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Using one chip's clock to drive other chips' clocks

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Endolith

Member
If a chip has an integrated clock driver that uses an external crystal in the MHz, with Xin and Xout pins, what are the difficulties of using one chip's Xout pin as a master clock to drive other chips' Xin pins?

Of course all the chips must use the same crystal frequency. What else? Does the signal need to be buffered? What would you use for that? Does the voltage level matter? Does fanout to multiple chips load down the source and degrade the signal? Would it be better to use a dedicated oscillator to drive all the chips' clock inputs?
 

Wp100

Well-Known Member
hi,

Its difficult to say without knowing what exactly the chips are and how many of them you are talking about ?

Looking in the chips datasheet specifications should also help.
 

Endolith

Member
Depends on the chip, what IC are you referring to?
Secret. :) I just want to know the general principles. Three different chips that all use 12 MHz clocks. I have one of them, and looking on a 300 MHz scope with a 150 MHz probe, the XOUT pin is just a 12 MHz sine wave from 0.4 V to 2.9 V (2.5 Vpp). Can you just connect this directly to the XIN pins of other chips, or does it need to be cleaned up or impedance buffered somehow first?

Looking in the chips datasheet specifications should also help.
There is very minimal information about the crystal inputs and outputs, other than two of the datasheets stating "this input can be used with either a crystal or an external oscillator" and "this output should not be used to drive external clock circuits". Obviously I am concerned about the latter.
 
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Speakerguy

Active Member
If you are hooking a simple crystal up to an IC which contains the actual drive circuitry, DO NOT hook anything else up to those pins. See if you can use a GPIO as an output for the internal clock of whatever IC you are using. If not, just used a canned oscillator that contains its own drive circuitry and crystal and then just distribute the clock output to all the chips in the system.
 
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AllVol

New Member
... just used a canned oscillator that contains its own drive circuitry and crystal and then just distribute the clock output to all the chips in the system.
This appears to be the best solution considering the limited information available.
 

Hero999

Banned
If you are hooking a simple crystal up to an IC which contains the actual drive circuitry, DO NOT hook anything else up to those pins.
Not even a buffer with an extremely high impedance?
 

Endolith

Member
Found one chip's documentation on using an external crystal. It has a voltage recommendation for the oscillator signal, which comes into the XTIn pin through a 22 pF cap, and then it has the XTOut pin connected through a 22 pF cap to ground. I hadn't realized the unused output had to be connected to something.

If you are hooking a simple crystal up to an IC which contains the actual drive circuitry, DO NOT hook anything else up to those pins.
Because it can affect the frequency of the oscillation due to load capacitance, etc? I was imagining that the output would go through a high input impedance buffer, but I guess FET inputs can have a relatively large capacitance...

See if you can use a GPIO as an output for the internal clock of whatever IC you are using.
Good idea. I can probably use a GPIO.

If not, just used a canned oscillator that contains its own drive circuitry and crystal and then just distribute the clock output to all the chips in the system.
Aren't those less accurate/stable in frequency?
 
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