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# Crystal oscillator as clock source

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#### abhishek.2138

##### Full Member level 2
In my onboard charger project, we are using 20MHz crystal oscillator for microcontroller.
The output of oscillator is 3.3V.

To limit the current at XTAL pin of microcontroller, I have added 68k resistor in series.
Due to the crystal pin capacitance (that can be in pF range), it forms RC low pass filter.

If capacitance considered as 1pF, R = 68Kohm, then cut off frequency = 2.34MHz.
But, my intended operating frequency = 20 MHz.
Here, it will add delay to signal rise time.

Can anyone help me here? What can be done or any suggestion?
Is it ok to go ahead with this combination ?

Hi,

We don´t kow what´s the input requirement for your microcontroller.
I guess 3.3V is the power supply for the oscillator. Output levels need to be 2: V_OL and V_OH.
We don´t know why you exactly chose 68k. Why don´t you choose a different value.

Can anyone help me here?
What help exactly do you expect?

What can be done or any suggestion?
.. regarding what?

Is it ok to go ahead with this combination ?
It´s functionally O.K. if the signal meets the input requirements of your microcontroller.

Klaus

Hi,

We don´t kow what´s the input requirement for your microcontroller.
I guess 3.3V is the power supply for the oscillator. Output levels need to be 2: V_OL and V_OH.
We don´t know why you exactly chose 68k. Why don´t you choose a different value.

What help exactly do you expect?

.. regarding what?

It´s functionally O.K. if the signal meets the input requirements of your microcontroller.

Klaus
The current limit for the XTAL pin is 70uA max. So, to limit the current less than 70uA, 68k is chosen.
I = 3.3V / 68k = 48.52uA.
Micro main supply = 5V. But its compatible with 3.3V oscillator.
Yes, oscillator has two o/p states - VOL max & VOH min are 0.1Vcc to 0.9Vcc respectively. Here, Vcc = 3.3V
--- Updated ---

The current limit for the XTAL pin is 70uA max. So, to limit the current less than 70uA, 68k is chosen.
I = 3.3V / 68k = 48.52uA.
Micro main supply = 5V. But its compatible with 3.3V oscillator.
Yes, oscillator has two o/p states - VOL max & VOH min are 0.1Vcc to 0.9Vcc respectively. Here, Vcc = 3.3V
What can be impact of RC filter created at the micro pin ?

Hi,

The current limit for the XTAL pin is 70uA max.
I guess you got that one wrong.
* It either is the nominal/ maximum input current the microcontroller draws ... Means: The microcontroller itself limits it, no external limit needed
* or it is the limit for the protrection diodes if your input signal goes beyond the rail voltage.

Is it asking too much for a link to the datasheets? So we can cross check what values you refer to...

are 0.1Vcc to 0.9Vcc respectively. Here, Vcc = 3.3V
Do you ask for others doing the math?

if the signal meets the input requirements of your microcontroller.
in datasheet they are V_IH, VIL of the osc input and the maximum limits.

Klaus

Why would you "limit the current" on what is a digital clock
signal?

If you think you're going to protect a lower voltage I/O
logic pin, don't do that - you will push any overvoltage
as (V/R current) into the ESD protection ring. Maybe 50uA
into the clamp rail matters and maybe it doesn't. I guess
you can find out, at some point not of your own choosing.

3.3V into a 2.5V I/O rail through an ESD diode would not be
too terrible, but the lower your I/O rail, the harder it will hit.
A problem which most don't consider (or know) is that low
voltage ESD protection schemes are largely dynamic
triggered, and with a pre-pumped ESD clamp ring a little
HF noise could be gained way up by the designed "HF
gain above threshold"

If you need to translate levels, at speed, use a part made
for that.

Hi,

which MCU are you using? The reference manual or datasheet usually give an insight how the crystal should be configured. You might also check if a development board is available for this MCU family, and look how the XTAL is used their, if one is used at all.

BR

There is no reason to “limit the current” on a clock input. Your 68k is going to cause problems while not solving any. You might want to use, say, a 33 OHM resistor for impedance matching.

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