pin -> pot -> fixed resistor (3x ESR) -> crystal (and one load cap to ground at this node as well). Set the pot for MAX resistance.
(3) at this point, the crystal oscillator shouldn't start when supply voltage is applied to the uC. If it does, use a larger pot. You don't want the oscillator to start with the pot at maximum resistance.
(4) decrease the resistance of the potentiometer until the crystal starts up reliably. If your application involves working over a temperature range, then make sure you perform this test at the temperature extremes. Same goes for voltage range, too, if you're working off of an unregulated supply.
(5) remove the pot from the circuit and measure its value.
(6) replace the fixed resistor (4x ESR) with a fixed resistor of the same value as the pot's measurement.
So, you should have IC output pin -> fixed resistor (equal to the value measured from the potentiometer) -> crystal (and one load cap to ground).
What you've just done is ensure that you've got sufficient "oscillation margin" to guarantee startup - is desireable to have the "negative resistance" in the loop be about 3 ~ 5 times the ESR of the crystal. That helps ensure that your crystal isn't overdriven and runs at a spurious mode frequency. Unfortunately, the ESR value is really dependent upon the drive level, but without ESR data at this particular IC's drive level, this is all you can go on.
Good luck! TJL
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Reply to
Tom LeMense

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